Maya Tutorial - Expressions

Expressions

Expressions 3 Introducing Expressions About expressions

12

Where you create expressions

4 Quick Start

11

13

15

Preparing for the examples

15

Creating a simple expression

17

Controlling multiple attributes of an object Controlling attributes in two objects Controlling attributes conditionally Notes on the predefined time variable

5 Expression Syntax Expressions and MEL

32 43

46 47

49

Static attributes

49

Dynamic attributes

49

Custom attributes Attribute names

50 51

Data types of attributes

51

Assigning a value to an attribute Variables

28

45

Elements of an expression Attributes

23

55

56

Data types of variables Predefined variables Custom variables

57 57

59

Using Maya: Hypergraph, Sets & Expressions

3

Expressions Contents Constants

62

Arithmetic, logic, and relational operators Arithmetic operators

63

Relational operators

65

Logical operators Operator precedence

67 68

Conditional statements if statements

69

69

if-else statements

70

else if statements

71

General syntax rules

73

Comments in expressions Programming features

75

75

Notes for C programmers

75

Expression language keywords Flow control statements Flow control errors String usage

63

76

77

88

90

Shortcut assignment operators

91

Shortcut increment and decrement operators Arrays

93

Boolean symbolic constants Common expression errors

95

Error message format

95

Common error messages

6 Editing Expressions Finding expressions

97

99

99

Finding by expression name Finding by selected object

4

95

100 101


92

Expressions Contents Finding by item type

102

Using the Selection list

103

Filtering attributes from the Selection list Editing an expression in the text field Deleting and copying text

104

105

105

Clearing the expression text field

106

Reloading an expression’s previous contents Editing an expression with a text editor

106

Using an editor listed in the Editor menu

107

Using an editor not listed in the Editor menu Changing an editor’s operation settings Selecting an editor for default startup Creating a new expression Deleting an expression

106

109

110 110

111

112

Using attribute names in expressions

112

Using attribute name abbreviations

113

Omitting an object name in expressions

115

Combining the abbreviation techniques

116

7 Beyond the Basics

117

How often an expression executes

118

Using custom attributes in expressions

118

Displaying attribute and variable contents Reproducing randomness

123

123

Speeding expression execution

127

Reducing redundant expression execution

130

Removing an attribute from an expression

131

Disconnecting an attribute

132

Displaying disconnected attributes in expressions

132

Connecting an attribute to a symbolic placeholder

135


5

Expressions Contents Renaming an object

136

Executing MEL commands in an expression Understanding path names

137

140

Understanding unexpected attribute values Values after rewinding Increment operations

141

141 142

Data type conversions

143

8 Particle Expressions

147

Understanding particle expressions

148

Understanding creation expression execution Setting the dynamics start frame

149

Setting attributes for initial state usage Writing creation expressions

149 150

150

Understanding runtime expression execution Writing runtime expressions

152

153

Working with particle attributes

159

Adding dynamic attributes

159

Understanding per particle and per object attributes Understanding initial state attributes

160

162

Example of assigning to a dynamic per particle attribute Example of assigning to a dynamic per object attribute Assigning to a custom attribute

Using creation expression values in a runtime expression Working with position, velocity, and acceleration 178

Working with emitted particles Working with collisions

6

183

183

Working with specific particles

167

169

Assigning to a particle array attribute of different length

Working with color

164

189


175

172 174

Expressions Contents Assigning to vectors and vector arrays List of particle shape attributes

9 Functions

193

196

203

Understanding functions Function syntax

205

206

Data types

208

Understanding function examples in this chapter Limit functions

209

abs

209

ceil

210

floor clamp

210 211

min

212

max

212

sign

212

trunc

213

Exponential functions exp

214

log

214

log10

215

sqrt

215

Trigonometric functions cosd sin sind tan tand

214

214

pow

cos

208

216

216 218 219 224 224 225


7

Expressions Contents acos

225

acosd

226

asin

226

asind

226

atan

227

atand

227

atan2

227

atan2d

228

hypot

228

Vector functions

229

angle

229

cross

230

dot

231

mag rot

231 232

unit

233

Conversion functions

234

deg_to_rad

234

rad_to_deg

234

hsv_to_rgb

235

rgb_to_hsv

235

Array functions clear

236

236

size

237

sort

237

Random number functions gauss

239

noise

241

dnoise rand

242 243

sphrand

8

239

244


Expressions Contents seed

246

Curve functions linstep

249 249

smoothstep hermite

254

General commands eval

259

print

261

system

252 259

263

Other functions and commands

264


9

Expressions Contents

10


3

Introducing Expressions Expressions are instructions you type to control an object attribute over time. An attribute is a characteristic of an object, for instance, X scale, Y translate, visibility, and so on. Though you can create an expression to animate attributes for any purpose, they’re ideal for attributes that change incrementally, randomly, or rhythmically over time.

An expression gives the manta ray’s wings a fluid, rhythmic motion.

Expressions

Eric Saindon

Expressions are also useful for linking attributes between different objects— where a change in one attribute alters the behavior of the other. For instance, you can make the rotation of a tire dependent on the forward or backward movement of a car. This chapter has the following topics: •

“About expressions” on page 12

•

“Where you create expressions” on page 13


11

Introducing Expressions About expressions

About expressions Expressions offer an alternative to difficult keyframing tasks. In keyframing, you set the values of attributes at selected keyframes in the animation, and Maya interpolates the action between the keyframes. With expressions, you write a formula, then Maya performs the action as the animation plays. Expressions are often as simple as a few words or lines. In the following example expressions, note the variation in length and detail (rather than their purpose).

Example Ball.translateX = Cube.translateX + 4;

Example if (frame == 1) Cone.scaleY = 1; else { Cone.scaleY = (0.25 + sin(time)) * 3; print(Cone.scaleY + "\n"); }

Though many expressions look like math or a programming language, you don’t need to be a mathematician or programmer to learn how to use them. If you’re fond of programming, expressions offer unlimited animation techniques that would challenge the skill of keyframing experts. You can use an expression to animate any keyable, unlocked object attribute for any frame range. You can also use an expression to control per particle or per object attributes. Per particle attributes control each particle of an object individually. Per object attributes control all particles of an object collectively. You cannot apply an expression to an attribute already animated with any of these techniques:

12

•

keys

•

set driven key

•

constraint

•

motion path


Introducing Expressions Where you create expressions •

another expression

•

any other direct connection If you do so, you’ll see an error message in the Script Editor and the Command Line’s response area. Though you can’t control a single attribute with two of the preceding techniques, you can control one attribute with keyframes, another with an expression, another with a constraint, and so on. Also, you can use a single expression to assign values to several attributes of one or more objects.

Where you create expressions You create and edit an expression in the Expression Editor. There are several ways to start the Expression Editor: From the main menu bar or Hotbox, choose Window→Expression Editor.

•

From the Channel Box, click the right mouse button in an attribute text field and select Expressions.

•

From the Attribute Editor, click the right mouse button in an attribute text field and select Create New Expression, Edit Expression, or Expression Editor. You cannot start the Expression Editor from every attribute text field in the Channel Box and Attribute Editor. Use Window→Expression Editor if necessary.


13

Expressions

•

Introducing Expressions Where you create expressions The Expression Editor follows:

Expression text field

The expression text field expands as you type text, so you can write expressions of unlimited length. You can also edit expressions with a text editor such as jot by selecting it from the Editor pull-down menu above the text field.

14


4

Quick Start The easiest way to learn about expressions is to work through examples. For this reason, we provide the following introductory lessons. Expressions that control particle attributes are more complex than for other objects. For examples, see Chapter 8, “Particle Expressions.”

You can use an expression to link attributes in different objects—so a change in one attribute alters the behavior of the other.

Expressions

In this chapter, you’ll learn about the following topics: •

“Creating a simple expression” on page 17

•

“Controlling multiple attributes of an object” on page 23

•

“Controlling attributes in two objects” on page 28

•

“Controlling attributes conditionally” on page 32

•

“Notes on the predefined time variable” on page 43

Preparing for the examples A few preparatory steps will simplify your understanding of the examples in this chapter. Before starting the examples, do these steps:

To prepare for the examples: 1

Select Options→General Preferences. Using Maya: Hypergraph, Sets & Expressions

15

Quick Start Preparing for the examples The General Preferences window appears. Drag either side of the General Preferences window to expand its width. You must do this to display the Units tab in the window. 2

Click the Units tab.

3

In the Units tab, make sure Time is set to Film (24 fps). This makes your animation play at the default rate of 24 frames per second.

4

In the General Preferences window, choose the Animation tab.

5

Enter 0 for the starting frame of the Time Slider and the Range Slider, and enter 300 for the ending frame of the Time Slider and Range Slider.

Important For the lessons to work correctly, you must enter 0 for the starting frame of the Time Slider and Range Slider. Press your keyboard’s Enter key after each entry. Rewind the animation to frame 0. After doing the lessons, read “Notes on the predefined time variable” on page 43 for details on why the lessons require the starting frame to be 0. Specifying a range of 300 frames gives ample time to see the effects you’ll create in the examples. 6

In the General Preferences window, click Save and Close.

7

At the top edge of the workspace, select Shading→Smooth Shade All to display all objects you create in the scene with smooth shading. This will enhance the look of the objects you create in the examples.

8

16

From the menu bar, choose Window→Expression Editor to display the Expression Editor.


Quick Start Creating a simple expression 9

Make sure these default Expression Editor menu options are selected: Select Filter→By Object/Attribute Name Object Filter→Selected Objects Attribute Filter→All “Finding expressions” in Chapter 6 gives details on these options.

Creating a simple expression The following steps show how to control an attribute of a single object. An attribute is a characteristic of an object, for example, X scale, Y scale, X rotation, and so on. In this example, you’ll learn how to stretch a sphere along its Y-axis by controlling its scaleY attribute as the animation plays.

To prepare the scene: 1

From the Modeling menu, choose Primitives→Create NURBS→Sphere. This creates a NURBS sphere with an X scale, Y scale, and Z scale of 1. In the Channel Box or elsewhere, name the sphere Ball.

3

Select Display→NURBS Smoothness→Fine to set Ball’s display smoothness to fine resolution. Maya displays the object with extra polygons to make it look smoother in the workspace. This doesn’t affect the underlying model’s geometry. It alters only its display.

4

Rewind the animation to frame 0.


17

Expressions

2

Quick Start Creating a simple expression

To create the expression: 1

Make sure Ball is selected.

2

Choose Window→Expression Editor to display the Expression Editor. The selected object’s name, Ball, is highlighted in the Objects list of the Expression Editor.

3

Enter ScaleBallHeight in the Expression Name box. Entering an expression name lets you find the expression easily in a later work session if you decide to alter it. Use alphabetical and numerical characters for expression names. If you use space characters or special characters such as a hyphen (-), Maya deletes them or replaces them with an underscore character (_) after you finish creating the expression.

4

Notice the Attributes list. It displays Ball’s keyable, unlocked attributes—the attributes you’ll most likely want to animate with an expression. Use the scroll bar to see the entire list.

5

Enter this expression in the expression text field:

Ball.scaleY = time + 1;

Enter the expression with the same upper and lowercase spelling shown. Entries in the expression field are type case sensitive. The semicolon (;) signifies the end of the expression statement. Each statement in an expression must end with a semicolon. The only exception is when the expression has a single statement. An error message appears in the Script Editor and Command Line’s response area if the expression has incorrect syntax or typing mistakes. Edit text the same way you edit other text fields in Maya.

18



Click Create to compile the expression. Compiling the expression checks it for syntax errors and converts it to a form Maya can execute when you rewind or play the animation. Clicking this button also executes the expression for the current frame. The expression sets Ball’s scaleY attribute to the value of time + 1. Ball.scaleY is the full name of the attribute. A period separates the name of the object and attribute. You must spell them with uppercase and lowercase letters as they appear in the Expression Editor’s Objects and Attributes lists. See “Using attribute names in expressions” in Chapter 6 for more details. The word time is a predefined variable in Maya that updates as an animation plays. It contains the elapsed number of seconds from the first frame to the current frame. The value increases with the increasing frame number. At the default animation playback rate of 24 frames per second, time has these values, rounded to four decimal places: Time (seconds)

0

0

1

0.0417

2

0.0833

3

0.125

24

1.0

240

10.0

Expressions

Frame

If you ever need to change the playback rate, you can do so by choosing Options→General Preferences. Expand the General Preferences window, display the Units folder, and choose the desired rate from the Time menu. Regardless of what animation playback rate you choose, you can find the time elapsed in the animation at any frame with this formula:

frame time = --------------rate For example, if the frame rate is 24 frames/second and the animation is at frame 1, the elapsed time is 1 divided by 24, or 0.0417. At frame 6, the elapsed time is 6 divided by 24, which equals 0.25. Using Maya: Hypergraph, Sets & Expressions

19

Quick Start Creating a simple expression If the frame rate is 30 frames/second and the animation is at frame 1, the elapsed time is 1 divided by 30, which equals 0.0333. At frame 6, elapsed time is 6 divided by 30, which equals 0.2.

To see the result of the expression: 1

Rewind and play the animation. Ball’s scaleY attribute increases as the time increases: Frame

Time (seconds)

Ball.scaleY (time + 1)

0

0

1

1

0.0417

1.0417

2

0.0833

1.0833

3

0.125

1.125

24

1.0

2.0

240

10.0

11.0

Maya executes the expression each frame. This causes the object size to scale along its Y-axis, stretching its height during playback.

The scaling is smooth because the geometry stretches in synch with the small time increments of the animation playback.

20



Stop and rewind the animation. Rewinding the animation returns Ball to its original shape. This occurs because the expression executes with time equal to 0. The value of time + 1 is 1, the original scaleY value of Ball.

3

Play the animation. This repeats the increasing scale.

4

Stop and rewind the animation.

5

Close the Expression Editor window. This complete the steps to creating an expression. To further your understanding of expressions, we’ve included the following steps to show how to edit the expression you just completed.

To edit the expression: Suppose you decide that Ball scales too quickly as the animation plays. You can change the expression to see how the animation looks when you scale Ball half as fast. 1

Choose Window→Expression Editor to display the Expression Editor again. You can find the expression you created earlier by: remembering the name of the expression

•

remembering the name of the object and attribute you controlled with the expression

•

examining each expression in the scene that’s controlled by an expression In this example, you’ll find the expression ScaleBallHeight by its name. See Chapter 6, “Editing Expressions” for details on the other methods.

2

Choose Select Filter→By Expression Name.

3

Click ScaleBallHeight in the Expressions list. The expression appears in the expression text field.


21

Expressions

•

Quick Start Creating a simple expression

Ball.scaleY = time + 1;

4

Change the previous expression to this: Ball.scaleY = time/2 + 1;

Use the same editing techniques you use with other text fields in Maya. By dividing time by 2, you’ll make the Y scaling increase half as fast as with the previous version of the expression. 5

Click Edit to compile the modified expression. Compiling the expression checks it for syntax errors and converts it to a form Maya can execute when you rewind or play the animation. After being compiled, the expression executes for the current frame. Clicking the Edit button does the same action as clicking the Create button. The Create button exists only for new expressions. The Edit button replaces the Create button when you display an existing expression.

To see the result of the edited expression: 1

Play the animation. Ball scales its Y dimension half as fast as with the previous expression contents.

2

Stop and rewind the animation. Feel free to experiment with other values in the expression. This concludes the first example. Before starting the next example, delete the scene and create a new scene.

Summary In this lesson, you learned how to: •

22

name an expression and type it in the expression text field


Quick Start Controlling multiple attributes of an object •

compile an expression to a form Maya can execute

•

work with the predefined variable time

•

find an expression you previously created

•

modify an expression

Controlling multiple attributes of an object You can use a single expression to control two or more attributes of an object. In the following steps, you’ll use an expression to increase the X, Y, and Z scale attributes of a sphere as the animation plays.

Expressions


From the Modeling menu, choose Primitives→Create NURBS→Sphere. This creates a NURBS sphere with an X scale, Y scale, and Z scale of 1.

2

In the Channel Box or elsewhere, name the sphere Planet.

3

Select Display→NURBS Smoothness→Fine to set the Planet’s display smoothness to fine resolution.

4

Display the Expression Editor and select Select Filter→By Object/Attribute Name.

5



Select the Planet object. Planet becomes the selected object in the Expression Editor.

2

In the Expression Editor, enter ScalePlanet in the Expression Name box. Using Maya: Hypergraph, Sets & Expressions

23

Quick Start Controlling multiple attributes of an object This names the expression so you can find it more easily later. 3

Enter these statements in the expression text field:

The expression has three statements. Each statement sets an attribute to the value of the predefined variable time. 4

Click Create to compile the expression. An error message appears in the Script Editor and Command Line’s response area if the expression has incorrect syntax. Planet disappears because clicking Create also executes the expression at the current frame after compiling. At frame 0, time is 0, so the value of the scaleX, scaleY, and scaleZ attributes becomes 0. Planet has no size, so it disappears.

To see the result of the expression: 1

Play the animation. The expression executes each frame, so Planet grows quickly as the animation plays.

24


Quick Start Controlling multiple attributes of an object

Because animation playback increases time, the value of Planet’s scaleX, scaleY, and scaleZ attributes increase at the rate of the increasing time. The object increases its scale until the last frame of the Time Slider plays. 2

Stop and rewind the animation. The following two series of steps show how to see the same result with other methods.

To see the result by linking attribute values: 1

Change the expression to this: Expressions

Planet.scaleX = time; Planet.scaleY = Planet.scaleX; Planet.scaleZ = Planet.scaleX;

2

Click Edit to compile the expression.

3

Play the animation. The expression works the same as the previous one. The first statement sets Planet.scaleX to the value of time. The second statement sets Planet.scaleY to the value of Planet.scaleX. Because you’ve set Planet.scaleX to the value of time, Planet.scaleY also has the value of time. You’re simply transferring one attribute’s value to another. The third statement also sets Planet.scaleZ to the value of the attribute Planet.scaleX.


25

Quick Start Controlling multiple attributes of an object The advantage of this expression is that if you assign a different value to Planet.scaleX in the first statement, the second and third statements automatically receive the new value. In other words, you’ve linked Planet.scaleY and Planet.scaleZ to the value of Planet.scaleX—whatever its value is. 4


5

In the first statement of the expression, divide time by 5 as follows: Planet.scaleX = time/5; Planet.scaleY = Planet.scaleX; Planet.scaleZ = Planet.scaleX;

6


7

Play the animation. Planet increases its scale attributes one-fifth as fast of the previous expression. By assigning the value of Planet.scaleX to Planet.scaleY and Planet.scaleZ, Planet.scaleY and Planet.scaleZ were automatically assigned the value time/ 5 in the second and third statements.

8

Stop and rewind the animation. You can get the same result using a variable in an expression.

To see the results using a variable: 1

Change the expression to this: float $increment; $increment = time/5; Planet.scaleX = $increment; Planet.scaleY = $increment; Planet.scaleZ = $increment;

The expression has the same result as the previous one. The first statement defines a variable named $increment to be used as storage for the value of a time increment. You define it as a floating point number—a number that can have a decimal point. The second statement assigns $increment the value of time divided by 5. As the animation plays and the time increases each frame, the value of $increment increases by the value of time divided by 5. The $increment therefore increases in smaller units than time increases.

26


Quick Start Controlling multiple attributes of an object The next three statements assign the contents of $increment to the three scale attributes of Planet. The scale attributes therefore receive the value of time divided by 5 each frame. This makes the object increase uniformly in scale slowly as the animation plays. 2

Click Edit.

3

Play the animation. The result is the same as with the previous expression

4

Stop and rewind the animation. You can make a change to the variable assignment in the second statement without altering the other statements.

To modify the variable: 1

Change the expression to this: float $increment; $increment = time * 2; Planet.scaleX = $increment; Planet.scaleY = $increment; Planet.scaleZ = $increment;

Click Edit.

3

Play the animation. Because you assigned time * 2 to the variable $increment, the expression sets all three attributes to the value of time * 2 as the animation plays. This makes the three scale attributes increase at a rate twice as fast as would occur if you assigned them the value of time alone.

4

Stop and rewind the animation. This concludes the example. Before starting the next example, delete the scene and create a new scene.


link multiple attributes of the same object with a single expression

•

use a variable you defined in an expression

•

modify a single assignment to an attribute without changing other statements


27

Expressions

2

Quick Start Controlling attributes in two objects

Controlling attributes in two objects You can write an expression to control attributes in two or more objects. In the following steps, you’ll create a cylinder and cone, then rotate each around its local X-axis as the animation plays. In other words, each object will spin around in place.

To see an object’s local rotation axes, select the object, then choose Display→Object Components→Local Rotation Axes.


From the Modeling menu, choose Primitives→Create NURBS→Cone.

2

In the Channel Box or elsewhere, name it Cone.

3

Choose Primitives→Create NURBS→Cylinder from the Modeling menu.

4

In the Channel Box or elsewhere, name the cylinder Can. The exact translation and scale of Cone and Can is unimportant in this example. Give them roughly the same translation and scale as in the above figure.

5

Select both objects, then choose Display→NURBS Smoothness→Fine. This sets the display smoothness of both objects to fine resolution.

28

6


7





Select Can. To control attributes in both objects, you can select either object to write the expression. In fact, any object or node in a scene can be selected when you write an expression to control an object other than a particle object. For details on creating expressions to control particles, see Chapter 8, “Particle Expressions.”

2

In the Expression Editor, enter RotCanAndCone in the Expression Name box. This names the expression so you can find it more easily later.

3

Enter this expression: Can.rotateX = time * 10; Cone.rotateX = time * 10;

This assigns Can’s rotateX attribute and Cone’s rotateX attribute to the value of time multiplied by 10. 4

Click Create to compile the expression.

To see the results: 1

Play the animation. Expressions

Each object rotates around its local X-axis by the degree value resulting from time * 10. After 1 second, for example, the rotateX attribute of each object is one degree times 10, or 10 degrees. After 2 seconds, it’s 2 degrees times 10, or 20 degrees. Maya works in degree angle units, by default. You can change the angular units to radians by choosing Options→General Preferences and displaying the Units folder.


29

Quick Start Controlling attributes in two objects With the animation playing at 24 frames per second, each object’s rotateX attribute has these values: Frame

Time

Can.rotateX (degrees)

0

0

0

1

0.0417

0.417

2

0.0833

0.833

3

0.125

1.25

24

1.0

10

240

10.0

100

The values in this chapter are rounded to four significant digits. The actual values might have many more digits. To see the degree value of Can.rotateX at different frames, select Can, display the Channel Box, and stop the animation at selected frames. The Channel Box updates its values after you stop the animation. To see the degree value of Cone.rotateX at different frames, select Cone instead of Can. The Channel Box displays values for the selected object. 2

Stop and rewind the animation. You can edit the expression to make Can rotate slower than Cone.

To get different results: 1

Change to the expression to this: Can.rotateX = time * 5; Cone.rotateX = time * 10;

2


3

Play the animation. Can rotates half as fast as Cone as the animation plays.

30



4


5

Close the Expression Editor window. This concludes the example. This example showed how to write a single expression to control attributes of two different objects. However, you could have written two expressions, one that rotates Can and one that rotates Cone. The advantage of creating separate expressions is that you’ll have two expression names, each presumably named after the object and attribute you’re controlling. Having two expression names makes it easier to find the expression that controls the desired attribute.

Before starting the next example, delete the scene and create a new scene.


control rotateX attributes in two objects using the same expression

•

increase the rotation of each object in synch with animation time

•

rotate one object at half the speed of the other object


31

Expressions

The advantage of using a single expression to control the attributes is that all statements are in a single expression. You don’t need to edit two expressions.

Quick Start Controlling attributes conditionally

Controlling attributes conditionally You can write an expression that takes different actions depending on the value of attributes or variables it examines as an animation plays. In the following steps, you’ll increase the scale of a sphere for the first two seconds of animation, then stop scaling and move it in a global Y-axis direction for the remainder of the animation.


From the Modeling menu, choose Primitives→Create NURBS→Sphere. By default, this creates a NURBS sphere at the origin with an X scale, Y scale, and Z scale of 1.

2

From the Channel Box or elsewhere, name the sphere Balloon.

3

Select Display→NURBS Smoothness→Fine to set Balloon’s display smoothness to fine resolution.

4


5



Select Balloon.

2

In the Expression Editor, enter RisingBalloon in the Expression Name box.

3

Enter this expression: if (time < 2) Balloon.scaleY = time;

This expression is an if statement. The if keyword causes the expression to make a decision based on a comparison of two or more items. In this case, the expression compares the value of time to the value 2.

32



Important When you compare the value of time to a number in an expression, Maya interprets time as seconds rather than milliseconds, minutes, or any other unit of time. In the example, Maya interprets 2 as 2 seconds. The expression checks whether the value of time is less than two seconds. If so, it does the assignment Balloon.scaleY = time. If time is not less than two seconds, the assignment doesn’t occur. Notice the indentation of Balloon.scaleY = time under if (time < 2). Maya ignores all indentation, extra spaces, and blank lines between statements. We used the indentation to make the expression easier to read. You could have also written the expression as follows: if (time < 2) Balloon.scaleY = time;

This isn’t as easy to read. Consistent, organized spacing is a good habit to develop. This book shows examples of good spacing style. 4

Click Create to compile the expression. Balloon flattens. Expressions

The expression executes when you click the Create button. Because the animation is at frame 0, animation time is 0. Because time is less than 2, Maya sets Balloon.scaleY equal to the value of time, which equals 0. A scaleY value of 0 flattens the object in the Y dimension. 5

Play the animation. The flattened Balloon’s scale increases along its Y-axis. It inflates as the animation plays.


33


At 2 seconds, Balloon stops inflating. An expression executes each frame as an animation plays. The if statement sets the scaleY attribute of Balloon to the value of time each frame when the time is less than 2. When time equals 2 or more, the if condition is no longer true. The statement that follows it, Balloon.scaleY = time, no longer executes. The value of the scaleY attribute stays at the last value it had before time became 2, specifically, 1.9583. At 2 seconds of animation time and every moment thereafter, scaleY continues to be 1.9583. Recall that this example uses a frame rate of 24 frames/second. The time and Balloon.scaleY have these values at various frames: Frame

Time (seconds)

Balloon.scaleY (time)

0

0

0

1

0.0417

0.0417

2

0.0833

0.0833

3

0.125

0.125

24

1.0

1.0

47

1.96

1.9583

48

2.0

1.9583

49

2.04

1.9583

The if statement’s condition, (time < 2), is a comparison. The condition must be surrounded by parentheses to isolate it from assignment that follows it.

34


Quick Start Controlling attributes conditionally The < in the condition is a relational operator. A relational operator tests how one value relates to another. In the example, the < tested whether time is less than 2. Besides the < operator shown in this example, there are several other relational operators such as >, >=, ==, and so on. See “Arithmetic, logic, and relational operators” in Chapter 5. 6

Stop and rewind the animation. Balloon flattens again because the scaleY attribute becomes 0 when you rewind the animation. Time is 0, so scaleY is 0. You can make Balloon rise after it inflates by adding a second if statement to the expression.

To add another if statement to the expression: 1

Change the expression to this: if (time < 2) Balloon.scaleY = time; if (time >= 2) Balloon.translateY = time;


3

Play the animation. Balloon inflates for 2 seconds. After 2 seconds, Balloon stops inflating and its position skips from a Y-axis position of 0 to 2. You’ll eliminate the motion skip in a later step. The second if statement increases the translateY position of Balloon after the animation time rises above two seconds. The >= symbols mean greater than or equal to. Whenever time is greater than or equal to 2, the expression assigns Balloon’s translateY the value of time. The translateY value therefore increases for the rest of your animation’s playback range. Notice that a semicolon ends each statement. Forgetting a semicolon after each statement causes a syntax error, and the changes you’ve made to the expression won’t take effect.


35

Expressions

2


Important Always examine the Script Editor for error messages after you edit an expression and click the Create button. If you alter a previously successful expression and a syntax error occurs, Maya executes the previous successful expression when you play the animation. This might make you believe your editing changes took effect. 4

Stop and rewind the animation. Balloon flattens but doesn’t return to the origin. (If Balloon has risen out of view, adjust your camera to see it.)

Balloon doesn’t return to the origin because the expression doesn’t assign Balloon a starting point for the beginning of the animation. 5

To make Balloon return to the origin, change the expression to this: if (time == 0) Balloon.translateY = 0; if (time < 2) Balloon.scaleY = time; if (time >= 2) Balloon.translateY = time;

The new first statement sets Balloon.translateY to 0 whenever time equals 0. The == symbols mean is equal to. In conditional statements, be careful to type == rather than =. The = symbol means assign the value to.

36


Quick Start Controlling attributes conditionally Note that you can put the three statements in any order in this example. When Maya plays each frame, it executes each statement in the expression in the order listed. In this example, the statements work independently, so their order doesn’t matter. We put the statements in the order of time execution because it’s easier to see the logic of the expression. If you ever need to change the expression, you’ll be able to grasp the expression’s actions more quickly. 6

Click Edit.

7

Stop and rewind the animation again. The flattened Balloon returns to its correct position at the origin.

8

Play the animation. Balloon inflates for two seconds, then rises. Expressions

As mentioned before, Balloon skips from Y-axis position 0 to 2 after two seconds of animation play. You can eliminate the skipping and make Balloon rise smoothly from the origin.

To eliminate the motion skip: 1


2

Change the expression to this: if (time == 0) Balloon.translateY = 0; if (time < 2) Balloon.scaleY = time; if (time >= 2) Balloon.translateY = time - 2;

3

Click Edit.

4

Play the animation.


37

Quick Start Controlling attributes conditionally Balloon inflates for 2 seconds, then rises slowly with time from its position at the origin. When time is greater than or equal to 2, the translateY position of Balloon becomes 2 minus 2, which is 0. As time increases beyond 2 seconds, the translateY position increases in the same increments that time increases.

5

Stop and rewind the animation. The expression achieved the desired result, but it’s more complicated than necessary. You can use an if-else statement to make the statement more compact and easier to read.

To use an else statement instead of multiple if statements: 1

Change the expression to this: if (time == 0) Balloon.translateY = 0; if (time < 2) Balloon.scaleY = time; else Balloon.translateY = time - 2;

2

Click Edit.

3

Play the animation. The else keyword sets Balloon.translateY to time - 2 when (time < 2) is false. In English terms, the combination of the if and else statements says, “If time is less than two seconds, set Balloon.scaleY to the value of time. Otherwise (when time is greater than or equal to two seconds), set Balloon.translateY to time minus two.”

38


Quick Start Controlling attributes conditionally At any instant in the animation’s playback, either Balloon.scaleY = time executes or Balloon.translateY = time - 2 executes. Under no circumstances can they both execute. The else statement executes only when the if condition that precedes it is false. Note that we added a blank line between the first if statement and the if-else statement combination. This has no effect on the execution of the statements. We put it there to emphasize that the two if statements are unrelated. The first if statement executes whenever time equals 0. It is unrelated to the if-else statements. Using else statements instead of multiple if statements makes an expression simpler to read. If you use an if-else construction instead of a lengthy list of if statements, you’ll also improve the execution speed of the expression. This improves your animation’s playback and rendering speed. Either expression is valid. If using the if-else construction seems confusing, stick with multiple if statements. You can accomplish most expression animation tasks with several if statements strung after one another. 4

Stop and rewind the animation. You can refine the expression to make it even easier to read. Expressions

To make the expression easier to read: 1

Change the expression to this: if (time < 2) { Balloon.translateY = 0; Balloon.scaleY = time; } else Balloon.translateY = time - 2;

We removed this statement from the previous version of the expression: if (time == 0) Balloon.translateY = 0;

In its place, we put the statement Balloon.scaleY = time in a segment enclosed by the braces { and }. Maya evaluates both the statements between the braces if the condition (time < 2) is true. 2

Click Edit. Using Maya: Hypergraph, Sets & Expressions

39

Quick Start Controlling attributes conditionally 3

Play the animation. The animation plays exactly as before with the new expression. When the animation time is less than two seconds, not only does Maya set Balloon.scaleY to time, it sets Balloon.translateY to 0. Balloon has a position at the origin until the animation time is greater than or equal to 2 seconds. Setting Ball.translateY to 0 here instead of in a separate if statement makes the expression easier to read and comprehend. As in the previous version of the expression, if time is greater than or equal to 2, Maya executes the else statement. Note that you can put multiple statements between braces for an else statement, just as you do for an if statement.

4

Stop and rewind the animation. You can further refine the animation’s appearance by expanding Balloon more slowly.

To slow Balloon’s expansion: 1

Change the expression to this: if (time < 2) { Balloon.translateY = 0; Balloon.scaleY = time * 0.6; } else Balloon.translateY = time - 2;

Only one statement is different, Balloon.scaleY = time * 0.6. The asterisk (*) multiplies time by 0.6.

40

2

Click Edit.

3

Play the animation.



The scaleY attribute increases at 60% of the value of time, so Balloon expands slower during playback. (The number 0.6 equals 60%.) By the time Balloon starts to rise, it has expanded to the size of a typical balloon. How do you know whether to multiply time by 0.6 or some other number? You don’t. In cases like this, you need to experiment. For example, you might multiply by various percentages such as 0.2, 0.5, 0.75, and finally 0.6. The 0.6 creates a life-like balloon shape at two seconds. 4

Stop and rewind the animation. You can further refine Balloon’s appearance by eliminating the flattened Balloon that appears at the origin when you rewind the animation. You can also scale Balloon at different rates along each of its three axes. Expressions

To further refine Balloon’s appearance: 1

Change the expression to this: if (time < 2) { Balloon.translateY = 0; Balloon.scaleY = time * 0.6; Balloon.scaleX = time * 0.5; Balloon.scaleZ = time * 0.5; } else Balloon.translateY = time - 2;

2

Click Edit. Balloon disappears from view because its scale attributes are 0. The scaleX, scaleY, and scaleZ attributes are 0 at frame 0 because time is 0. Any number multiplied by 0 is 0.

3

Play the animation. Using Maya: Hypergraph, Sets & Expressions

41

Quick Start Controlling attributes conditionally As time increases, the value of Balloon’s scale attributes increase. Because the expression sets scaleX and scaleZ to 50% of the value of time, these dimensions scale slower than scaleY, which is set to 60% of the value of time. Balloon scales faster in height than in width or depth. This is true for many real balloons.

4

Stop and rewind the animation. This concludes the example.

Summary Using an expression is a combination of logic and experimentation. Problem solving starts with breaking a task into smaller problems you can solve and later refine. In this lesson, you learned how to:

42

•

control an attribute conditionally with an if statement

•

use good spacing and indentation for expressions

•

use relational operators such as <, <=, and ==

•

use multiple if statements to control different conditions

•

use an if-else statement in place of multiple if statements to make an expression easier to read and comprehend

•

refine an expression with a combination of analysis and experimentation


Quick Start Notes on the predefined time variable In the preceding examples we didn’t include comments in expressions because we explained them line by line. When you write your own expressions, include comments with statements to help document how the expression works. This will help you or someone else understand how your expression works later if the need to enhance it arises. See “Comments in expressions” in Chapter 5.

Notes on the predefined time variable The lessons in this chapter use a starting frame number of 0. In your work, you’ll typically create an animation with a starting frame number of 1. Because the examples use Maya’s default frame rate of 24 frames per second, time is 0.0417 at frame 1. Because of this small offset from 0, the examples would have required more steps and instructions to work with frame 1 as the starting frame. For instance, in the first example of the chapter, suppose you set the starting frame of the animation to 1. The expression in the example follows: Ball.scaleY = time + 1;

This discrepancy means the Ball scaleY is larger than its scaleX and scaleZ attributes in the first frame of the animation. Though the difference is not substantial in this example, other cases might be more significant. To start your animation at frame 1 and get the same result as the example, you can subtract 0.0417 from the attribute: Ball.scaleY = (time - 0.0417) + 1;

When you rewind the animation, the expression sets Ball’s scaleY value to (0.0417 - 0.0417) + 1. This equals 1, its original scaleY value. When you use the predefined time variable, be aware of the starting frame number and the associated time value.


43

Expressions

If you rewind the animation, the expression executes and sets the initial value of Ball’s scaleY attribute to time + 1, which equals 0.0417 + 1, or 1.0417. Because Ball’s scaleY attribute was 1 when you created it, rewinding the animation sets scaleY to a value 0.0417 larger than its initial value.

Quick Start Notes on the predefined time variable After doing the lessons in this chapter, remember to change your Time Slider’s starting frame, ending frame, and frame rate to the desired values when you start other projects. To do this, select Options→General Preferences and display the appropriate tabs in the General Preferences window.

44


5

Expression Syntax Expressions use the syntax of the Maya Embedded Language (MEL). Though MEL is a scripting language, you’ll find the syntax easy to learn even if you’ve never programmed. Mastering the rules of syntax is essential to writing expressions without errors.

Tristan Ikuta

This chapter describes the following topics: •

“Expressions and MEL” on page 46

•

“Elements of an expression” on page 47

•

“Attributes” on page 49

•

“Variables” on page 56

•

“Constants” on page 62

•

“Arithmetic, logic, and relational operators” on page 63

•

“Operator precedence” on page 68

•

“Conditional statements” on page 69

•

“General syntax rules” on page 73

•

“Comments in expressions” on page 75 Using Maya: Hypergraph, Sets & Expressions

45

Expressions

You can use an expression to rotate each child joint based on the root joint’s keyframed rotation.

Expression Syntax Expressions and MEL •

“Programming features” on page 75

•

“Common expression errors” on page 95

Expressions and MEL Expressions have a different purpose from MEL commands and MEL scripts. You enter a MEL command to do a single action, for example, to create a sphere. A MEL script is a list of commands you create to do a sequence of actions, for instance, create a wall-shaped object and apply a brick texture to it. Because you store a MEL script in a file on disk, you can run a script in different scenes and different work sessions, whether today or next year. An expression animates one or more attributes over time in a single scene. By default, an expression executes each frame as an animation plays. After you create an expression, it executes whenever you play the animation— including when you play the animation after saving, closing, and reopening the scene. A MEL command or script is not part of a scene after you execute it. You must execute it again to repeat the action. Sometimes it’s useful to exectute MEL commands and scripts in expressions. See “Executing MEL commands in an expression” on page 137 for details. Action

Typical use

MEL command

Does one action

MEL script

Does several actions

Expression

Animates attribute values as an animation plays

The following pages describe expression syntax elements such as arithmetic operators you can use to set and compare attributes. If you have trouble understanding the syntax descriptions, refer to a C programming guide for beginners. Except for attribute names, the syntax elements have the same definitions as their counterparts in C. Attribute names do not exist in C. If you’re familiar with a programming language such as C, Pascal, or Basic, be sure to see “Programming features” on page 75.

46


Expression Syntax Elements of an expression

Elements of an expression An expression is made of one or more statements. Statements follow the rules of algebra, so they’ll seem familiar if you’ve studied math. Each statement has several elements as in the following example: Assignment operator Arithmetic operator Attribute name

Function Constant

Ball.rotateZ = sin(time) + 6;

Terminator

Statement Variable

Detailed explanations follow the summary definitions: Attribute name The name of the attribute set by the statement. In the

example, Ball.rotateZ is the attribute name of the rotateZ attribute of an object named Ball. Assignment operator

A special word that you provide with an entry called an argument. In this example, the argument is time. Based on the value of the argument, Maya does a calculation for the function and returns a new value or takes some other action. In the example, the function sin(time) returns the sine of the value of time, which evaluates to a number between -1 and 1. Maya has many convenient built-in functions and commands that do math calculations, conversions, and so on. See Chapter 9 for details.


47

Expressions

Function

The = to the right of an attribute name. This symbol assigns the attribute the result of the statement on the right side of =. In the example, Ball.rotateZ receives the value of the statement sin(time) + 6. (Ignore what this assignment does; it’s for illustration only.) You can also use = to assign a value to a variable.

Expression Syntax Elements of an expression A variable is a symbolic name that stands for a changing value. You can assign a value to a variable or read a variable’s value. The variable time is a predefined Maya variable that contains the animation time at the current frame. You can read but not set the value of time.

Variable

Arithmetic, logic, or relational operator An operation such as + or < (less than). Constant

An unchanging number, for example, 6.1 or 90.

Terminator

A semicolon (;) that marks the end of a statement. An expression can have an unlimited number of statements. You must end each statement with a semicolon.

Each expression usually has an attribute name, assignment operator (=), expression value assigned to the attribute, and a statement terminator (;). Other elements are optional.

Example Here’s an expression with the fewest elements possible: Value assigned

Ball.scaleY = 5;

The expression has an attribute, assignment operator (=), value assigned to the attribute, and a statement terminator (;). The expression sets Ball’s Y scale to 5 grid units. When you play the animation, Ball’s Y scale stays fixed at 5 regardless of the Y scale value you gave it when you created it.

48


Expression Syntax Attributes

Attributes An attribute is a characteristic of an object or other item in a scene. There are many ways to set attributes in Maya—with the Attribute Editor, Channel Box, menu selections, and of course, expressions. You can set attributes to control virtually anything in your animation. There are three types of attributes you work with in Maya: •

static

•

dynamic

•

custom Static and dynamic attributes have a predefined purpose. They are standard attributes Maya provides for objects and items that make up a scene. Custom attributes are attributes you define for an object.

Static attributes Static attributes are attributes an object has by default. They exist the moment you create the object and throughout its lifetime.

Expressions

For example, the transform node of a NURBS sphere has static attributes scaleX, scaleY, scaleZ, rotateX, and so on. You can set the values of these attributes with the Attribute Editor, Channel Box, expressions, and other techniques after you create the object.

Dynamic attributes Dynamic attributes have predefined names and purposes, but Maya adds them to an object in response to your user interface selections. For example, suppose you create a particle object and display its particle shape folder in the Attribute Editor. If you click one of the following buttons in the Add Dynamic Attributes section of the Attribute Editor, Maya adds a dynamic attribute to the node:

Clicking the General button lets you add a custom attribute (see the next topic). Clicking any of the other buttons lets you add one or more dynamic attributes with names that are the same or similar to the button name. Using Maya: Hypergraph, Sets & Expressions

49

Expression Syntax Attributes An object has no dynamic attributes unless your actions cause Maya to add them to the object. By adding only required attributes, Maya runs faster. When you add a dynamic attribute to an object, the attribute appears in the Attribute editor for the selected object or node.

Note Because soft body geometry is a particle shape node coupled with geometry, a soft body has the same static and dynamic attributes as a particle object.

Custom attributes Custom attributes are attributes you optionally add from the New folder of the Add Attribute window.

50


Expression Syntax Attributes Such attributes have no direct effect on any characteristic of an object. They’re often used to control a combination of other attributes. You might also use a custom attribute as a variable—a place to store a value temporarily to be read by other attributes. When you add a custom attribute to an object, it appears in the Attribute Editor and Channel Box for the object or node. Though custom attributes are dynamically added to an object, we refer to them as custom to distinguish them from the built-in dynamic attributes. See “Assigning to a custom attribute” in Chapter 8 for details on how to add and use a custom attribute.

Attribute names Static, dynamic, and custom attributes follow the same naming conventions and represent the same types of data. A full attribute name has this format: object.attribute where object is the name of the object node, and attribute is the name of the attribute. A period (.) separates the name of the object and attribute.

See “Using attribute names in expressions” in Chapter 6 for more details.

Example Ball.scaleY

Data types of attributes Each attribute has a data type that specifies the type of values you can use to control it in an expression. This is true for static, dynamic, and custom attributes.


51

Expressions

You must spell the object and attribute name with uppercase and lowercase letters as they appear in the Expression Editor’s Objects and Attributes lists. You cannot spell attribute names with the common English spellings shown in the Attribute Editor or by default in the Channel Box.

Expression Syntax Attributes Attributes you’ll work with in expressions have these data types: Data type

Meaning

Example attribute

Example data

float

floating point numbers

Balloon.scaleY

-2.3333333333

integer

signed whole numbers

Ball.sections

16

Boolean

on or off selection

Ball.visibility

on

The most common attribute data type is floating point. In mathematics, floating point numbers are also called real numbers. Often, such numbers have a decimal point. Booleans are also common data types in attributes. Integer data types are rarely used. Particle shape nodes have these additional attribute data types: Data type

Meaning

Example attribute

Example data

vector array

array of vectors

FireShape.position

<<3.2, 7.7, 9.1>> <<4.5, 9.2, 3.1>> <<3.8, 4.4, 2.1>>

float array

array of floating point numbers

FireShape.lifespan

1.333 1.666 2.333 1.333

Note Scientists often refer to a vector as a quantity that specifies both a magnitude and direction. In Maya, a vector is simply a related group of three floating point numbers that set an attribute or variable.

52


Expression Syntax Attributes Vector array data types are useful for animating position, velocity, acceleration, color, and other particle attributes made of three components. Float array attributes are useful for setting lifespan, opacity, and other particle attributes that have a single number value. Attributes having a vector array or float array data type are also called per particle attributes. See Chapter 8 for details on working with particle attributes. If you have programming experience, note that for vector array data types, Maya represents the specified attribute for each particle of the object with a single element of an array. Each element is made of three floating point numbers. In a float array, Maya represents the specified attribute for each particle with a vector array element that’s a floating point number.

Note In expressions, you must type a vector in double angle brackets (<< >>). For example, type <<3,0,5>> for a vector having 3, 0, and 5 as its left, middle, and right component.

Data types of static and dynamic attributes


Expressions

Static and dynamic attributes have predefined data types. To learn the attribute’s data type, select the node containing it. In the Attribute Editor, find the attribute name and examine its data format.

53

Expression Syntax Attributes Here’s an example display of attributes with floating point, Boolean, and integer data types:

Floating point

Boolean Integer

A floating point attribute shows a value that includes a decimal point. Most numerical attributes in Maya are floating point. A Boolean attribute has a checkbox or other user interface item for turning it on or off. An integer attribute has no decimal point. Integer attributes are rare in Maya. The data type of an attribute limits what type of value you can enter for the attribute in the Attribute Editor and in expressions. For example, because a directional light’s Depth Map Filter Size attribute is an integer, you cannot enter a decimal point in its text entry box or assign it a decimal quantity in an expression. For a floating point attribute, you can omit the decimal point. The Attribute Editor automatically inserts a decimal point in the attribute’s text field after you press the Enter key. For example, if you type 3 for a floating point entry, the Attribute Editor replaces 3 with 3.0000.

54


Expression Syntax Attributes An expression also automatically converts an integer to a floating point value when appropriate. See “Data type conversions” in Chapter 7 for details. Only particle objects, not geometric objects, have vector array and float array attributes. The static vector array attributes for particle objects are position, velocity, and acceleration. These are also called per particle attributes because you can set the attribute for each particle to different values. Maya has other attribute data types that are irrelevant to the use of expressions. For example, Maya has a matrix data type that is useful only in MEL scripting and API programming.

Data types of custom attributes When you add a custom attribute to an object with Modify→Add Attribute, you choose whether its data type is floating point, integer, Boolean, or vector. Vector attributes are commonly used with particle shape nodes.

Assigning a value to an attribute You assign a value to an attribute using the = assignment operator. Static and dynamic attributes have data types established by Maya. You do not define their data type.

You can assign a value to any attribute. If the attribute is dynamic or custom, though, you must add the attribute to the object before you can assign it a value in an expression. Become familiar with the purpose of an attribute by working with it in the Attribute Editor, Channel Box, or other parts of Maya before assigning it a value in an expression. It’s best to know the behavior you can expect from the attribute in case you write your expression incorrectly.

Note For rigid bodies, you can read but not write the velocity, angularVelocity, and force attributes.


55

Expressions

Because you choose the data type of custom attributes when you add them with Modify→Add Attribute, you do not define their data type either.

Expression Syntax Variables

Assigning to a float or integer attribute An assignment operation is a statement, so you must terminate it with a semicolon (;).

Examples Cone.scaleY = 5.3;

This assigns 5.3 to the floating point scaleY attribute of Cone. Ball.translateY = time;

This assigns the value of time to the floating point translateY attribute of Ball. Ball.scaleX = Ball.scaleY = Ball.scaleZ = 2;

This assigns 2 to the floating point scaleX, scaleY, and scaleZ attributes of Ball. As the example shows, you can use an assignment operator several times in a statement to set multiple attributes to the same value.

Assigning to a vector attribute You can assign values to all three components of a vector attribute, or just to a single component. See “Assigning to vectors and vector arrays” in Chapter 8 for details on assigning values to vector attributes. Only particle shape nodes have vector attributes. Note that you cannot assign a vector to three related scalar attributes such as scaleX, scaleY, and scaleZ. For example, you can’t do this: Ball.scale = <<1,2,0>>;

You must assign to each attribute separately: Ball.scaleX = 1; Ball.scaleY = 2; Ball.scaleZ = 0;

Variables A variable is a symbolic name that stands for a constant or changing value. There are two types of variables, predefined and custom. Maya creates and maintains predefined variables. Custom variables are variables you can create to store data in an expression.

56


Expression Syntax Variables Keep in mind that attributes, not variables, set object and component behavior in Maya. You can use variables to as temporary storage for working with the attributes.

Data types of variables Variables can be one of these types of data: Data type

Meaning

Examples

Defining keyword

float

floating point numbers

392.6, -0.667

float

integer

signed whole numbers

10, -5, 0

int

vector

vector made of three floating point numbers

<<3.2, 7.7, 9.1>>

vector

string

one or more characters

“What’s up, chief?”

string

The most common data type of variables is floating point. Integer data types are rarely used. Booleans are commonly used in attributes, but not allowed in variables. Vector variables are useful in expressions for particle shape attributes.

For a custom variable you create in an expression, you must declare the data type as described in “Custom variables” on page 59.

Predefined variables Maya maintains values in two predefined variables as an animation plays: Variable

Contents

Data type

frame

number of frames the animation has played

float

time

time in seconds the animation has played

float


57

Expressions

Animators with programming experience sometimes use string variables. Quote marks (" ") are required with strings. See “String usage” on page 90 for details.

Expression Syntax Variables Your expressions can read, but not set, the value of time and frame. These variables are floating point values that are useful for animating an attribute as an animation plays. The time updates as an animation plays. It contains the elapsed number of seconds from the first frame to the current frame. The value increases with the increasing frame number. At the default animation playback rate of 24 frames per second, time has these values, rounded to four decimal places: Frame

Time (seconds)

0

0

1

0.0417

2

0.0833

3

0.125

24

1.0

240

10.0

If you need to change the playback rate, choose Options→General Preferences. Expand the General Preferences window, display the Units folder, and choose the desired rate from the Time menu. Regardless of what animation playback rate you choose, you can find the time elapsed in the animation at any frame with this formula:

frame time = --------------rate For example, if the frame rate is 24 frames/second, and the animation is at frame 1, the elapsed time is 1 divided by 24, or 0.0417. At frame 6, the elapsed time is 6 divided by 24, which equals 0.25. If the frame rate is 30 frames/second and the animation is at frame 1, the elapsed time is 1 divided by 30, which equals 0.0333. At frame 6, elapsed time is 6 divided by 30, which equals 0.2.

58


Expression Syntax Variables There’s an advantage to writing an expression using the value of time rather than frame: You won’t need to modify your expression if you change your animation’s frame rate. Note that time is always 0 at frame 0. As each frame plays, the time increases in increments resulting from the frame rate. It’s impossible to set time to a value other than 0 at frame 0. If you set Maya’s frame range to begin at a negative number, time has a negative value.

Examples Ball.translateY = time/2;

This sets the Ball’s Y translation equal to the value of time divided by 2 as the animation plays. This make the Ball move in a Y direction as the animation time increases. Ball.scaleY = frame/2;

This sets the Ball’s Y scale equal to the value of frame divided by 2 as the animation plays. The Ball scales along its Y axis as the animation frame number increases.

Custom variables

Though programming languages use such variables abundantly, you might not need to use them at all in many expressions.

Declaring variables Each custom variable name must begin with a dollar sign character ($). After the $, you can use alphabetical, numerical, and underscore characters. You cannot include spaces in the names. Variable names are type case sensitive. In other words, $temp is a different variable name than $Temp.


59

Expressions

You can declare and use variables to store a constant or changing value. These work like their counterparts in programming languages and spreadsheet programs.


Examples float $object_height;

This declares $object_height as a floating point variable. int $counter;

This declares $counter as an integer. vector $top_velocity;

This declares $top_position as a vector variable.

Assigning a value to an integer or float variable To assign a value to a variable, you use = as an assignment operator. An assignment operation is a statement, so you must end it with a semicolon (;).

Examples float $counter = 5.3;

This declares a floating point variable named $counter and gives it an initial value of 5.3. $height = 6;

This declares a floating point variable named $height and gives it an initial value of 6. This example shows you can skip declaring the variable’s data type. When you assign a variable a value, Maya assumes the variable is floating point unless you specify a different data type. $pi = 3.1415927; $twist = $pi;

These statements show you can assign the value of one variable to another variable. The first statement assigns 3.1415927 to $pi. The second statement assigns the contents of $pi, 3.1415927, to $twist.

60



Important If you misspell an existing declared variable name and assign it a value, a syntax error won’t be generated for the undeclared variable. Because Maya automatically provides a data type for an undeclared variable if it’s on the left side of the assignment operator, the misspelled variable will be interpreted as a newly added variable. Undeclared variables on the right side of the assignment operator do generate error messages. Check spellings of variables if your expression isn’t working as expected. In the following example, the misspelling in the final statement generates an error but not the misspelling in the statement before it: int $start; int $end; int $interrupt; $starrrt = 1; $end = $interrupppt;

Assigning a value to a vector variable You can assign values to all three components of a vector variable, or just to a single component. Expressions

See “Assigning to vectors and vector arrays” in Chapter 8 for details on assigning values to vector variables. Such variables are useful for working with particle shape node attributes.

Using custom variables globally Typically, you’ll use variables within a single expression. If you want to create and maintain a custom variable in one expression, but use it in another expression, you must declare it as a global variable.

Example global float $counter;

You can thereafter set or read the value of this variable in any other expression in the scene. If you create a variable with the same name in two expressions, the two variables are separate and unrelated. For example, suppose you create a variable named $timer in two expressions. Assigning a value to one of the $timer variables has no effect on the other’s value. Using Maya: Hypergraph, Sets & Expressions

61

Expression Syntax Constants If you declare and initialize a global variable in a single statement, you can initialize it to a numerical constant or string only.

Examples global float $counter = 3;

This initializes $counter to 3. global float $counter = time;

This causes an error because time is a variable. If you declare and initialize a global variable in a single statement, the statement executes only when Maya compiles the expression. Maya compiles an expression when you click the Create or Edit button in the Expression Editor, or when you open a scene containing a previously created expression.

Example global float $counter = 3; print($counter+"\n"); $counter = 1000; print($counter+"\n");

When Maya compiles the expression, it sets $counter to 3, prints 3, sets $counter to 1000, then prints 1000. During playback, each execution of the expression skips the first statement, so $counter never receives the value 3. The expression prints 1000, sets $counter to 1000 again, and prints 1000 again.

Constants A constant is an unchanging number or variable.

Examples Ball.translateY = 6.1.

This statement sets Ball’s translateY attribute to the constant number 6.1. float $pi = 3.1415927; Ball.rotateY = $pi;

These statements set the value of Ball’s rotateY attribute to the value of the variable $pi. The variable $pi represents the constant 3.1415927.

62


Expression Syntax Arithmetic, logic, and relational operators

Arithmetic, logic, and relational operators You can use the following operator symbols to add, subtract, multiply, compare, and do other actions to variables and attributes.

Arithmetic operators Symbol

Meaning

Used with these data types

+

plus

integer, float, vector, string

-

minus or negation

integer, float, vector

*

for integers and floats: multiply for vectors: dot product


/

divided by

integer, float

%

remainder of division

integer, float

Integers and floats

Examples Car.translateX = time / 2.0;

This moves the Car in an X direction as the time increases in the animation. By dividing time by 2.0, you move the object half as fast as if you used time alone. Car.translateX = 7 % 3;

This assigns Car.translateX the value 1, the remainder of 7 divided by 3. The number 7 divided by 3 equals 2 with a remainder of 1. Car.translateX = 8.8 % 4.2;

This assigns Car.translateX the value 0.4, the remainder of 8.8 divided by 4.2. The number 8.8 divided by 4.2 equals 2 with a remainder of 0.4. Car.translateX = 0.5 % 3;


63

Expressions

For integer and floating point attributes and variables, the above arithmetic operators work according to the rules of basic math. Note that the modulus operator (%) calculates the remainder of division.

Expression Syntax Arithmetic, logic, and relational operators This assigns Car.translateX the value 0.5, the remainder of 0.5 divided by 3. The number 0.5 divided by 3 equals 0, with a remainder of 0.5.

Vectors For operations between vector attributes and variables, the * operator performs the dot product. The dot product multiplies corresponding components of each vector, then adds the components to create a single floating point number result. For + and - operators, each component of one vector is operated on by its counterpart component in the other vector. For operations between a vector and an integer or floating point number, each component of the vector is operated on by the integer or floating point number.

Examples Suppose you’ve initialized these vectors: vector $A = <<1,2,3>>; vector $B = <<2,3,4>>; vector $C; float $myfloat;

You then use the following statements (in different expressions, not in sequential order): $C = $A + $B;

This assigns $C the value << 3, 5, 7>>. $C = $B - $A;

This assigns $C the value <<1, 1, 1>>. $myfloat = $A * $B;

This assigns $myfloat the value (1*2) + (2*3) + (3*4), which equals 20. Multiplying two vectors gives the dot product of the vectors. $C = 3 * $A;

This assigns $C the value <<3, 6, 9>>. Each component of the vector is multiplied by 3 to create a vector result.

64



Strings For details on how to use the + operator with strings, see “String usage” on page 90.

Note Maya handles integer and Boolean attributes in an expression mathematically as floating point numbers. After the expression executes, Maya converts the floating point number to the proper data type. If your expression does arithmetic on an integer or Boolean attribute and you display the attribute’s contents in the Script Editor, you’ll see floating point values. After the expression executes, Maya assigns an appropriate integer or Boolean value to the attributes you set in the expression text field. Maya handles integer and Boolean variables within an expression mathematically as integer and Boolean data types.

Relational operators You’ll often use relational operators to compare the value of variables and attributes in conditional statements. See “Conditional statements” on page 69. Meaning

Used with these data types

<

less than


>

greater than


==

equal to


!=

not equal to


>=

greater than or equal to


<=

less than or equal to


Expressions

Symbol

Integers and floats For integer and floating point attributes and variables, the above relational operators work according to the rules of algebra.


65


Examples if (time > 10) Sphere.translateX = 3;

When the animation time is greater than 10 seconds of play, the expression sets the Sphere’s translateX attribute to 3. It stays fixed in this position thereafter. See “Conditional statements” on page 69 for details on the if condition in this and following examples. if (Ball.scaleY == 3) Cone.scaleY = 6;

If Ball’s scaleY attribute is equal to 3, Maya sets Cone’s scaleY attribute to 6.

Important Be careful to type == rather than = for the equal to operator. For example, suppose you type if (Ball.scaleY = 3) in the previous example. Rather than test whether Ball.scaleY is equal to 3, the statement assigns 3 to Ball.scaleY. Maya evaluates the assignment statement Ball.scaleY = 3 as a true condition, so it executes Cone.scaleY = 6. This statement doesn’t cause an error message, but it gives unintended results.

Vectors If you use the == or != operators between two vector attributes or variables, Maya compares the corresponding components of each vector. In contrast, the >, >=, <, and <= operators compare the magnitude of two vectors. Use this formula to calculate a vector’s magnitude: 2

2

x +y +z

2

The x, y, and z numbers in the formula represent the three components of the vector.

Examples vector $A = <<1,2,3>>; vector $B = <<1,2,3>>; if ($A == $B) Sphere.translateX = 3;

66


Expression Syntax Operator precedence Notice that each condition is grouped in parentheses, and the pair of conditions are enclosed again in parentheses. If you use multiple conditions with logical operators, you must enclose all the conditions in parentheses for the if statement. If you omit the outer pair of parentheses as in the following example, an error message occurs: if (time > 5) && (time < 10) Ball.scaleZ = time;

Example 2 if ((Ball.translateX < 5) || (Ball.translateY > 10)) Ball.scaleZ = time;

This sets Ball’s scaleZ attribute to the value of time in either of two conditions: when Ball’s translateX attribute is less than 5 or greater than 10.

Operator precedence The precedence of operators in expressions follows: Highest

() [] ! ++ - * / % ^

+

Lowest

-

< <= > >= == != && || = += -= *= /=

This figure includes operators used mainly by individuals experienced in programming. See “Programming features” on page 75 for details. In the figure, operators on the same row have equal precedence. If a statement has two or more operators from the same row, the operator furthest to the left is evaluated first. The parentheses at the top of the figure are for grouping a condition or elements of a statement. As shown in a following example, parentheses are useful for altering the order of operator evaluation.

68


Expression Syntax Conditional statements

Examples Ball.scaleY = 8 + 2 * 4;

This assigns Ball.scaleY the value 16. Ball.scaleY = (8 + 2) * 4;

This assigns Ball.scaleY the value 40. Ball.scaleY = 8 + 6 - 4;

This assigns Ball.scaleY the value 10. The + executes first because it’s further to the left in the statement than the -.

Conditional statements Conditional statements set one attribute or variable based on the condition of another attribute or variable. For example, you might increase the scale of a balloon after frame 48 plays. The if and if-else statements are the most commonly used conditional statements in expressions. You’ll often use relational and logical operators in conditional statements. See page 65 and page 67 for details. Expressions

If you have programming experience, be aware you can use loop and flow control statements such as while and for. See “Programming features” on page 75.

if statements The if conditional statement has this format: if ( condition ) statement;

If condition is true, statement executes.

Example if (time > 3) Ball.scaleY = 2;

This sets the scale of Ball’s scaleY attribute to 2 after the animation plays three seconds.


69


if-else statements The if-else conditional statement has the following format: if ( condition ) statement1; else statement2;

If condition is true, statement1 executes. Otherwise statement2 executes.

Example 1: Simple if-else statement if (time > 3) Ball.scaleY = 2; else Ball.scaleY = 1;

This sets Ball’s scaleY attribute to 2 if animation time is greater than 3 seconds. If animation time is less than 3, scaleY is set to 1. You can use more than one statement after a condition with this format: if ( condition ) { statement; statement; } else { statement; statement; }

Notice you must enclose the multiple statements between braces ({ }).

Example 2: Braces in if-else statement if (time < 2) { Balloon.translateY = 0; Balloon.scaleY = time * 0.6; } else { Balloon.translateY = time - 2; Balloon.scaleY = 1; }

70


Expression Syntax Conditional statements If the animation time is less than 2 seconds, the expression sets Balloon’s translateY attribute to 0, and sets its scaleY attribute to the value of time multiplied by 0.6. If animation time is greater than or equal to 2 seconds, the expression sets Balloon’s translateY attribute to time minus 2, and sets its scaleY attribute to 1.

Important You cannot set the same attribute in two different expressions. If you try to do so, an error message results and your second expression has no effect.

else if statements The else if statement works with the if-else conditional statement and has this format: if (condition1 ) statement1; else if ( condition2 ) statement2;

If condition1 is true, statement1 executes and the else if statement after it is skipped.

You can add an else condition to the previous format as follows: if (condition1 ) statement1; else if ( condition2 ) statement2; else statement3;

If neither condition is true, statement3 executes.


71

Expressions

If condition1 is false, the else if statement executes. If condition2 is true, statement2 executes. If neither condition is true, neither statement executes.


Example if (time < 3) Ball.scaleY = else if ((time >= 3) Ball.scaleY = else Ball.scaleY =

1; && (time =< 6)) 2; 3;

This sets Ball’s scaleY attribute to 1 if animation time is less than 3 seconds. If animation time is between 3 and 6 seconds, scaleY is 2. Otherwise, when time is greater than 6 seconds, scaleY is 3. Note that you can add multiple else if statements and multiple statements within braces ({ }) using this format: if (condition1 ) { statement; statement; } else if ( condition2 ) { statement; statement; } else if ( condition3 ) { statement; statement; } else if ( condition4 ) { statement; statement; } else { statement; statement; }

72


Expression Syntax General syntax rules

General syntax rules Use these guidelines to avoid syntax errors while writing expressions: •

Terminate each statement in an expression with a semicolon (;). Here’s an example expression with two statements: if (time < 3) nurbsSphere1.translateX = time/2; else nurbsSphere1.translateX = time*3;

A semicolon marks the end of each statement. •

Enclose each conditional item in an expression within parentheses. In the preceding example, (time < 3) is a conditional item enclosed in parentheses.

•

Match each opening parenthesis with a closing parenthesis. For example, this statement causes an error: Ball.rotateZ = deg_to_rad(-6 * (floor(time));

If you look closely, you’ll see that there are three opening parentheses, but only two closing parentheses. The next statement causes no error: Ball.rotateZ = deg_to_rad(-6 * (floor(time)));

•

Expressions

There are three matching closing parentheses for the three opening parentheses. When you use { and } as opening and closing braces, make sure you use them in matching pairs: if (time > 3) { Ball.rotateZ = deg_to_rad(-6 * (floor(time)); Ball.rotateY = Ball.rotateZ * 3; }

•

Enclose a vector in double angle brackets as in this example: <<3,4,8>>

Spaces before and after the numbers and commas are optional. •

Begin any variable you use with a dollar sign ($), and do not to use spaces or special characters other than underscores in the name. Here’s an acceptable example: float $my_Rotate; $my_Rotate = 3.14;


73

Expression Syntax General syntax rules •

In conditional statements, be careful to type == rather than = for the equal to operator. The = symbol means assign the value. For example, make sure you type: if (Ball.scaleY == 3) Cone.scaleY = 6;

instead of this: if (Ball.scaleY = 3) Cone.scaleY = 6;

•

You can use as many spaces, tab characters, and blank lines as you like when separating words, operators, or statements. Maya ignores white space in an expression. For example, suppose you’ve written this expression: if (time < 2) { Balloon.translateY = 0; Balloon.scaleY = time * 0.6; } else Balloon.translateY = time - 2;

Though the following expression has different spacing and is unpleasant to read, Maya interprets the expression the same as the previous one. if(time<2){Balloon.translateY=0; Balloon.scaleY=time*0.6;} else Balloon.translateY=time-2;

You must include at least one space between any two keywords, variables, or attribute names (or combination of these). So a space is required after the else keyword but in no other place in this expression. To simplify spacing considerations, remember to put at least one space before and after a keyword, variable, operator, attribute, assignment operator, and so on. Consistent use of white space makes expressions easier to read. Examples throughout this chapter show examples of good spacing style.

74


Expression Syntax Comments in expressions

Comments in expressions Add comments to your expressions to explain the purpose of each statement within. You’ll appreciate this later if you need to modify the expression. Maya ignores comments.

To add a comment to the right of a one-line statement: Enter two forward slashes (//) at the end of the statement, then enter the comment: Ball.translateX = time; // Moves ball in X dir. with time

To write a multi-line comment: Enter two forward slashes (//) before the comment: // This is an example of a // comment spanning two lines.

Programming features Expressions

The following topics describe programming features available in expressions. Discussion is brief and assumes you’re familiar with programming. Most of the syntax features described work like their C counterparts.

Notes for C programmers Some important differences between expression and C syntax follow: •

A C program consists of one or more functions, each containing multiple statements. An expression is simply a single block of statements. You don’t declare main( ) or your own functions in an expression. You also don’t include the C standard library of functions. You will, though, sometimes include a built-in Maya function such as sin( ) to accomplish time-saving tasks. See Chapter 9, “Functions,” for details.

•

After you type an expression in the Expression Editor, clicking the Create or Edit button compiles the expression.

•

The first character of variables must be a dollar sign ($).

•

ANSI C has 32 keywords. The expression language has less, as listed in the following topic. Using Maya: Hypergraph, Sets & Expressions

75

Expression Syntax Programming features •

Maya’s integer data type has the same numerical range as ANSI C’s integer data type, -2,147,483,648 to 2,147,483,648.

•

Maya’s float data type has the same numerical range as ANSI C’s double data type.

Expression language keywords The expression language keywords follow:

Data type keywords int

float

vector

string

matrix

on

off

true

false

in

Boolean constant keywords yes

no

Flow control keywords if

else

for

while

do

break

continue

default

switch

case

source

catch

alias

Other keywords global

return

proc

The return, proc, and matrix keywords are useful for writing MEL scripts, not for expressions. Other keywords above are described throughout this chapter. Type keywords in lowercase letters exactly as shown. Do not name a custom attribute with any of these keywords.

76


Expression Syntax Programming features

Flow control statements Besides the if and if-else statements described previously, you can control the flow of statement execution with while, do, for, break, continue, and ?: instructions. These work like their C language counterparts. You’ll often use logical and relational operators in conditional statements. See page 65 and page 67 for details.

Important Using a while, do, or for loop incorrectly might halt Maya. See “Flow control errors” on page 88 for details.

while A while loop has this format: while ( condition ) { statement; statement; ... }

Example float $test = 0; while ($test < 5) { print("$test equals: " +$test+"\n"); $test = $test + 1; }


77

Expressions

Use condition to compare variable, attribute, or constant values. If condition is true, Maya executes each statement between braces. Maya then evaluates condition again. If true, it executes each statement again. This cycle continues until condition is false, whereupon execution resumes with the statement after the loop.

Expression Syntax Programming features This expression displays the following lines in the Script Editor: $test $test $test $test $test

equals: equals: equals: equals: equals:

0 1 2 3 4

These lines are followed by a status message similar to this: expression -e -s "" -o Ball -an 1 Expr

This message indicates that a MEL command executed when you clicked the Create or Edit button in the Expression Editor. Specifically, an expression command executed. This is unrelated to the exact statements in the expression.

do A do loop has this format: do

{ statement; statement; ... }

while (condition);

Here Maya executes each statement between braces, then evaluates condition. The condition compares variable, attribute, or constant values. If condition is true, each statement executes again. The loop terminates when condition is false. In contrast to a while loop, a do loop executes the statements in the loop at least once. It tests the termination condition after the loop. A while loop tests the termination condition before executing the statements in the loop.

Example float $test = 0; do

{ print("$test equals: " +$test+"\n"); $test = $test + 1; } while ($test < 5);

78


Expression Syntax Programming features This expression displays the following lines in the Script Editor: $test $test $test $test $test


0 1 2 3 4

for A for loop has this format: for (initialization; condition; change of condition) { statement; statement; ... }

A for loop evaluates the termination condition before executing each statement. The condition compares variable, attribute, or constant values.

Example float $i;

Expressions

for ($i = 0; $i < 5; $i = $i + 1) { print("$i equals: " +$i+"\n"); }

This expression displays the following lines in the Script Editor: $i $i $i $i $i


0 1 2 3 4

break The break instruction exits a loop from any point within its body, bypassing the normal termination at the loop’s beginning or end. Expression execution resumes at the next statement after the loop. You can use a break instruction with a while, do, or for loop.


79


Example float $f = 0; while( $f < 10 ) { print("$f equals: "+$f+"\n"); if ( $f > 5 ) break; $f = $f + 1; }

This expression displays the following lines in the Script Editor: $f $f $f $f $f $f $f

equals: equals: equals: equals: equals: equals: equals:

0 1 2 3 4 5 6

Suppose the example didn’t have this statement: if ($f > 5) break;

The loop would execute ten times and display the numbers 0 through 9. The break statement terminates the loop after $f is greater than 5. So the expression displays only numbers 0 through 6.

continue The continue instruction works inside loops. It forces the next iteration of the loop to occur, skipping any statements between itself and the loop’s test condition. The condition compares variable, attribute, or constant values.

80



Example float $f = 0; for( $f = 0; $f < 10; $f = $f + 1) { print("$f equals: "+$f+"\n"); if( $f > 5 ) continue; print(" got here.\n"); }

This expression displays the following lines in the Script Editor: 0 here. 1 here. 2 here. 3 here. 4 here. 5 here. 6 7 8 9

Expressions

$f equals: got $f equals: got $f equals: got $f equals: got $f equals: got $f equals: got $f equals: $f equals: $f equals: $f equals:

Suppose the example didn’t have this statement: if( $f > 5 ) continue;

The loop would display got here after each line of $f equals: n. Maya ignores the continue instruction until $f increases to a value greater than 5. When $f becomes 6 or greater, the continue instruction executes and skips the remaining statement in the loop, so got here isn’t printed.

for-in The for-in loop is a specialized for loop that simplifies manipulation of all elements of an array. A for-in loop with an array element variable lets you omit the initialization, condition, and change of condition components of a for loop. Using Maya: Hypergraph, Sets & Expressions

81

Expression Syntax Programming features The for-in loop has this format: for (array-element in array) { statement; statement; ... }

Example string $carType[3] = {"Porsche", "Ferrari", "Fiesta"}; string $car; for ($car in $carType) { print("I want a new "); print($car + ".\n"); }

The expression displays this in the Script Editor: I want a new Porsche. I want a new Ferrari. I want a new Fiesta.

The loop executes three times, once for each array element in $carType. The first loop execution copies array element $carType[0] into $car, then prints, “I want a new Porsche.” Array element $carType[0] is Porsche. The second loop execution copies $carType[1] into $car, then prints the second line shown. The third execution copies $carType[2] into $car, then prints the third line shown. When the for-in statement finishes reading all array elements, the loop terminates.

82



switch A switch instruction executes one of several groups of statements based on a control value. The control value can be a variable value or an attribute other than an array (per particle) attribute. The format follows:

The switch executes with a variable control-value. If the variable contents match value1, value2, or another value in the switch, the statements under the associated case statement execute. The control-value can be an int, float, string, or vector. Be careful if you use a float control-value. Because of the way floating point arithmetic rounds numerals, a case value might fail to match a control-value as you expect. A break statement within a switch causes execution to skip subsequent case statement groups within the switch instruction.


83

Expressions

switch (control-value) { case value1: statement; statement; ... break; case value2: statement; statement; ... break; case value3: statement; statement; ... break; ... default: statement; statement; ... break; }


Example 1: Break statement within a switch int $sway = rand(3); switch ($sway) { case 0: print("Case 0\n"); // Executes if $sway = 0 break; case 1: print("Case 1\n"); // Executes if $sway = 1 break; case 2: while (rand(10) < 7)// These statements print("I say!\n");// execute only print("Case 2\n");// if $sway = 2 break; }

When the expression executes a few times, it might display this random selection of entries in the Script Editor: Case 0 Case 1 I say! I say! I say! Case 2 Case 0 Case 1

The last case instruction in a switch doesn’t need a break statement because the switch is finished. Still, it’s best to add the break statement to avoid future problems that might result from adding other cases to the switch. For details on the purpose of rand(3), see “rand” on page 243.

84



Example 2: Omitted break statement within a switch The following expression omits a break statement to make the switch continue execution after the first case: int $argo = rand(2); switch ($argo) { case 0: print("Food\n"); // Runs if $argo is 0. case 1: print("Fight\n");// Runs if $argo is 0 or 1. break; }

When the expression executes a few times, it might display this random selection of entries in the Script Editor: Fight Fight Fight Food Fight Food

Whenever Food appears, Fight also appears after it. Fight can appear without Food being displayed.

int $argo = rand(4); switch ($argo) { case 0: case 1: print("Food\n"); // Runs if $argo is 0 or 1 case 2: case 3: print("Fight\n");// Runs if $argo is 2 or 3 break; }

This works like the preceding expression, except that a match of 0 or 1 displays Food and Fight, and a match of 2 or 3 displays Fight.


85

Expressions

Note that you can make more than one case statement execute the same statements:


Example You can use the default keyword to make a block of statements execute when none of the case values match the control value label. Generally, you put this label after all the case statements, though you can put it anywhere in the switch statement. If the switch has no default label and none of the case values match the control value, the switch does nothing. vector $mgb = <<1,1,0>>; switch ($mgb) { case <<0,1,1>>: print("Who?\n");// Runs if $mgb is <<0,1,1>> break; case <<1,0,1>>: print("What?\n");//Runs if $mgb is <<1,0,1>> break; default: print("Why?\n"); // Executes if $mgb is not break; // <<0,1,1>> or <<1,0,1>> }

The expression executes the default case, which displays the following line in the Script Editor: Why?

?: operator The ?: operator lets you write a shorthand if-else statement to set an attribute or variable in one statement. Because of its cryptic appearance, many programming style experts suggest not using it. Here’s its format: attribute = condition? statement1: statement2;

The condition compares variable, attribute, or constant values. If condition is true, Maya evaluates statement1 and assigns its value to attribute. (You can also assign the statement’s value to a variable.) Maya evaluates either statement1 or statement2, never both. You can optionally enclose statement1 and statement2 in parentheses to make the expression easier to read.

86



Example Balloon.scaleY = (time < 2) ? time / 2: time * 2;

This statement sets Balloon’s scaleY attribute to time divided by 2 if time is less than 2, and time multiplied by 2 if time is greater than or equal to 2. This causes the scaleY attribute to increase slower for the first two seconds than after two seconds. This is the same as the following if-else statement: if (time < 2) Balloon.scaleY = time / 2; else Balloon.scaleY = time * 2;

Use this format because it’s easier to read.

Important If you use an integer value as statement1 and a floating point value as statement2, the ?: operator truncates the floating point value of statement2 to an integer. In the expression Balloon.scaleY = (time < 2) ? 0: time;, for example, 0 is an integer, and time is a floating point value. When time is 2 seconds or more, Maya sets Balloon’s scaleY attribute to the integer value of time.

If you have problems using the ?: operator, use an if-else statement instead.

! operator You can use the not logical operator (!) with integer, float, and vector data types. For vector values, ! is true only when the vector magnitude is 0. A vector’s magnitude is the value resulting from this equation: 2

2

x +y +z

2

The x, y, and z numbers in the formula represent the three components of the vector.


87

Expressions

Because Maya sets scaleY to the integer value of time (without the decimal part), scaleY jumps in one-second increments at time 2, 3, 4, and so on.


Examples if (!$count) Ball.scaleY = 2;

The !$count condition is true only if $count is 0. If true, Ball.scaleY is set to 2. vector $myvector = <<0,0,0>>; if (!$myvector) Ball.scaleY = 2;

Because the magnitude of $myvector is 0, the !$myvector condition is true and Ball.scaleY is set to 2.

Flow control errors The following topics describe solutions to common mistakes in expression flow control statements.

Modifying variable values in test conditions If you use a while, do, or for loop in an expression, remember to change the variable or attribute being tested in the test condition of the loop. Failing to do so can halt Maya operation.

Example 1 Suppose you create an object named Balloon and decide to use a while loop to increase its Y scaling after three seconds of animation play. while (time > 3) Balloon.scaleY = time;

Though you might think this expression sets Balloon’s scaleY attribute to the increasing value of time after the animation time exceeds 3 seconds, it actually halts Maya operation as soon as time exceeds 3. At that moment, the while condition is true, so the while loop statement Balloon.scaleY = time executes repeatedly and endlessly. Even though a statement sets an attribute within an expression, Maya updates the attribute only after the expression finishes executing. Because the expression never finishes executing, Maya halts. Unless you change Balloon.scaleY within the while loop to a value less than or equal to 3, the statement executes infinitely.

88


Expression Syntax Programming features To get the desired result without halting Maya, use this expression: if (time > 3) Balloon.scaleY = time;

Example 2 Suppose you create objects named Cone and Ball, then use a while statement to link the Ball’s translateY attribute to the Cone’s translateY attribute: while (Cone.translateY > 0) Ball.translateY = Cone.translateY;

At first glance, the expression seems to set Ball’s translateY position to the value of the Cone’s translateY position whenever Cone’s translateY is greater than 0. In fact, the expression halts Maya as soon as you translate the Cone to a Y position greater than 0. At that moment, the while condition is true, so the while loop statement Ball.translateY = Cone.translateY executes endlessly. Nothing you do in the user interface can change the Cone’s translateY position. It stays at translateY value of 0. Unless you change Cone.translateY within the while loop to a value less than or equal to 0, the statement executes infinitely.

if (Cone.translateY > 0) Ball.translateY = Cone.translateY;

Comparing floating point values to 0 with == If you use the == operator to compare a floating point variable or attribute to 0, your expression might not work correctly. This typically occurs when you assume the value returned by a built-in function such as cosd will be exactly 0.

Example float $x = cosd(90); if ($x == 0) print("This equals 0.\n"); else print("This doesn’t equal 0.\n");


89

Expressions

To get the desired result without halting Maya, use this expression:

Expression Syntax Programming features The expression displays the following text: This doesn’t equal 0.

Though the cosine of 90 degrees is mathematically 0, the cosd(90) function returns the value 6.123e-17, which is extremely close to 0 but not exactly equal. Though the number for practical purposes is the same as 0, it’s stored in the computer as a fractional quantity above 0 because of the way computers handle floating point numbers. To fix the problem, compare the values as in this expression: float $x = cosd(90); if (($x > -0.0001) && ($x < 0.0001)) print("This equals 0.\n"); else print("This doesn’t equal 0.\n");

The expression displays the following text: This equals 0.

By checking that $x is between -0.0001 and 0.0001, the appropriate print statement executes. The value returned by cosd(90) is so close to 0 that it’s within the small range specified in the if statement’s numerical comparison.

String usage A string is a sequence of alphabetical, numerical, and special characters. You can display strings in the Script Editor, for example, to check the contents of attributes or variables. You can also create strings in the Expression Editor to execute MEL commands in an expression. See Chapter 7 for details. Guidelines for using strings follow: •

Enclose a literal string with double quotes as in this example: print("asteroid2");

This displays the following text: asteroid2

•

You can use the + operator to concatenate strings as in this example: print("Ball’s scaleY attribute equals: " + Ball.scaleY);

90


Expression Syntax Programming features This displays the following text: Ball’s scaleY attribute equals: 0.3333333333

•

The following table lists how Maya converts data types if you use arithmetic operators with strings in an expression. Arithmetic operation

Resulting data type

string operator integer

string

string operator float

string

string operator vector

string

For example, suppose you type the following statement: print("Hi there, "+007);

This displays the following text: Hi there, 007

•

If you’re familiar with C programming, be aware you can assign a string to a vector as in these examples: vector $i = (vector) "<<1,2,3>>";

Expressions

vector $i = vector ("<<1,2,3>>");

•

You can execute a MEL command in an expression statement. See “Executing MEL commands in an expression” in Chapter 7.

Shortcut assignment operators You can use shorthand assignment operators to save typing time compared to their longhand counterparts. In place of a statement like this: $height = $height + 3;

You can use this statement: $height += 3;


91

Expression Syntax Programming features The following table shows the shorthand operators and the valid data types for each. The shorthand operators work like their counterparts in C. Symbol

Data type

+=

integer, float, vector, string

-=


/=


*=


%=

integer, float

Do not insert a space between the operator and =.

Example $counter += 1;

This adds 1 to $counter each time the statement executes.

Shortcut increment and decrement operators You can use the ++ and -- shortcut increment and decrement operators to increase or decrease floating point and integer variables by 1. The following table shows the shortcut syntax and its equivalent expanded syntax: Shortcut syntax

Expanded syntax

++variable;

variable = variable + 1;

--variable;

variable = variable - 1;

variable++;

variable = variable + 1;

variable--;

variable = variable - 1;

When the increment or decrement operator precedes the variable, the increment or decrement occurs before the statement executes. When the operator follows the variable, the increment or decrement occurs after the statement executes.

92



Examples float float $crab $crab $crab

$eel = 32.3; $crab = $eel++; = $eel--; = --$eel; = ++$eel;

// // // //

$crab $crab $crab $crab

= = = =

32.3; 33.3; 31.3; 32.3;

$eel $eel $eel $eel

= = = =

33.3; 32.3; 31.3; 32.3;

Important To avoid unexpected results, do not use more than one shortcut increment or decrement operator on the same variable in the same statement. The evaluation order of the operators is unpredictable.

Arrays You can create arrays of float, vector, integer, or string values. You can clear an array using a clear function. You can find the size of an array with the size function. See “Array functions” in Chapter 9 for details.

Expressions

When you assign a value in an array, Maya reserves memory for all elements less than that number. This means you can exceed the capacity of your computer with a single array declaration. For example, do not use a statement like this: $newarray[12312323123] = 1;

Examples: Defining an array float $myarray[]; vector $myposition[]; int $p[];

Note that an array expands its size automatically as you assign values to its elements. You don’t need to declare its size. If your array assignment exceeds the size of the array, the array expands to that size. If you reference an element of the array beyond the array size, a 0 is returned. Suppose you include these statements in an expression: int $p []; $p[1500] = 3; $p[2000] = 5;


93

Expression Syntax Programming features The second statement makes the array contain 1501 elements and assigns element 1500 the value 3. The third statement expands the array to 2001 elements and assigns element 2000 the value 5.

Example: Initializing and printing an array’s contents float $fa[]; print("$fa size: "+size($fa)+"\n"); for( $i = 0; $i < 10; $i = $i + 1) { $fa[$i] = $i; print($fa[$i]+"\n"); } print("fa size: "+size($fa)+"\n");

This expression displays the following: $fa size: 0 0 1 2 3 4 5 6 7 8 9 $fa size: 10

The first statement creates an array of floating point variables named $fa[ ]. The next statement displays the size of the array, which has 0 elements after its definition. The for loop executes the statements between the braces 10 times, once for each increment of $i from 0 to 9. The first statement between the braces ({ }) initializes and sets the value of one element of the array. Array element $fa[0] is set to floating point value 0, element $fa[1] is set to 1, element $fa[2] is set to 2, and so on. The print statement between the braces displays the value of each element of the array after you initialize it. In other words, the Script Editor displays 0 through 9.

94


Expression Syntax Common expression errors The for loop stops executing after $i becomes equal to 10. Then the final print statement displays the number of elements of the initialized array, 10. The array increased in size as you assigned values to its elements.

Boolean symbolic constants You can use the symbolic constants on, true, or yes for the Boolean numeric value 1. You can use off, false, or no to represent 0. Be aware that on, true, and yes are equal only to 1. They aren’t equal to nonzero values.

Example if (Monster.visibility == on) Lance.scaleY = time / 3;

This causes Lance’s scaleY attribute to increase only if Monster’s visibility attribute is on. The on represents 1. print(3 + on);

This displays 4 in the Script Editor. Again, on represents the value 1.

Common expression errors

Logic errors are mistakes in your reasoning that cause unexpected animation results. The syntax of your expression is valid, but errors in your logic prevent Maya from doing what you intended. In the worst cases, Maya might halt operation because your statements lock it into a permanent loop. Because Maya can’t detect logic errors, it can’t display error messages. As such, these errors are harder to find and require more analysis to solve. To resolve logic errors, it’s often helpful to display the contents of relevant attributes and variables. See “Displaying attribute and variable contents” in Chapter 7.

Error message format A syntax error displays one or more messages in the Script Editor.


95

Expressions

There are two types of errors you can make when writing expressions: syntax errors and logic errors. Syntax errors include mistakes in spelling, incomplete attribute names, omitted semicolons, and other oversights that prevent the expression from compiling and executing. For syntax errors, Maya explains the error in a message to the Script Editor.

Expression Syntax Common expression errors

You’ll often need to scroll or increase the size of the Script Editor to see an entire message. When the Script Editor displays a syntax error, the response area of the Command Line displays the same error with a red background.

Command line’s response area turns red if error occurs

If an expression executes a valid statement after the erring statement, the error message with the red background flashes briefly. You won’t notice it unless you’re looking directly at it and have quick eyes. The best way to know when an error has occurred is to look for a new message prefixed by // Error: in the Script Editor.

96


Expression Syntax Common expression errors Before clicking the Create or Edit button to create an expression, you might want to select Edit→Clear History in the Script Editor to remove previous messages in the window. This makes it easier to see when a new error message appears.

Common error messages Here are some common syntax errors and their explanations: Attribute not found or variable missing '$': Ball.goof.

You misspelled an attribute name, the attribute doesn’t exist in the scene, or you forgot to prefix a variable name with $. Attribute of a particle object can only be used with dynExpression command: particleShape1.position

You used a particle array attribute in the expression, but a particle shape node is not the Selected Object in the Expression Editor. A particle shape node must be selected to use particle array attributes. A particle array attribute is also called a per particle attribute. Attribute already controlled by an expression, keyframe, or other connection: Balloon.tx.

•

set driven key

•

constraint

•

motion path

•

another expression

•

any other direct connection More than one attribute name matches. Must use unique path name: Ball.tx.

You used an object.attribute name that exists in two or more parent objects. Two objects in a scene can have the same object name if they have different parent objects. For example, a scene might have a child of GroupA named Ball.tx and a different child of GroupB named Ball.tx. If you write a statement such as “Ball.tx = time;”, Maya won’t know which Ball.tx to set.


97

Expressions

You tried to set the value of an attribute that has already been set by one of these techniques:

Expression Syntax Common expression errors To eliminate the error in this example, you must enter the full pathname of the attribute as GroupA|Ball.tx. The pipe symbol (|) specifies that the object to its left is the parent of the object on the right. Cannot set 'time' or 'frame'

You can read the value of the predefined time and frame variables, but you cannot set them. Attributes must be of float, integer, or boolean types: Ball.worldMatrix

You tried to set or read the value of an attribute that was a string or matrix type. For instance, you might have tried to use an attribute named translate rather than translateX, translateY, or translateZ attribute. In the error message above, worldMatrix is an attribute that exists for transforms, but you can’t use it. It’s for Maya’s internal use. Cannot divide by zero

You tried to divide by an attribute or variable that equals 0. This typically happens in an expression statement that divides by an object’s translateX, translateY, or translateZ attribute when the Snap to grids button is on and you drag the object to past the X-, Y- or Z-axis. When Snap to grids is on, the translateX, translateY, or translateZ attribute becomes exactly equal to 0 at the point where you drag the object across the axis. To prevent this error, turn Snap to grids off. With snapping off, the attribute is unlikely to become exactly 0 as you drag across the axis.

Note If you compile an expression for a particle shape node and see the same error message once for each particle in the object, it’s likely that some attribute name, variable, or function is undefined or misspelled.

98


6

Editing Expressions The Expression Editor offers convenient techniques for editing the text of expressions. There are filters that help you search for expressions you previously created, as well as techniques for entering and modifying the text of an expression.

You can edit an expression directly in the text box or with a text editor such as vi.

This chapter describes the following topics: “Finding expressions” on page 99

•

“Editing an expression in the text field” on page 105

•

“Editing an expression with a text editor” on page 106

•

“Creating a new expression” on page 111

•

“Deleting an expression” on page 112

•

“Using attribute names in expressions” on page 112

Expressions

•

Finding expressions After you’ve created an expression, you might decide later to alter it to create a different animation result. To edit an expression, you display it in the Expression Editor. The following sections describe how to find and display an expression for editing.


99

Editing Expressions Finding expressions

Finding by expression name To find an expression, you can choose from a list of all expressions in the scene.

To search for an expression by name: 1

From the Expression Editor, choose Select Filter→By Expression Name. An Expressions list appears in the Expression Editor. This list shows all expressions created for the scene.

List of expressions

2

Click the expression in the list. The expression contents appear in the expression text field. If you don’t remember the name of the expression, click each name on the list until the desired expression appears in the expression text field.

Note For a particle shape node, you can create a creation expression, a runtime expression, or both. Both expressions are listed under a single name—the name of the particle shape node. You can’t name or rename such expressions. To find such expressions, look for the particle shape node’s name in the Expressions list. Click the appropriate Runtime or Creation checkbox to display the desired expression.

100



Finding by selected object If you can’t remember the name you gave an expression, you can find it by selecting the affected object. For a nonparticle shape node, you can also select an affected attribute from the Attributes list to narrow the search for the expression.

To search for an expression by object and attribute name: 1

Select the object or other node in the Outliner, Hypergraph, or workspace.

2

Choose Select Filter→By Object/Attribute Name in the Expression Editor. This is the default search setting for the Expression Editor.

3

Choose Object Filter→Selected Objects. The selected object’s name and appropriate attributes appear in the window.

Object name Object’s attributes

For an object other than a particle shape node, click the name of the attribute controlled by the expression. If you’ve forgotten the name of the attribute controlled by the expression, choose Attribute Filter→Connected to Expressions. The Attributes list displays only the attributes controlled by expressions for the selected object. Click each attribute in the Attributes list until you see the desired expression in the expression text field. You can’t write a different expression for each attribute of a particle shape as you can for other types of objects. Because you can write only one creation expression and one runtime expression per particle shape, you don’t need to select an attribute from the Expression Editor’s Attributes list. See “Understanding particle expressions” on page 148 for details on particle expressions.


101

Expressions

4


Note The Attributes list shows only unlocked, keyable attributes. You can choose whether an attribute is keyable or locked with View→Object→ Editors→Channel Control. To write an expression for any nonkeyable attribute not shown in the list, enter object.attribute name in the Selected Obj & Attr text box.

Finding by item type You can find an expression based on the type of object or item the expression affects. For example, if you can’t remember an expression’s name but remember you applied it to a shader node, you can narrow your search to expressions that control shader nodes in the scene.

To search for an expression by item type: 1

In the Expression Editor, choose Select Filter→By Object/Attribute Name.

2

From the Object Filter menu, select the type of object or item the expression affects.

3

Choose Attribute Filter→Connected to Expressions.

4

Select the affected object or item from the Objects list.

5

Select the affected attribute from the Attributes list. The expression that controls the attribute appears in the expression text field.

Example Suppose you’ve written an expression that controls the rotateZ attribute of a spotlight transform node named Searchlight. Do this to find the expression: 1

Choose Select Filter→By Object/Attribute name.

2

Select Object Filter→Transforms. Note that you don’t select Object Filter→Lights in this example. The rotateZ attribute is an attribute of a light’s transform node, not of the light object itself.

102

3

Choose Attribute Filter→Connected to Expressions.

4

Select the object Searchlight from the Objects list.


Editing Expressions Finding expressions 5

Click rotateZ from the Attributes list. The expression appears in the expression text field.

Using the Selection list The Expression Editor displays a Selection list by default. This list displays either a list of objects and attributes, or a list of expressions you’ve created. To display the list of objects and attributes, choose Select Filter→By Object/ Attribute Name. This is the default display. To display the list of expressions you’ve created in the scene, choose Select Filter→By Expression Name.

Selection list triangle

Expressions list

Expressions

Using the Objects and Attributes list The objects listed in the Objects list depend on which entry you’ve selected from the Object Filter menu. If you select Object Filter→Lights, for instance, all lights in the scene appear in the list. The appropriate attributes of the object selected in the Objects list appear in the Attributes list. For example, if spotLightShape1 is selected in the Objects list, the attributes of spotLightShape1 appear in the list. When searching for an expression to edit, you can click an object and attribute from this list to find and display an expression that affects the chosen attribute. You can edit the displayed expression in the expression text field.


103

Editing Expressions Finding expressions For a particle shape node, you don’t need to select an attribute from the Attributes list. You can create only one creation expression and one runtime expression per particle shape node. The same expression appears for each attribute. When you create a new expression, you can click an object from this list to choose the default object to which the expression applies. When you select the default object in the Expression Editor, you can skip omit the object name and period that’s part of a full attribute name (see “Omitting an object name in expressions” on page 115.)

Using the Expressions list The Expressions list shows all expressions you’ve created in the scene. When searching for an expression to edit, click an expression from this list to display and edit its contents.

Hiding the Selection list You can hide the Selection list to lessen clutter in the window. To do so, click the triangle next to Selection (see previous figure). This triangle collapses and expands the list.

Filtering attributes from the Selection list If a selected object has several attributes controlled by expressions but you’re not sure which attributes, you can select a filter to list only attributes controlled by an expression.

To filter attributes from the Attributes list: 1

Select the object containing the attributes.

2

Choose Select Filter→By Object/Attribute Name.

3

Choose Object Filter→Selected Objects.

4

Choose Attribute Filter→Connected to Expressions. Only the object’s attributes controlled by expressions appear in the Attributes list. To see all attributes you can control with an expression again, choose Attribute Filter→All.

104


Editing Expressions Editing an expression in the text field

Editing an expression in the text field The Expression Editor provides techniques for deleting and copying text in the expression text field. There are also techniques for clearing and restoring the text of an expression.

Expression text field

Important

Deleting and copying text To delete text: 1

Drag the mouse to select the text.

2

Press your keyboard’s Backspace key to delete it.

To copy and paste text: 1

Drag the mouse to select the text to be copied.

2

At the point in the text where you want to copy the text, click with the middle mouse button. This technique takes a little practice. If you find this frustrating, you might prefer using a text editor native to your operating system, for example, vi or jot. See “Editing an expression with a text editor” on page 106.


105

Expressions

If you close the Expression Editor window without successfully compiling an expression with the Create or Edit button, Maya discards any editing changes you’ve made to the expression.

Editing Expressions Editing an expression with a text editor

Clearing the expression text field You can erase the entire expression text field by clicking a button rather than dragging and deleting text.

To clear the expression text field: Click the Clear button.

Important To erase an expression and make sure its previous contents no longer control an attribute, click the Edit button after clicking the Clear button.

Reloading an expression’s previous contents Clicking the Create or Edit button compiles an expression. If you’ve made an editing change and haven’t yet clicked the Edit button, you can reload the previous expression if you don’t like the results.

To reload the expression: Click the Reload button. This restores the expression to the contents last present when you clicked the Create or Edit button.

Editing an expression with a text editor From the Expression Editor, you can start a text editor such as vi to create and edit an expression. Text editors have features useful for editing big expressions. When you start the text editor for an expression, you can edit only that expression with that instance of the text editor. However, you can start the text editor once for each of several expressions if you want to examine or edit several expressions at the same time. Once you start a text editor for an expression, the Expression Editor’s text field dims to indicate you can’t work there while the text editor runs. You can, though, work in the expression text field for another expression.

106


Editing Expressions Editing an expression with a text editor There is no file on disk you can edit independently of the Expression Editor. When you use the text editor through the Expression Editor, you’re working with a temporary file that’s linked to the expression stored in the scene. You can, however, read an independent text file containing expression text into the temporary file. If you save an expression without specifying a filename, Maya reads the saved expression and stores it with the scene. You’ll see it dimmed in the expression text field while you’re working with the text editor. When you close the text editor, the expression text field entry no longer is dim. The text expression field becomes active after you close the text editor. If you quit the text editor without saving the expression, Maya does nothing. Because the expression hasn’t changed, Maya’s copy of the expression doesn’t need to change either.

Tip You can use a text editor to save an expression to a filename in the directory of your choice. This gives you a way to archive an expression you want to use in a different scene.

Using an editor listed in the Editor menu Expressions

By default, you can start one of these editors from the Editor menu in the Expressions Editor: •

jot

•

vi

•

vim

•

xemacs To run a different editor, see “Using an editor not listed in the Editor menu” on page 109.

To start an editor listed in the menu: 1

From the Editor pull-down menu in the Expression Editor, select an editor.

2

Double-click an object name, expression name, or attribute name from the Selection list.


107

Editing Expressions Editing an expression with a text editor The editor appears. An example display of vi follows:

The editor’s title bar shows a filename that’s temporarily created while you work on the expression. When you write or save the file, its contents are copied to the Maya scene containing the expression. The expression text field is inactive while the text editor is open. You can optionally close the Expression Editor window. If you single-click the name of an object, attribute, or expression, the text editor doesn’t appear. You can single-click to browse the contents in the expression text field without opening a text editor. If you double-click an attribute that’s already been assigned a value in an expression, the expression that controls that attribute appears in the text editor. For nonparticle expressions, you can assign to any attribute in the scene, not just to the double-clicked attribute. In fact, you don’t even need to work with the double-clicked attribute at all. If you double-click an attribute that has not yet been assigned a value, the text editor appears with no contents. If you double-click that attribute again, a new instance of the editor appears. After you assign a value to an attribute in an expression, you can start the editor only once for the attribute.

108

3

Create or edit the expression with the editor.

4

Save the file.

5

Confirm that the Expression Editor detected no syntax errors.

6

Quit the editor.



Note If you’ve created a UNIX command alias for jot, vi, vim, or xemacs, the Expression Editor tries to launch this command. If the arguments provided in the command alias are unusable by the Expression Editor, the editor might operate unexpectedly or fail to launch. Avoid using an alias to customize your editor’s operation settings. Do the steps in “Changing an editor’s operation settings” on page 110.

Using an editor not listed in the Editor menu If your workstation has a text editor that’s not listed in the Editor menu, you can use it after doing a few preliminary UNIX system administration tasks.

To start an unlisted editor: 1

In your UNIX .cshrc file, set the WINEDITOR environment variable to specify the desired editor and options. See “Changing an editor’s operation settings” on page 110 for examples. You can choose any valid options for the editor, but you must specify that the editor runs in the foreground (if this option is relevant to the editor).

2

Log out and log into your user account.

3

Restart Maya.

4

Choose Other from the Editor pull-down menu.

5

Double-click an object name, expression name, or attribute name from the Selection list. The editor appears.

6


7

Save the file.

8


9

Quit the editor.


109

Expressions

If the editor normally appears in the shell where you launched it, you must make the WINEDITOR setting display the editor in a shell.


Changing an editor’s operation settings Maya launches the editors listed in the Editor menu with default operation settings. You can change the operation settings with a few preliminary system administration tasks.

To change an editor’s operation settings: 1

Set the WINEDITOR environment variable to specify the desired editor options. You can choose any valid options for the editor, but you must specify that the editor runs in the foreground (if this option is relevant to the editor). For example, jot requires the option -f, vim requires -g -f, and xemacs requires the option -nw. An example of setting WINEDITOR for vi follows: setenv WINEDITOR “xwsh -name mayaEditor -e vi”

An example for vim follows: setenv WINEDITOR “xwsh -geometry 80x57+350+130 -bg 97 -e vim”

2

Log out and log into your user account.

3

Restart Maya.

4

Choose Other from the Editor pull-down menu.

5

Double-click an object name, expression name, or attribute name from the Selection list. The editor appears.

6


7

Save the file.

8


9

Quit the editor.

Selecting an editor for default startup You can make an external text editor start by default each time you start a text editor.

110


Editing Expressions Creating a new expression

To start an editor by default: 1

Choose Options→UI Preferences.

2

In the UI Preferences window, click the Misc folder.

3

Choose the editor in the Expression Editor menu. To choose an editor specified with the WINEDITOR environment variable, select Other.

4

Click Save Changes to close the window.

5

In the Expression Editor, double-click an object name, expression name, or attribute name from the Selection list. The editor appears. The next time you start the Expression Editor, the editor’s name appears in the Editor pull-down menu by default. If you’ve chosen different text editors in UI Preferences and the Editor menu, the one chosen in UI Preferences appears.

Important If you’ve specified a text editor through Options→UI Preferences or with the Expression Editor’s Editor menu, starting the Expression Editor from the Channel Box or Attribute Editor displays the text editor instead of the Expression Editor.

Creating a new expression You can create a new expression after you’ve been editing an existing one.

To create a new expression: 1

Make sure you click the Create or Edit button to compile the existing expression.

2

Choose Select Filter→By Expression Name.

3

Click the New Expression button. This clears the Expression Name box and expression text field so you can create a new expression.


111

Expressions

Note the text editor appears when you click the New Expression button.

Editing Expressions Deleting an expression When you create the expression, the Expression Editor associates the object name with the expression. This means you can narrow your search for the expression using the object’s name in addition to the expression name. You do not need to select an attribute in the Attributes list. You can associate the expression with an object only. For a particle shape node, you don’t need to select an attribute, as you can create only one creation expression and one runtime expression per particle shape. For nonparticle shape objects, you can create one expression per attribute.

Deleting an expression If you want to stop an expression from controlling attributes, you can delete the expression.

To delete an expression: 1

Display it in the Expression Editor.

2

Click the Delete button.

Using attribute names in expressions A full attribute name has this format: object.attribute where object is the name of the object node and attribute is the name of the attribute. A period separates the name of the object and attribute. Object and attribute names are case-sensitive. You must spell them with uppercase and lowercase letters as they appear in the Expression Editor’s Objects and Attributes lists. You cannot spell attribute names with the common English spellings shown in the Attribute Editor or by default in the Channel Box. The following topics show how you can abbreviate attribute names to save typing time.

112


Editing Expressions Using attribute names in expressions

Using attribute name abbreviations You can use an abbreviation in place of any full attribute name in the expression text field.

Example In place of this: Ball.translateY = time;

you can type this: Ball.ty = time;

Each attribute has at least one acceptable abbreviation. Here are some commonly used attribute name abbreviations for several types of object transform nodes: Abbreviation

translateX

tx

translateY

ty

translateZ

tz

rotateX

rx

rotateY

ry

rotateZ

rz

scaleX

sx

scaleY

sy

scaleZ

sz

visibility

v

Expressions

Long name

To see the abbreviations for attributes that can be keyframed: 1

Select the object or item containing the desired attributes.

2

Turn on Options→Channel Box to display the Channel Box.


113

Editing Expressions Using attribute names in expressions Common English equivalents for the long attribute names appear in the Channel Box by default. These names are different than the names you must use in the expression text field. If you use the long attribute name, use the name that appears in the Attributes list of the Expression Editor. Do not use the common English language equivalents displayed in the Channel Box.

Use either attribute long names or abbreviated names in expressions

Do not use these common English names

3

From the Channels menu at the top of the Channel Box, select Channel Names→Short. The abbreviated attribute names replace the common English attribute names in the Channel Box.

114



To see abbreviations for other attributes: Execute this MEL command in the Script Editor: listAttr -sn objectname

where objectname is the name of the object or other node.

Note After you click Create or Edit to compile an expression, Maya converts all attribute abbreviations in the expression to the full attribute name.

Omitting an object name in expressions If you select an object as the Default Object in the Expression Editor, you can omit the object name and period that’s part of a full attribute name.

Example Suppose you’ve selected Ball as the Default Object. In place of this: Ball.translateY = time;

you can type this: Expressions

translateY = time;

Maya interprets translateY as belonging to Ball, the object listed in the Default Object text box of the Expression Editor.

To make an object the Default Object: Enter the object’s name in the Default Object text box. By default, the selected object is also the default object. You can omit the object name only for attributes of the object in the Default Object text box. The Default Object text box is dim when a particle shape node is the selected object in the Expression Editor. Because a particle shape node’s attributes can be controlled by only one creation expression and one runtime expression, the particle shape node is always the default object when it is the selected object.


115


Combining the abbreviation techniques You can combine the abbreviation techniques mentioned in the two previous topics to minimize typing.

Example Suppose you’ve selected Ball as the Default Object. In place of this: Ball.translateY = time;

you can type this: ty = time;

Maya interprets ty as being the translateY attribute of Ball, the object listed in the Default Object text box of the Expression Editor. Attributes of other objects must be spelled out with the full object and attribute name.

116


7

Beyond the Basics This chapter describes advanced concepts for writing expressions. Unless otherwise noted, the topics within apply to expressions for attributes of all objects, including particles. For additional details on working with particles, see Chapter 8, “Particle Expressions.”

Rob Tesdahl

This chapter describes the following topics: •

“How often an expression executes” on page 118

•

“Using custom attributes in expressions” on page 118

•

“Displaying attribute and variable contents” on page 123

•

“Reproducing randomness” on page 123

•

“Speeding expression execution” on page 127

•

“Reducing redundant expression execution” on page 130

•

“Removing an attribute from an expression” on page 131

•

“Disconnecting an attribute” on page 132 Using Maya: Hypergraph, Sets & Expressions

117

Expressions

The falling cube is a soft body with goal and springs. As its points move below the floor, an expression assigns them a goal weight of 0. The cube appears to melt as it passes through the floor.

Beyond the Basics How often an expression executes •

“Renaming an object” on page 136

•

“Executing MEL commands in an expression” on page 137

•

“Understanding path names” on page 140

•

“Understanding unexpected attribute values” on page 141

How often an expression executes After you’ve typed an expression in the Expression Editor, you click the Create or Edit button to compile the expression. Compiling the expression checks it for syntax errors and converts it to a form Maya can execute when you rewind or play the animation. After being compiled, the expression executes for the current frame. When you select an object other than a particle shape node, the Expression Editor displays an Always Evaluate checkbox that affects when an expression executes. If you select a particle shape node, the Expression Editor dims this checkbox. For details on particle shape node expressions, see Chapter 8, “Particle Expressions”). Generally an expression executes whenever the current animation time or frame changes. For example, an expression executes when you rewind or play the animation. The expression executes once for each time the animation frame or time changes. An expression also generally executes when your interaction with Maya makes use of an attribute in the expression. For example, if your expression assigns a sphere’s translateX attribute to another attribute and you move the sphere in an X-axis direction, the expression executes upon each increment of the sphere’s movement. Occasionally, it’s useful to turn off Always Evaluate to diminish redundant expression execution and speed Maya operation. Before doing this, it’s best to understand the subtle details of expression execution. See “Reducing redundant expression execution” on page 130 for details.

Using custom attributes in expressions It’s often helpful to add a custom attribute to an object and use it in an expression. You can use a custom attribute to control a combination of other attributes. You can also use a custom attribute as a variable—a place to store a value temporarily to be read by other attributes.

118


Beyond the Basics Using custom attributes in expressions Custom attributes have no direct effect on any characteristic of an object. See “Assigning to a custom attribute” in Chapter 8 for details on how to add and use a custom attribute with particles.

Example Suppose you’ve given a NURBS sphere named Planet a circular, orbiting motion in the XY plane with this expression: Planet.tx = sin(time); Planet.ty = cos(time);

Expressions

Planet orbits the origin at a radius of 1 unit. In the following steps, you’ll create a custom attribute named distance to increase the radius of Planet’s orbit over time.

Note The small balls in the preceding figure show the circular path of Planet. They’re in the figure only to help you visualize the motion. They aren’t part of the animation or expression.

To add a custom attribute to alter the orbit: 1

Select Planet.

2

Choose Modify→Add Attribute.


119

Beyond the Basics Using custom attributes in expressions

3

In the Add Attribute window, enter distance in the Attribute Name text box.

4

Make sure Make attribute keyable is on.

5

Set Data Type to Float, and Attribute Type to Scalar.

6

Set Minimum to 1, Maximum to 10, and Default to 4. Minimum and Maximum set the lowest and highest values you can enter for the attribute in the Attribute Editor or Channel Box. Default sets the default value displayed for the attribute. An expression isn’t bound by the Minimum and Maximum values. The attribute receives whatever value you assign it in the expression. The expression can read the Default value or any other value you set in the Attribute Editor or Channel Box.

7

120

Click Add to add the attribute, then close the Add Attribute window.


Beyond the Basics Using custom attributes in expressions The distance attribute appears in the Attributes list of the Expression Editor for Planet. You can now set or read the value of the attribute in any expression. 8

Edit the expression to this: Planet.tx = distance * sin(time); Planet.ty = distance * cos(time);

Multiplying the sin(time) and the cos(time) by the distance attribute makes Planet circle the origin at a distance specified by the value of the distance attribute. See Chapter 9 for details on the sin and cos functions.

You can make the expression control the distance attribute over time. 9

Edit the expression to this: distance = time; Planet.tx = distance * sin(time); Planet.ty = distance * cos(time);

By setting distance to the value of time, Planet’s orbiting distance increases as playback time increases. Planet moves in a steady outward spiral as the animation plays.


121

Expressions

Because you gave the distance attribute a default value of 4 when you added it to Planet, playing the animation makes Planet circle the origin at a distance of 4 grid units from the origin.

Beyond the Basics Using custom attributes in expressions

Instead of using an expression to control distance, you can keyframe its value over time. For example, by keyframing a distance value of 1 at frame 1 and a value of 10 at frame 200, Planet moves in a steady outbound spiral as you play the 200 frames. Planet’s distance increases in a linear interpolation from 1 to 10 as the animation plays. You can animate the distance attribute with keyframes or with an expression, not with both.

Tip If an expression controls an attribute and you want to control it with keyframes instead, delete all statements that assign values to the attribute, then click the Edit button. Use the Channel Box to reset the attribute’s value to an initial value, then set keyframes as desired. If keyframes control an attribute and you want to control it with an expression instead, click the attribute’s text box in the Channel Box, then choose Channels→Delete Selected. Assign values to the attribute name in an expression as desired.

122


Beyond the Basics Displaying attribute and variable contents

Displaying attribute and variable contents The predefined print( ) function displays attribute contents, variable contents, and other strings in the Script Editor. This is often helpful for debugging an expression. See “print” on page 261 for more details. Note that for a nonparticle expression consisting of only print statements, Always Evaluate must be on in the Expression Editor for the expression to execute.

Reproducing randomness If you execute the rand, sphrand, and gauss functions repeatedly in an expression, Maya returns a sequence of random numbers. (See “Random number functions” on page 239 for details on these functions.) Each time you rewind and play your animation, the sequence of random numbers is different. Often, you’ll want to generate a sequence of random numbers that repeats each time your animation plays. For instance, suppose you use the rand function to assign a random radius to each particle in a stream of emitted particles rendered as Spheres. By default, Maya gives the particles a different sequence of random radius values each time your animation plays.

Important When you set a seed value in an expression or MEL script, the seed value affects the rand, sphrand, and gauss functions in other expressions and MEL scripts. Such functions are affected by this seed value in all scenes you open subsequently in the current work session. This seed value is unrelated to the Seed option available through Settings→Dynamics Controller in the Dynamics menus. The seed function therefore doesn’t affect randomness created with dynamics.


123

Expressions

To create the same radius values each time the animation plays, you can use the seed function in an expression before the rand, sphrand, or gauss functions execute. There’s no need to execute the seed function more than once per animation unless you need to generate several different repeating sequences of random numbers as your animation plays.

Beyond the Basics Reproducing randomness

Example Suppose you use the rand function to position several marbles at random translateX positions in your scene at frame 1: if (frame == 1) { marble1.tx marble2.tx marble3.tx marble4.tx }

= = = =

rand(-10,10); rand(-10,10); rand(-10,10); rand(-10,10);

The rand(-10,10) returns a random number between -10 and 10 each time it executes. When you rewind the animation to frame 1, Maya might assign these values to the translateX attributes of the marbles:

124

Attribute

Value

marble1.tx

2.922

marble2.tx

5.963

marble3.tx

-4.819

marble4.tx

7.186


Beyond the Basics Reproducing randomness The next time you rewind the animation to frame 1, each marble’s translateX attribute receives a different random value. Maya might assign these values: Attribute

Value

marble1.tx

-3.972

marble2.tx

9.108

marble3.tx

-7.244

marble4.tx

-3.065

You can use the seed function to keep the sequence of random values returned by the rand function consistent when you rewind the animation. if (frame == 1) { seed(10); marble1.tx marble2.tx marble3.tx marble4.tx }

= = = =


By setting the seed value to an arbitrary number, for instance, 10, the subsequent executions of the rand function return a repeating sequence of random numbers.


125

Expressions

You might prefer the marbles’ translateX values to stay the same when you rewind, for instance, so you can composite the marbles correctly among a foggy backdrop.

Beyond the Basics Reproducing randomness When you rewind the animation the first time, Maya might assign these values to the translateX attributes of the marbles: Attribute

Value

marble1.tx

8.020

marble2.tx

-2.973

marble3.tx

-7.709

marble4.tx

0.741

Each time you rewind the animation thereafter, Maya assigns these same values to the translateX attributes of the marbles. The marbles don’t move. Each time a statement sets the seed value to 10, the subsequent executions of the rand function return numbers from the sequence starting at the beginning number. In other words, resetting the seed value to 10 restarts the random number generation process to the first value in the sequence. Suppose you alter the expression to this: if (frame == 1) { seed(10); } marble1.tx marble2.tx marble3.tx marble4.tx

= = = =


When you rewind the animation to frame 1, the expression sets the seed to 10. Maya assigns values to the marbles’ translateX attributes as in the previous expression.

126


Beyond the Basics Speeding expression execution Because the expression doesn’t set the seed value in frames other than frame 1, playing the animation causes the rand function to return a new, yet repeating, sequence of random numbers each frame. If you play the animation several times, the translateX values will constantly change during animation, but the sequence of values will be identical each time you play the animation. You can assign the seed a different value to generate a different sequence of returned values. See “seed” on page 246 for details.

Speeding expression execution Maya does calculations internally in centimeters, radians, and seconds. A radian is an angular unit commonly used in mathematics. It equals 180 degrees divided by pi, or roughly 57.3 degrees. When you assign a number to an attribute whose value is a measurement unit, the expression interprets the number, by default, as the appropriate unit selected in the Units folder of the General Preferences window. By default, the Units folder selections are centimeters, degrees, and seconds. If a measurement unit you’ve chosen in the Units folder differs from the corresponding internal unit, Maya converts the number to the appropriate internal unit to do the assignment. Expressions

Example Suppose you’ve selected degrees from the Angular menu in the Units folder. You then write this expression for an object named Ball: Ball.rotateZ = 10;

Maya reads the 10 as being 10 degrees, then converts the value to the appropriate number of radians to make the assignment to Ball’s rotateZ attribute. The conversion happens automatically. From your standpoint, Maya is simply rotating Ball 10 degrees. In nonparticle expressions, these automatic conversions affect Maya performance. Because the expression executes slower, Maya slows when you play, rewind, or otherwise change the animation time. Saving, opening, and other file operations on the scene containing the expression are also slower. To boost Maya performance, you can turn off conversion to internal units. If you do so, you must convert units in expression statements.


127

Beyond the Basics Speeding expression execution

To speed expression execution: 1

Display the Expression Editor.

2

Choose one of these Convert Units options: None

Converts no units. You must assign values to attributes as centimeters, radians, or seconds, as appropriate. Execution is fastest with this option.

Angular Only

Converts angular units, but no others. You must assign values to attributes as centimeters, seconds, and degrees, as appropriate. (This assumes you’re using the default degree setting in the Units folder. If you’ve selected radians, you must enter radians.) If you’re confused by converting degrees to radians, select this option. Execution is fast with this option—unless the expression has many angular values.

To return to default conversions: 1

Display the Expression Editor.

2

For the Convert Units option, choose All. This lets you enter all measurement numbers in the same units specified in the Units preference settings. Execution is slowest with this selection, but expression writing is simplest. You can set a different conversion option for each expression.

Example Suppose, in the Units folder, you’ve set Linear units to millimeters and Angular units to degrees. You then write the following expression: Ball.translateX = 5; Ball.rotateZ = 10;

All causes Maya to read 5 as millimeters and 10 as degrees. None causes Maya to read 5 as centimeters and 10 as radians. Angular causes Maya to read 5 as centimeters and 10 as degrees.

To convert units in an expression statement: You must convert the units mathematically in a statement.

128


Beyond the Basics Speeding expression execution

Examples Suppose, in the Units folder, you’ve set Linear units to millimeters and Angular units to degrees. In the Expression Editor you set the Convert Units option to None and enter this expression: Ball.translateX = 5; Ball.rotateZ = 10;

None causes Maya to read 5 as centimeters and 10 as radians, which is not the result you’re seeking. To assign 5 millimeters to Ball’s translateX attribute, you must convert 5 to the appropriate number of centimeters. To assign 10 degrees to Ball’s rotateZ attribute, you must convert 10 to the appropriate number of radians. The following statements do this: Ball.translateX = 5.0 / 10.0; Ball.rotateZ = 10.0 / 57.3;

There are 10 millimeters per centimeter. In other words, a millimeter is a centimeter divided by 10. So 5 millimeters equals 5 centimeters divided by 10. You therefore use the operation 5.0 / 10.0.

When you divide floating point attributes or variables, enter the floating point value 5.0 for an even number such as 5. This ensures that the division works as expected. For more details, see the note in “Using mixed data types with arithmetic operators” on page 145. There are 57.3 degrees per radian. In other words, a degree is a radian divided by 57.3. So 10 degrees equals 10 radians divided by 57.3. You therefore use the value 10.0 / 57.3. If you need a more precise conversion to radians, divide a degree by 57.29578 instead of 57.3. You can instead use the deg_to_rad function as follows: Ball.rotateZ = deg_to_rad(10.0);

The deg_to_rad function converts 10.0 degrees to a precise radian equivalent. See “deg_to_rad” on page 234 for details.


129

Expressions

Important

Beyond the Basics Reducing redundant expression execution Turning off unit conversion affects only expressions. It doesn’t affect other Maya commands, options, or displays. For instance, the preceding example expression assigns centimeters to translateX and radians to rotateZ. The Channel Box still displays values for these attributes in millimeters and degrees. It displays values in whatever units you choose in the Units folder of the General Preferences window. Note that you can’t turn off unit conversion for particle shape node expressions. Maya handles unit conversion differently for such expressions with little impact on performance.

Reducing redundant expression execution If your expression has redundant statement calculations, you can turn off Always Evaluate to speed up scrubbing and playback of your animation. To understand when this feature is useful, you must understand the subtle details of expression execution. An expression generally executes whenever the animation time changes. An expression also executes whenever an attribute that’s read by the expression changes value, and either of the following two actions occurs: •

Some other node in Maya uses the value of an attribute the expression writes to. For example, a deformer or shader uses its value.

•

Maya needs the value of an attribute to which it writes in order to redraw the workspace contents. In this context, the predefined variables time and frame are also considered attributes the expression reads. Suppose you write an expression that moves a NURBS sphere along the Yaxis at twice the current value of its X-axis translation: nurbsSphere1.translateY = 2 * nurbsSphere1.translateX;

If you use the Move tool in the workspace to drag the sphere in an X-axis direction, Maya executes the expression for each incremental change to the translateX attribute as you drag. Dragging the sphere in the X direction changes the value of the translateX attribute in the expression. As you drag the sphere and Maya updates the workspace display, the value of the translateY attribute changes in the expression. This makes the expression execute.

130


Beyond the Basics Removing an attribute from an expression If you turn Always Evaluate off, an expression won’t execute if it contains only print function statements, variable assignments, or assignments that do not read attribute values.

Example global float $BallHeight = 5; print($BallHeight+"\n"); nurbsSphere1.tx = rand(1); print(nurbsSphere1.tx+"\n");

The first statement declares and assigns a value to the variable $BallHeight, which is not an attribute. The next statement prints the $BallHeight but assigns no value to an attribute. The next statement assigns an attribute a value, but the value is generated by the random number function rand. This function doesn’t read an attribute value. For details on the rand function, see “rand” on page 243. The last statement reads and prints the value of an attribute, but doesn’t assign a value to an attribute. None of these actions causes the expression to execute when Always Evaluate is off.

For most animations, expressions execute regardless of whether Always Evaluate is on. If in doubt, leave it on.

Removing an attribute from an expression If you do any of the following actions, an expression no longer sets or reads an attribute: •

Delete all occurrences of the attribute name in the expression.

•

Convert to comments all statements that use the attribute name in the expression.

•

Delete the expression that contains the attribute. Following these actions, the attribute keeps its value from the last time the expression executed and set its value.


131

Expressions

Always Evaluate affects only the expression you’re creating or editing. You can turn it on for one expression and off for another.

Beyond the Basics Disconnecting an attribute The attribute doesn’t return to the value it had before the expression set it. To return the attribute to its original value, use the Channel Box or Attribute Editor to set the attribute.

Disconnecting an attribute If you disconnect an attribute from an expression, the expression no longer reads or set its value. You might want to disconnect an attribute, for example, so you can keyframe the attribute rather than control it with an expression. These actions disconnect an attribute from an expression: •

Delete from the scene an object with an attribute that exists in the expression.

•

Use the Window→General Editors→Connection Editor to disconnect the attribute from the expression.

•

Use the MEL disconnectAttr command.

•

Use the MEL choice command.

Tip The MEL choice command lets you control an attribute alternately with two or more techniques in different frames. For example, you can keyframe an attribute for frames 1-48, control it with an expression for frames 48-96, and control it with a motion path for subsequent frames.

Displaying disconnected attributes in expressions The Expression Editor displays a disconnected attribute with a symbolic placeholder representing the attribute’s former existence in the expression.

Example Suppose your scene has two objects, Ball and Cone, and you’ve written this expression: Ball.translateX = Cone.translateX; Ball.translateY = Cone.translateY; Ball.translateZ = Cone.translateZ;

132


Beyond the Basics Disconnecting an attribute If you delete Cone from the scene, Cone.translateX, Cone.translateY, and Cone.translateZ attributes no longer exist for the expression to read and assign to Ball’s translateX, translateY, and translateZ attributes. If you display the expression again, it appears as follows: Ball.translateX = .I[0]; Ball.translateY = .I[1]; Ball.translateZ = .I[2];

The .I[0], .I[1], and .I[2] characters indicate you’ve disconnected Cone’s translate attributes from the expression. These symbols represent placeholders for the former use of the attributes in the expression. The .I means the placeholder represents an input to the expression. An input to an expression is an attribute with a value the expression reads for assignment to another attribute or variable. The number in brackets indicates the order in the expression the attribute was read. For example, .I[0] indicates the input is the first attribute read in the expression, .I[1] indicates the input is the second attribute read, and .I[2] indicates the input is the third attribute read.

Note that if you disconnect an attribute from an expression but the attribute still exists in the scene, the attribute keeps its value from the last time the expression executed and set its value.

Example Suppose you’ve written these statements among others: Ball.translateX = Cone.translateX; Ball.translateY = Cone.translateY; Ball.translateZ = Cone.translateZ;

If you delete Ball from the scene, Ball.translateX, Ball.translateY, and Ball.translateZ attributes no longer exist. The expression can no longer assign Cone’s translateX, translateY, and translateZ values to the corresponding Ball attributes.


133

Expressions

A floating point or integer attribute placeholder has a value of 0. A particle shape node’s vector attribute placeholder has a value of <<0,0,0>>. In the example, the placeholders .I[0], .I[1], and I[2] have the value 0. When the expression executes, it assigns Ball.translateX, Ball.translateY, and Ball.translateZ the value 0.

Beyond the Basics Disconnecting an attribute Symbolic placeholders replace Ball attributes in the expression. If you display the expression again, the statements appear as follows: .O[0] = Cone.translateX; .O[1] = Cone.translateY; .O[2] = Cone.translateZ;

Note If an expression assigns values to the attributes of only one object, deleting the object deletes the expression also. If your expression assigns values to attributes of several object attributes, deleting all those objects deletes the expression. To avoid deleting the expression in the preceding example, you would need have some statement that sets an attribute of an object other than the deleted Ball. For example, you might include this statement: Cone.visibility = 1;

The .O[0] characters indicate you’ve disconnected the attribute Ball.translateY from the expression. The .O indicates that the placeholder represents an output from the expression. An output from an expression is an attribute assigned a value by the expression. The number in brackets, for example, [0], indicates the order in which the attribute was assigned a value in the expression. Because Ball.translateX was the first output from the expression, the expression replaces it with .O[0]. The expression replaces Ball.translateY and Ball.translateZ with .O[1] and .O[2] because they were the second and third outputs from the expression. When the expression executes, it continues to assign values to the placeholder, though the placeholder has no effect on any object or component of scene. The expression assigns the placeholders .O[0], .O[1], and .O[2] the value of Cone.translateX, Cone.translateY, and Cone.translateZ, but these placeholders don’t control anything in the scene. The statements have no effect.

134


Beyond the Basics Disconnecting an attribute

Connecting an attribute to a symbolic placeholder After you’ve disconnected an attribute from an expression, a symbolic placeholder replaces it in the expression as described in the preceding topic. You can replace the placeholder with the attribute of your choice. The most obvious way to do this is to type the desired attribute name in every occurrence of the symbolic placeholder in the expression. If you have a lengthy expression that has lots of symbolic placeholders, you can use a single MEL connectAttr command to connect the new attribute to all occurrences of the same symbolic placeholder. You can also use Window→General Editors→Connection Editor.

Example 1 Suppose you have these statements among others in an expression named HorseController: WhiteHorse.translateX = Car.translateX; BlackHorse.translateX = Car.translateX; BrownHorse.translateX = Car.translateX;

Deleting the Car and reloading the expression shows this:

.I[0] is the symbolic placeholder for what was the Car.translateX attribute. You can connect a different attribute to this placeholder to assign its contents to the translateX attributes of WhiteHorse, BlackHorse, and BrownHorse. Suppose you want to control these attributes with the translateX attribute of an object named Cow. You can enter the following MEL command at the Command Line: connectAttr Cow.tx HorseController.input[0]

This command connects the attribute Cow.tx to the expression’s input[0]. The expression is named HorseController. The input[0] is abbreviated as .I[0] in the expression. You can see the spelled-out input name input[0] in the Graph→Up and Downstream Connections display of the Hypergraph. Reloading the expression shows the new attribute connection: WhiteHorse.translateX = Cow.translateX; BlackHorse.translateX = Cow.translateX; BrownHorse.translateX = Cow.translateX;


135

Expressions

WhiteHorse.translateX = .I[0]; BlackHorse.translateX = .I[0]; BrownHorse.translateX = .I[0];

Beyond the Basics Renaming an object

Example 2 You can also reconnect an expression’s output with the connectAttr command. Suppose you have these statements among others in an expression named HorseController: WhiteHorse.translateX = Car.translateX; BlackHorse.translateX = Car.translateX; BrownHorse.translateX = Car.translateX;

Deleting the BrownHorse object and reloading the expression displays this: WhiteHorse.translateX = Car.translateX; BlackHorse.translateX = Car.translateX; .O[2] = Car.translateX;

.O[2] is the symbolic placeholder for what was the BrownHorse.translateX attribute. It received the placeholder .O[2] because it’s the third output from the expression. (The first and second outputs from the expression are .O[0] and .O[1] .) You can connect a different object attribute to this placeholder to control it with the value in Car.translateX, as shown in the third statement. Suppose you want to control the attribute of a new object named RedHorse.translateX with the Car.translateX value. You can enter the following MEL command in the Command Line: connectAttr HorseController.output[2] RedHorse.tx

This command connects the HorseController expression’s output[2] to the attribute RedHorse.tx. The output[2] is abbreviated .O[2] in the expression. Reloading the expression shows the new attribute connection: WhiteHorse.translateX = Cow.translateX; BlackHorse.translateX = Cow.translateX; RedHorse.translateX = Cow.translateX;

Renaming an object If you rename an object whose attributes were used in an expression, the Expression Editor continues to read or set the attributes. Maya doesn’t disconnect the attribute from the expression. The Expression Editor converts to the new name of the object the next time you click the Reload button in the Expression Editor.

136


Beyond the Basics Executing MEL commands in an expression

Note When you reload an expression, the Expression Editor converts any short attribute names to their long attribute name equivalents. For example, if you originally type the attribute name Ball.ty, reloading the expression renames it as Ball.translateY.

Executing MEL commands in an expression You can execute MEL commands and procedures in an expression. However, if you make or break connections or add or delete nodes, your scene might malfunction. Rewinding your animation does not undo MEL command execution in an expression. For instance, if your expression executes MEL commands to create a pair of spheres, rewinding doesn’t delete the spheres. Moreover, playing the scene again creates another pair of spheres. Though you can usually undo executed MEL commands by selecting Edit→Undo repeatedly, this might not work if your scene is malfunctioning.

You can execute MEL commands in an expression with several techniques: •

MEL command alone in a statement

•

MEL command within left-hand single quote marks

•

MEL command used as an argument to an eval function

•

MEL procedure call to a procedure in a MEL script The following topics explain the techniques. See Using MEL for details on MEL.

Using a MEL command alone in a statement The simplest way to use a MEL command in an expression is to type it in a statement exactly as you would in the Script Editor or in a MEL script.

Example select -cl;


137

Expressions

When you execute a command from the Command Line, status information appears in the Script Editor and the Command Line’s response area. This information is not displayed when a command executes in an expression.

Beyond the Basics Executing MEL commands in an expression This example shows the use of a MEL command alone. The statement executes exactly as it would in the Script Editor, except no command output appears in the Script Editor.

Using a MEL command within single quote marks If you enclose a command within left-hand single quote marks (‘), Maya returns command output where the command is in the statement. You can assign this output to a variable to, for example, display it in the Script Editor.

Example string $a[]; $a = ‘ls -lights‘; print($a);

The first statement defines an array named $a. The second statement executes the MEL command within quotes, then assigns the command’s output to array $a. The third statement displays the contents of $a to the Script Editor as follows: ambientLightShape1 directionalLightShape1

Using a MEL command with the eval function Using a MEL command with the eval function has an advantage over the previous two techniques: you can build a command from a string.

Example string $mycommand = "sphere"; eval($mycommand+" -r 5");

The first statement assigns the string sphere to the variable $mycommand. The second statement appends -r 5 to sphere and executes the complete command sphere -r 5. This creates a sphere with a radius of 5 grid units. See “eval” on page 259 for more details.

Using a MEL procedure in an expression You can execute a MEL procedure in an expression by entering the procedure name in a statement.

138


Beyond the Basics Executing MEL commands in an expression

To execute a MEL procedure: 1

Give the script the same filename as the procedure it contains, but with the extension .mel. For example, if your procedure name is randspot( ), name your script file randspot.mel.

2

Put the script file in your Maya scripts directory.

3

Declare the procedure as global in the script file as in this example: global proc string randspot()

4

In an expression statement, use a statement that calls the procedure. You can use the statement within left-hand single quote marks with an eval function, or alone as in this example: randspot();

A complete example of calling a MEL procedure from an expression follow:

Example Suppose, in your Maya scripts directory, you’ve created a MEL script file named randspot.mel with the following contents:

Expressions

global proc string randspot() { string $mycommand; if (rand(2) < 1) $mycommand = "particle -p "+ sphrand(10); else $mycommand = "sphere -p "+ sphrand(10); return $mycommand; }

Further suppose you’ve created this expression: string $randcommand = randspot(); eval($randcommand);

When you rewind or play a frame in the animation, the expression executes. The first expression statement executes the randspot procedure in the randspot.mel script file. In the randspot procedure, the rand(2) part of the ifelse statement generates a random floating point value between 0 and 2, then compares its value to 1. For details on the rand function, see “rand” on page 243. Using Maya: Hypergraph, Sets & Expressions

139

Beyond the Basics Understanding path names If the rand(2) function returns a value less than 1, the if statement assigns a MEL command string such as particle -p -1.356 5.983 8.458 to $mycommand. The + sphrand(10) part of the statement appends to sphere -p the three floating point components of a randomly generated vector. Though sphrand(10) returns a vector, Maya converts the vector to a string upon assigning it to the string $mycommand. For details on the sphrand function, see “sphrand” on page 244. The converted string contains no double angle brackets or commas, but does contain a space character between the floating point components. A space between the floating point components is required syntax for the MEL particle command as used above. If the rand(2) function returns a value greater than 1, $mycommand receives a MEL command string such as sphere -p 4.926 -2.589 1.274. The procedure finishes executing and passes the value of $mycommand back to the expression’s calling procedure randspot( ). This assigns the command string to the variable $randcommand. The eval function executes the command string in $randcommand. For example, if the statement executes particle -p -1.356 5.983 8.458, it creates a particle with coordinates <<1.356, 5.983, 8.458>>. The expression executes each frame and creates a new particle or sphere at a random location within a spherical radius of 10 units from the origin.

Understanding path names If two objects in a scene have different parents, they can have the same object name. If you refer to an attribute of such an object in an expression, you must use a more complete name that includes the object’s path name. An object’s path name has this format: pathname|objectname.attributename where pathname is the parent node’s name, objectname is the object’s name, and attributename is the attribute’s name of the attribute. A pipe symbol (|) symbol divides the pathname from the object name. Don’t type spaces before or after the | symbol.

140


Beyond the Basics Understanding unexpected attribute values For example, a scene might have a child of GroupA named Ball.tx and a different child of GroupB named Ball.tx. If you write this statement: Ball.tx = time;

Maya generates an error because it doesn’t know which Ball.tx to set. To eliminate the error, you must enter the pathname of the attribute as in this example: GroupA|Ball.tx = time;

The | symbol between GroupA and Ball.tx indicates that the object to the left of the symbol is the parent of the object to its right. Use no spaces before or after the | symbol.

Understanding unexpected attribute values As you work with expressions, you’ll sometimes see attribute values you didn’t expect. The following topics describe a few common causes of confusion.

Important

Values after rewinding When you rewind a scene, an expression executes with the last settings made for attribute values. This sometimes gives unexpected results.

Example Ball.tx = $distance; $distance = time;

Assume for this example you’ve set the starting frame of the animation to frame 0. The first statement sets Ball.tx to the variable $distance. The second statement sets $distance to the value of time.


141

Expressions

Always examine the Script Editor for error messages after you edit an expression and click the Create button. If you alter a previously successful expression and a syntax error occurs, Maya executes the previous successful expression when you play the animation. This might lead you to believe your editing changes took effect.

Beyond the Basics Understanding unexpected attribute values When you play the animation, Ball moves along the X-axis with the increase in time. Ball’s X-axis position is 4 grid units, for example, when animation time equals 4 seconds. When you rewind the animation, Ball’s position along the X-axis doesn’t return to 0 as you might assume. The previous execution of the expression at time equals 4 set the $distance variable to 4. So rewinding sets Ball.tx to 4, then sets the value of $distance to 0, the value of time upon rewinding. If you rewind again, Ball’s position along the X-axis returns to 0 as desired. Because the previous execution of the expression upon rewinding set the $distance to 0, the expression now correctly sets Ball.tx to 0. To fix this problem, reverse the order of the statements and compile the expression: $distance = time; Ball.tx = $distance;

After you play and rewind the expression, the first statement executes and assigns the time to $distance. The next statement assigns Ball.tx the value of $distance, which the first statement set to the value of time. Because $distance is set to 0 as the first statement after rewinding, Ball returns to the desired translateX position.

Increment operations If you increment an attribute or variable during animation, you might be confused by its behavior.

Example Ball.ty = 0; Ball.ty = Ball.ty + 1;

Ball’s translateY position stays at 1 unit along the Y-axis. Ball’s translateY position doesn’t increase by 1 each frame as the animation plays.

Example Ball.ty = Ball.ty + 1;

Ball’s translateY position increases by 1 each frame as you play the animation. When you rewind the animation, translateY increases by 1 again.

142


Beyond the Basics Understanding unexpected attribute values When you play the animation again, the translateY position increases by 1 each frame. If you rewind the animation or drag the current time indicator, the translateY position continues to move up the Y-axis. The attribute never returns to its original position. To return Ball to a starting position each time you rewind, you must initialize the attribute to a starting value. For example, you could use the following expression: Ball.ty = Ball.ty + 1; if (frame == 1) Ball.translateY = 0;

This returns Ball to a Y position of 0 when you rewind to frame 1. When you drag the current time indicator, though, Ball doesn’t return to its Y position of 0. The if statement resets the value of translateY to 0 only when frame 1 plays. Frame 1 is the default frame that plays when you rewind an animation. You would need to use a different frame number in the if statement if you’ve set your animation to start at a different frame.

Data type conversions

The following topics describe the conversions that occur in such instances. Understanding these details might help you troubleshoot unexpected attribute and variable values. Unless you have programming experience, don’t intentionally convert data types. You might be confused by unexpected attribute and variable values.

Assigning to a floating point attribute or variable If you assign a vector to a floating point attribute or variable, Maya converts the vector to a floating point value according to this equation: 2

2

x +y +z

2

The x, y, and z numbers in the formula represent the three components in the vector. The resulting value is the magnitude of the vector.


143

Expressions

Maya is flexible in its handling of data types. If you do assignment or arithmetic operations between two different data types, Maya converts data type as necessary and doesn’t report a syntax error.

Beyond the Basics Understanding unexpected attribute values

Example Ball.scaleY = <<1,2,0>>;

Maya assigns the floating point scaleY attribute the converted vector: 2

2

2

1 +2 +0 =

5 = 2.236

If you assign an integer to a floating point attribute or variable, Maya makes no conversion. None is necessary.

Example Ball.scaleY = 1;

Maya assigns the value 1 to Ball.scaleY.

Assigning to an integer attribute or variable If you assign a floating point value to an integer attribute or variable, Maya deletes the decimal part of the number. If you assign a vector to an integer attribute or variable, Maya converts the vector to an integer using the square root equation in the previous topic. However, it deletes the decimal component of the result.

Example int $pi = 3.14;

Maya assigns the integer variable $pi the value 3. int $temp = <<1,2,0>>;

Maya assigns the integer variable $temp this vector value: 2

2

2

1 +2 +0 =

5 = 2.236 ≈ 2

It deletes the decimal component .2360607. The $temp variable receives the truncated value 2.

144



Assigning to a vector attribute or variable If you assign an integer or floating point value to a vector attribute or variable, Maya puts the integer or floating point value into each component of the vector.

Example vector $speed = 1.34;

Because $speed is a vector, Maya assigns it <<1.34,1.34,1.34>>.

Using mixed data types with arithmetic operators The following table lists how Maya converts data types when you use arithmetic operators between different types in an expression. Operation

Resulting data type

integer operator float

float

integer operator vector

vector

vector operator float

vector

Suppose you multiply a vector variable named $velocity by a floating point number 0.5 as follows: $race = $velocity * 0.5;

If $velocity is <<2,3,0>> when the preceding expression executes, the $race variable is assigned the resulting vector value <<1,1.5,0>>.


145

Expressions

Example


Important When Maya does arithmetic operations on literal constants and variables without a declared data type, it guesses the data type based on the values present. In the statement Ball.scaleY = 1/3;, for example, Maya treats 1 and 3 as integers because they have no decimal points. The expression divides integer 1 by integer 3. The integer result is 0 with a remainder of 1. Maya discards the remainder. Because Ball.scaleY is a floating point attribute, Maya converts the integer 0 result to floating point 0 (which is the same value), then assigns it to Ball.scaleY. To get the intended result of 1/3, you must type Ball.scaleY = 1.0/3.0; Maya treats 1.0 and 3.0 as floating point numbers because they have decimal points. The number 1.0 divided by 3.0 results in 0.33333333333.

146


8

Particle Expressions Particle expressions are more complex than other types of expressions. For example, you can write an expression to control all particles in an object the same way, or you can control each particle differently. Execution of expressions differs for particles than for other types of objects. To become proficient with particle expressions takes more study than for other expressions, but the resulting effects are worth the effort. This chapter guides you through the intricacies of working with particle expressions.

Claude Macri

This chapter has the following topics: •

“Understanding particle expressions” on page 148

•

“Understanding creation expression execution” on page 149

•

“Writing creation expressions” on page 150

•

“Understanding runtime expression execution” on page 152

•

“Writing runtime expressions” on page 153

•

“Working with particle attributes” on page 159

•

“Assigning to vectors and vector arrays” on page 193

•

“List of particle shape attributes” on page 196 Using Maya: Hypergraph, Sets & Expressions

147

Expressions

An expression moves a particle emitter in a corkscrew pattern and changes the color of trailing emitted particles as they age. The particles are displayed as Spheres render type.

Particle Expressions Understanding particle expressions

Understanding particle expressions If you select a particle shape node, the Creation and Runtime buttons in the Expression Editor are no longer dim and you can select them.

Buttons lit for particle shapes

These buttons let you write two types of expressions: creation and runtime. You can use both types for any attribute of a particle shape node. Though the details of execution are subtle, a creation expression generally executes when you rewind an animation or when a particle is emitted. A runtime expression typically executes for each frame other than the rewind frame or the frame in which a particle is emitted. By default, either type of expression executes once for each particle in the object. Creation and runtime expressions don’t execute at the same time. The age of each particle in the object determines whether a runtime expression or creation expression executes. Execution details are in “Understanding creation expression execution” on page 149 and “Understanding runtime expression execution” on page 152. The Default Object, Always Evaluate, and Convert Units options become dim when you select a particle shape node, and you can’t use them. Default Object is dim because a particle shape node’s attributes can be controlled by only one creation expression and one runtime expression. The particle shape node is always the default object when it’s the selected object. Always Evaluate is dim for particle shape node expressions because it has no effect on particle shape node expressions. See “How often an expression executes” in Chapter 7 for details on the checkbox.

148


Particle Expressions Understanding creation expression execution Convert Units is not selectable because you can’t alter how Maya handles unit conversions for particle shape node expressions. See “Speeding expression execution” on page 127 for details on how Maya converts units for other types of expressions.

Important You can’t write a different expression for each particle shape attribute as you can for other types of objects. Because you can write only one creation expression per particle shape, you don’t need to select an attribute from the Expression Editor’s Attributes list.

Understanding creation expression execution For a particle you create with the Particle Tool, a creation expression executes when you rewind the animation. For an emitted particle, a creation expression executes in the frame where the particle is emitted. However, there are exceptions to these rules as described in the following topics. Note that rewinding an animation two or more times in succession without playing the animation doesn’t execute a creation expression. Because no attribute value changes when you rewind several times in succession, the expression doesn’t execute.

Setting the dynamics start frame A creation expression executes once for each particle whose age is 0 when Maya evaluates dynamics. Maya evaluates dynamics whenever the animation time changes and it’s greater than or equal to the dynamics Start Frame setting—frame 1 by default. The Start Frame specifies when dynamic calculations begin for your animation. This option is available through Settings→Dynamics Controller, in the Extra Attributes section of the Attribute Editor. The animation time changes when you rewind, play, or otherwise change the current frame displayed. An emitted particle’s age is 0 in the frame where it’s emitted.


149

Expressions

You might also notice that all expressions in your scene are compiled and executed each time you open the scene. This occurs for architectural reasons and is unimportant to your work with expressions.

Particle Expressions Writing creation expressions Particles created with the Particle Tool have an age of 0 on and before the Start Frame. With the default animation frame range and Start Frame, rewinding an animation to frame 1 returns such particles to age 0. If you set the Time Slider’s start frame higher than the dynamics Start Frame, be aware that rewinding the animation might cause the age of particles to be greater than 0. If this occurs, the creation rule for the particles won’t execute.

Tip You can set options in the Attribute Editor to display the age of an object’s particles in the workspace. Set the particle shape’s Render Type to Numeric, click Add Attributes For Current Render Type, and enter age in the Attribute Name box. The age appears next to each particle. You can also examine the age of an object’s particles by entering print(age+“\n”) in a particle expression. See “print” on page 261.

Setting attributes for initial state usage If, at some frame, you’ve saved a particle shape’s attributes for its initial state, rewinding an animation does not return the age of the particles to 0. Suppose you’ve created a particle grid having an opacity attribute that fades gradually as the animation plays. You stop the animation at some frame where you decide the grid’s opacity looks good as a starting point for the animation. You then choose Settings→Initial State→Set For Current to cause the current value of the object’s attributes—including age—to become the initial state values. If you rewind the animation, the age of the particles in the grid is equal to age at the time you chose Set For Current. The age of the particles therefore is not equal to 0 when you rewind the scene. See “Understanding initial state attributes” on page 162 for more details on initial state attributes.

Writing creation expressions A creation expression is useful for attributes that don’t need to change during animation. For example, you might want all particles in an object to have a single velocity for the duration of an animation.

150


Particle Expressions Writing creation expressions A creation expression is also useful for initializing an attribute’s value for the first frame before a runtime expression takes control of the attribute value in subsequent frames. See “Writing runtime expressions” on page 153 for an example of the interaction between a runtime and creation expression.

Example Suppose you’ve used the Particle Tool to place a collection of particles in the workspace. You then create the following creation expression to control their velocity: particleShape1.velocity = <<0,1,0>>;

Important To use an expression to control particle attributes, make sure the selected object in the Expression Editor is a particle shape node, not the transform node of the particle object. If a particle object’s transform node is selected rather than the particle shape node, move the mouse pointer to the workspace and press your keyboard’s down arrow. This selects the particle shape node.


151

Expressions

All the particles move in a Y-axis direction at one grid unit per second as the animation plays.

Particle Expressions Understanding runtime expression execution

Understanding runtime expression execution For a particle you’ve created with the Particle Tool, a runtime expression typically executes in each frame after the frame that appears upon rewinding. For an emitted particle, a runtime expression typically executes in each frame after the first one where the particle was emitted. More specifically, a runtime expression executes once for each particle whose age is greater than 0, each time Maya evaluates dynamics. Maya evaluates dynamics whenever the Time Slider time changes and the time is greater than or equal to the dynamics Start Frame. To set the dynamics Start Frame, select Settings→Dynamics Controller from the Dynamics menu bar. Time changes when you rewind, play, or otherwise change the current frame displayed. A runtime expression executes once per oversample level per frame as you play or otherwise change the animation time. For example, if the oversample level is 4, Maya executes a particle shape expression four times per frame for each particle in the object. Use Settings→Dynamics Controller from the Dynamics menu to set the Oversample Level. Maya’s default setting is 1. In addition to executing when animation time changes, a runtime expression executes when the value of an attribute it reads changes, and when either of these actions occurs for an attribute the expression writes to: •

Some other node in Maya uses its value.

•

Maya needs the value to redraw the workspace contents. In this context, the predefined variables time and frame are also considered attributes the expression reads.

Important There are no creation expressions for nodes other than particle shape nodes. Such objects have only one type of expression. (It’s similar to a runtime expression.) For a particle shape node, you can write only one runtime expression for all its attributes. You don’t need to select an attribute from the Attributes list. You can create only one runtime expression per particle shape.

152


Particle Expressions Writing runtime expressions

Writing runtime expressions A runtime expression controls an attribute as an animation plays. Maya updates any attribute that’s assigned a value in a runtime expression each time the expression executes. This typically occurs once per frame. If an attribute is not set by a runtime expression, the attribute uses the creation expression value for subsequent frames of the animation.

Example Suppose you’ve created a grid of particles, then create this runtime expression for its velocity attribute: particleShape1.velocity = <<0,1,0>>;

The expression moves the grid of particles up at 1 grid unit per second as the animation plays.


Expressions

Constant upward velocity

153


Note To make the illustrations of particles easier to see in this and other chapters, we show them as small, shaded spheres rather than points.

To display particles as spheres: 1

Select the particle shape node.

2

In the Attribute Editor’s Render Attributes section, choose Spheres for the Render Type.

3

Click the Current Render Type button next to Add Attributes For. A Radius slider appears below the button.

4

Adjust the Radius to set the size of the spheres.

5

Turn on Shading→Smooth Shade All (at the upper left of the workspace).

With the default frame rate of 24 frames/second, the particles move 1/24 of a grid unit each frame. With the default oversampling level of 1, the runtime expression executes once per frame. Maya calculates the runtime expression once for each particle of an object. Because the expression sets the velocity to <<0,1,0>> each frame, the expression executes redundantly. This expression would therefore be more appropriate for a creation expression. However, either type of expression has the same effect in this example.

Example Suppose you’ve created a grid of particles, and your animation’s starting frame number is 0. You create this runtime expression for its velocity attribute: particleShape1.velocity = <<0,time,0>>;

The expression increases the Y component of velocity with the increasing value of time as the animation plays. This makes all particles in the grid rise with increasing velocity as the time increases. An increasing velocity is the same as acceleration.

154



Increasing upward velocity

You need to use the statement in a runtime expression rather than a creation expression, because you’re increasing a value in the assignment each frame. Using the statement in a creation expression would instead set the velocity to a constant value <<0,0,0>>, because time equals 0 when the creation expression executes for the particle grid.

Example The previous examples gave all particles the same value for the velocity attribute. You can instead give each particle a different value for an attribute. Expressions

Suppose you’ve created a grid of 121 particles.

Suppose further you create this runtime expression for its acceleration attribute: particleShape1.acceleration = sphrand(2);


155

Particle Expressions Writing runtime expressions The expression executes once for each of the 121 particles each time the runtime expression executes. The sphrand(2) function provides a vector whose randomly selected components reside within an imaginary sphere centered at the origin and with a radius of 2. Each particle receives a different vector value. For details on the sphrand function, see “sphrand” in Chapter 9. Because each particle receives a different random vector for its acceleration each frame, the particles accelerate individually in a constantly changing direction and rate as the scene plays. This gives the acceleration abrupt changes in direction.

Important To give particles a constant acceleration, assign the acceleration attribute a constant value in a runtime expression rather than in a creation expression. Maya simulates the physics of acceleration. It initializes acceleration to <<0,0,0>> before each frame, or if the oversample level is greater than 1, before each timestep. If the oversample level is 2, there are 2 timesteps per frame. If the oversample level is 3, there are 3 timesteps per frame, and so on.

156



Example Suppose you’ve set your animation’s starting frame to 0, and you’ve used the Particle Tool to place a single particle at the origin:

You then create a runtime expression to control its position: particleShape1.position = <<3,time,0>>;

When you play the animation, the runtime expression takes control of the attribute. In the first frame that plays, the particle jumps to <<3, time, 0>>. At the default frame rate of 24 frames/second, the position is <<3, 0.0417, 0>>, because the value of time is 0.0417. Expressions


157

Particle Expressions Writing runtime expressions Each subsequent frame moves the particle upward at a rate set by the incrementing value of time.

When you stop and rewind the animation, the particle moves back to the origin, the particle’s original position when you created it with the Particle Tool. When you created the particle, Maya stored its original position in an internally maintained initial state attribute named position0. For details, see “Understanding initial state attributes” on page 162. Because the attribute has no creation expression controlling its value, Maya sets the attribute to its initial state position0 value of <<0,0,0>>. To prevent the particle from jumping back to the origin after rewinding, you can write a creation expression that’s the same as the runtime expression: particleShape1.position = <<3,time,0>>;

When you rewind the animation, the particle moves to position <<3,time,0>>. Because time is 0 at frame 0, the particle starts at position <<3,0,0>> when you rewind the animation. In the second and following frames, it moves upward synchronized with the increasing value of time. Though this example showed how to initialize the position attribute with a creation expression, you could have gotten almost the same result by saving the object’s current attribute values for initial state usage:

To save the current attributes for initial state usage: 1

Select the particle shape node.

2

Advance the animation to frame 1. Here the position of the particle is <<3, 0.0417, 0>>.

3

158

Select Settings→Initial State→Set for Current.


Particle Expressions Working with particle attributes When you rewind the animation, Maya positions the object at the initial state setting of its position attribute. This setting is <<3, 0.0417, 0>> because you selected Set for Current while the position was equal to <<3, 0.0417, 0>>.

Working with particle attributes When you create a particle object, it has two types of static attributes: •

attributes for its transform node

•

attributes for its particle shape node These attribute are permanently part of a particle object. You typically won’t work with the static attributes that are part of its transform node, for example, scaleX, translateX, and so on. These attributes control the position and orientation of the transform node of the entire particle object, not the position and orientation of the individual particles. You’ll instead work with the static attributes of the particle shape node, for example, position, velocity, acceleration, and age. These attributes appear in the Attributes list of the Expression Editor’s when you choose Object Filter→Dynamics→Particles for the selected particle object.

Adding dynamic attributes

When you add a dynamic attribute to an object, the attribute names appear in the Expression Editor’s Attributes list.

Note See “List of particle shape attributes” on page 196 for attributes you can use with particle objects.


159

Expressions

You also use expressions to control dynamic and custom attributes you add to a particle shape node. See “Attributes” in Chapter 5 for details on the differences between static, dynamic, and custom attributes. See “Assigning to a custom attribute” on page 169 for details on working with custom attributes.

Particle Expressions Working with particle attributes

Understanding per particle and per object attributes You can dynamically add two types of attributes to a particle shape node: •

per particle

•

per object A per particle attribute lets you set the value of the attribute individually for each particle of the object. A per object attribute lets you set the attribute value for all particles of the object collectively with a single value. For example, a per particle opacityPP attribute lets you set a unique opacity value for each particle of an object. With a per object opacity attribute, you must give all particles of the object the same opacity. A per particle attribute holds the attribute values for each particle in the object. For example, though there is only one opacityPP attribute in a particle object, the attribute holds the value for each particle’s opacity value. The attribute holds the values in an array. In simple terms, an array is a list. Though per particle attributes are best for creating complex effects, you can’t keyframe them. You can keyframe per object attributes. You can add per particle or per object attributes for opacity, color, lifespan, and other effects. For a particle shape node attribute, you can tell whether it’s a per particle or per object attribute by examining the Attribute Editor’s particle shape folder. All per particle attributes appear in the Per Particle (Array) Attributes section of the folder. The per object attributes appear elsewhere in the folder. Most appear above the Per Particle (Array) Attributes section, for example, in the Particle Attributes and Render Attributes sections. For many dynamically added attributes, you can also tell whether they are per particle or per object by their names in the Expression Editor. If a name ends with PP, it’s per particle. Otherwise, it’s usually per object. Note that position, velocity, and acceleration are per particle attributes, though their names don’t end with PP.

160


Particle Expressions Working with particle attributes The most common way to create dynamic per object or per particle attributes for a particle shape is by clicking one of the following buttons in the Add Dynamic Attributes section of the Attribute Editor:

For example, if you click the Opacity button, a window appears and lets you choose whether to add the opacity characteristic as a per object attribute or a per particle attribute. If you choose per particle, the Attributes list of the Expression Editor displays a new attribute for the selected particle shape node: opacityPP. If you choose per object, an opacity attribute is displayed instead. For attributes other than lifespan, if you add both a per particle attribute and a per object attribute for a characteristic, the per particle attribute takes precedence. For instance, if you add opacity and opacityPP, the opacityPP attribute controls the opacity of the particles of the specified object. When you click Lifespan and add both a per particle and per object attribute, Maya adds an additional attribute named useLifspanPP that lets you choose whether lifespanPP or lifespan controls the characteristic.

If you click the Goal button in the Add Dynamic Attributes section of the Attribute Editor, Maya adds a per object attribute and a per particle attribute. The attributes are named goal and goalPP. Neither attribute has precedence. Maya multiplies the value of the per object goal attribute by the per particle goalPP attribute to create the final goal effect for each particle.


161

Expressions

By default, the Attribute Editor setting of useLifespanPP is on, so the lifespanPP attribute control the characteristic. If you turn useLifespanPP off in the Attribute Editor, lifespan controls the characteristic.


Important You can use per particle attributes only in particle expressions. You can use per object attributes in particle or nonparticle expressions. If you use a runtime expression to read or write a per object attribute of a particle object with many particles, you can speed up expression execution by reading or writing the attribute in a nonparticle expression. Nonparticle expressions execute only once per object. Particle expressions execute once for each particle in the object. Because reading or writing a per object attribute more than once per frame is redundant, you can save processing time by working with them in nonparticle expressions.

Understanding initial state attributes For all static per particle attributes, Maya keeps a corresponding attribute with a name ending in 0. For example, the static attributes position, velocity, and acceleration have counterparts position0, velocity0, and acceleration0. An attribute name that ends in 0 holds the initial state value of the attribute. When you save a particle object’s current attribute values for initial state usage, Maya assigns those values to the initial state attributes. To save a particle object’s attribute values for initial state usage, use either of these commands: •

Settings→Initial State→Set for Current This saves all per particle attribute values for the selected particle shape node or rigid body.

•

Settings→Initial State→Set for All Dynamic This saves all per particle attribute values for all dynamic objects in the scene—in other words, all particle shape nodes and rigid bodies. When you dynamically add a per particle attribute by clicking one of the buttons in the Add Dynamic Attributes section of the Attribute Editor, Maya also adds a corresponding initial state attribute with name ending in 0. For example, when you click the Lifespan button in the Attribute Editor, Maya adds lifespanPP0. Though an initial state attribute doesn’t appear in the Expression Editor, you can read its value to retrieve the initial state.

162


Particle Expressions Working with particle attributes When you use the Add Attribute window to add a custom per particle (array) attribute to a particle shape, you must choose whether you want to add it with Add Initial State Attribute on or off. If you choose on, Maya creates a corresponding initial state attribute for the added attribute. If you choose off, Maya doesn’t create a corresponding initial state attribute for the added attribute. Without this corresponding attribute, you can’t save a particle object’s current attribute values for initial state usage. You must write a creation expression if you decide to initialize the custom attribute’s value upon rewinding the animation.

Note A per particle attribute is called an array attribute in the Add Attribute window. The two terms have the same meaning. See “Assigning to a custom attribute” on page 169 for details. You can see whether a custom attribute was added with Add Initial State Attribute on or off by using the MEL listAttributes command. (See the online MEL documentation for details.)

When you add a custom attribute to a particle shape, do not end the name with a 0 character. You’ll subvert Maya’s naming scheme for the initial state attribute associated with an attribute. For any attribute, if you don’t initialize its value with a creation expression or save its value for initial state usage, Maya gives the attribute a default value at the animation’s first frame. It typically assigns the attribute the value 0 or <<0,0,0>>, as appropriate for the data type. In other cases, for instance, opacityPP and opacity, Maya assigns the attribute a default value of 1. If you know you’re going to write a creation expression for a custom attribute, you can set Add Initial State Attribute off when you add the attribute. Otherwise, set Add Initial State Attribute on whenever you add a custom attribute. When a creation expression assigns a value to an attribute, the value overrides the attribute’s initial state value for all particles whose age is 0.


163

Expressions

You might want to read the value of an initial state attribute in an expression, for instance, to use its original (rewind) value for some calculation. If you assign a value to an initial state attribute. Maya will overwrite the value if you save the attribute value for initial state usage.


Example of assigning to a dynamic per particle attribute Suppose you’ve used the Particle tool to create a small number of particles named Bubbles:

The following steps show how to assign a different lifespanPP value for each of the particles to make them disappear as the scene plays.

To use a per particle lifespanPP attribute: 1

Select the particle shape node for Bubbles in the Outliner or Hypergraph.

2

In the Add Dynamic Attributes section of the Attribute Editor, click the Lifespan button. A window appears that prompts you to choose whether to add the attribute per object or per particle.

3

Select Add Per Particle Attribute, then click the Add Attribute button. This adds a lifespanPP attribute to the particle shape node for Bubbles. You can set this attribute to give each particle a different value for how long it lives.

4

Select Bubble’s particle shape node in the Expression Editor.

5

Turn on Creation in the Expression Editor.

6

Create the following expression: BubblesShape1.lifespanPP = rand(5); print("Hello\n");

7

164

Play the animation.


Particle Expressions Working with particle attributes Because lifespanPP is a per particle attribute and the object’s particle shape node is selected in the Expression Editor, the expression does an execution loop of both statements once for each particle in the object. Because the expression is a creation expression, it executes after the expression compiles. It also executes when you rewind the animation after playing it. For each of the particles, the first statement assigns the lifespanPP attribute a random floating point number between 0 and 5. The rand function returns a different random number each time it executes, so each particle has a different lifespanPP value between 0 and 5. For details on the rand function, see chapter 9, “Functions.” The second statement displays Hello in the Script Editor, once for each particle. The creation expression gives each particle a random lifespanPP of less than 5 seconds. The particles disappear from the scene at random times between 0 and 5 seconds of scene play. (Maya gives particles created with the Particle tool an age of 0 in the first frame of the animation.)

Expressions

Particles remaining after three seconds, with creation expression for lifespanPP.

When you rewind the animation, the particles reappear in the scene. Playing the scene again makes them disappear at random times within 5 seconds.


165

Particle Expressions Working with particle attributes It’s important that you use a creation expression for this effect. If you were to use a runtime expression, the particles would disappear more quickly as the animation plays, and Hello would appear 100 times each frame. The reason for this is subtle: In each frame, a runtime expression would assign a different random value between 0 and 5 seconds to the lifespanPP of each particle. The expression would likely assign one or more of the particles a lifespanPP near 0. Meanwhile, the age of each particle increases from 0 at the first frame of play. Maya checks the age of each particle every frame. If the age is greater than the lifespanPP value, Maya removes the particle. Because the expression would reassign new random lifespanPP values to each remaining particle in each frame, the new assignments would likely give a few particles a lifespanPP that’s less than their current age value. Maya deletes such particles. This causes the object’s particles to disappear quickly from the scene.

Particles remaining after one second, with runtime expression for lifespanPP.

166



Example of assigning to a dynamic per object attribute Suppose you’ve used the Particle tool to create the same Bubbles particle object described in the previous topic.

The following steps show how to give the particles a single lifespan. All particles disappear at the same time when you play the scene.

To use a per object lifespan attribute: Select the particle shape node for Bubbles in the Outliner or Hypergraph.

2

In the Add Dynamic Attributes section of the Attribute Editor, click the Lifespan button. A window appears that prompts you to choose whether to add the attribute per object or per particle.

3

Select Add Per Object Attribute, then click the Add Attribute button. This adds the lifespan attribute to the particle shape node for Bubbles.

4

In the Expression Editor, turn on Creation.

5

Create this creation expression: BubblesShape1.lifespan = 1.33; print("Hello\n");

Because this is a creation expression, it executes after the expression compiles. It also executes when you rewind the animation after playing it.


167

Expressions

1

Particle Expressions Working with particle attributes For each of the 100 particles, the first statement assigns the lifespan attribute the value 1.33. The second statement displays Hello in the Script Editor once for each particle. 6

Play the animation. Because all particles have a lifespan of 1.33, they disappear from the scene after 1.33 seconds of animation play. When you rewind the animation, the particles reappear in the scene. Playing the scene again makes them disappear again after 1.33 seconds.

1.32 seconds of animation

1.33 seconds of animation

If you had put the preceding statements in a runtime expression, the particles would still disappear in 1.33 seconds. The expression would assign a lifespan of 1.33 seconds to all 100 particles redundantly each frame. The age of the particles is 0 in the first frame of their creation. (Maya gives particles created with the Particle tool an age of 0 in the first frame of the animation.) Reassigning lifespan 1.33 each frame has no effect on the age of the particles. Their age increases from the first frame regardless of the lifespan value. Note that, unlike particles created with the Particle Tool, the age of emitted particles starts in the frame where they’re emitted.

168



Assigning to a custom attribute You can add a custom attribute to a particle shape node and control its value in an expression.

To add a custom attribute: 1

Select the object’s particle shape node rather than its transform node. Use the Hypergraph or Outliner to select the shape node.

2

Choose Modify→Add Attribute. or In the Add Dynamic Attributes section of the Attribute Editor, click the General button. The Add Attribute window appears:

Expressions

3

Enter a name for the attribute in the Attribute Name box.


169

Particle Expressions Working with particle attributes 4

Make sure Make attribute keyable is on.

5

Select one of the following data types:

6

•

Vector

Creates a vector attribute consisting of three floating point values.

Float

Creates a floating point attribute.

Integer

Creates an integer attribute.

Boolean

Creates an attribute consisting of an on/off toggle.

Select one of the following attribute types: Scalar

Creates a per object attribute that you can set to a single value that applies to every particle in the object. A vector scalar is considered a single value with three numbers.

Array

Creates a per particle attribute. You can set this type of attribute to different values for each particle.

If you select Scalar, you can specify Minimum, Maximum, and Default values for a Float or Integer attribute. Minimum and Maximum set the lowest and highest values you can enter for the attribute in the Attribute Editor or Channel Box. Default sets the default value displayed for the attribute. Because you’re going to control the attribute’s value with an expression, you might want to skip entering values for these options. An expression isn’t bound by the Minimum and Maximum values. The attribute receives whatever value you assign it in the expression. The expression can read the attribute’s Default value or any other value you give it in the Attribute Editor or Channel Box. When you select Scalar, you can’t create a counterpart initial state attribute by turning on Add Initial State Attribute.

•

If you select Array, you can also create a counterpart initial state attribute by turning on Add Initial State Attribute. See “Understanding initial state attributes” on page 162 for details. You can’t set Minimum, Maximum, or Default values for an Array attribute.

7

170

Click Add if you want to add more attributes. Click OK to add the attribute and close the Add Attribute window.


Particle Expressions Working with particle attributes The new attribute appears under the Dynamic Attributes section of the Attribute Editor.

To assign values to a custom attribute: You can assign values to a custom attribute with the same techniques you use to assign values to static or dynamic attributes.

Example Suppose you’ve created a 100-particle object named sunspot, and you add to its particle shape node a vector per object attribute named glow. You assign the glow attribute a vector value in a creation expression as follows: sunspotShape1.glow = <<3,0,0>>; print(sunspotShape1.glow + "\n");

When you rewind the animation, the glow attribute of sunspotShape1 receives the value <<3,0,0>>. The print statement displays the values in the Script Editor.

Example

float $randomNumber = rand(1); sunspotShape1.heat = <<$randomNumber,0,0>>; print(sunspotShape1.heat + "\n");

When you rewind the animation, the expression loops through 100 executions, once for each particle. The first statement sets the $randomNumber variable to a random number between 0 and 1. The next statement assigns a vector to the heat attribute of a single particle. The left component of the vector assigned to heat is a different random number each time the statement executes. The middle and right components are always 0. One particle might have the value <<0.57, 0, 0>>, another <<0.32, 0, 0>>, another <<0.98, 0, 0>>, and so on. The print statement displays the values in the Script Editor.


171

Expressions

Suppose you add a vector per particle attribute named heat to the 100particle sunspot shape node. You can give each particle a different value as in this creation expression:


Note If you add a custom vector attribute to an object, Maya displays the attribute in the Attribute Editor, but you can’t enter its value there. You must enter a value for it in an expression or with the Component Editor available from the Attribute Editor.

Assigning to a particle array attribute of different length You can assign the array attribute of one particle shape node to another node having a different number of particles. The assignment is affected by which node you select in the Object Selection list in the Expression Editor. The number of particles in the selected particle shape node sets the number of statement executions, and, therefore, affects the assignment.

Example Suppose your scene contains an object named ThreePts made of three particles and an object named TwoPts made of two particles. The three particles in ThreePts are at these positions: -5 0 0 -4 0 0 -3 0 0

The two particles in TwoPts are at these positions: 5 0 0 6 0 0

172


Particle Expressions Working with particle attributes Suppose you write this runtime expression with TwoPtsShape2 selected in the Object Selection list: ThreePtsShape1.position = TwoPtsShape2.position; print(ThreePtsShape1.position+"\n");

In the first frame of runtime expression execution, this assigns the position attribute of TwoPts to the position attribute of ThreePts. In other words, the expression repositions the three particles to the position of the two particles. Because you selected TwoPtsShape1 in the Object Selection list, the expression will execute once for each of its two particles. When you play the scene, the runtime expression executes. The first particle of ThreePts now is at the position of the first particle of TwoPts. The second particle of ThreePts is at the position of the second particle of TwoPts. The third particle of ThreePts doesn’t change position. The expression executes only twice each frame. In summary, the particles in ThreePts are at these positions: 5 0 0 6 0 0 -3 0 0

Expressions

Suppose you write the preceding runtime expression instead with ThreePtsShape1 selected in the Object Selection list. Again, the expression repositions the three particles to the position of the two particles. Because you selected ThreePtsShape1 in the Object Selection list, the expression executes once for each of its three particles. When you play the scene, the runtime expression executes. The first particle of ThreePts moves to the position of the first particle of TwoPts. The second particle of ThreePts moves to the position of the second particle of TwoPts. Using Maya: Hypergraph, Sets & Expressions

173

Particle Expressions Working with particle attributes Because TwoPts lacks a third particle, the third particle of ThreePts is repositioned at the position of the first particle of TwoPts. You can’t see this in the workspace because the particles are in the exact same position. The three particles in ThreePts are at these positions: 5 0 0 6 0 0 5 0 0

As this example shows, the assignment statement executes three times. When it runs out of counterpart particles to assign to, it loops around and assigns to the previous particles. It starts with the first particle in the object, and continues through the other particles. For example, suppose you create a five-particle object named FivePts with the Particle Tool, and position the particles somewhere in the workspace. Suppose further you select the particle shape node of FivePts in the Expression Editor, then make this assignment in a runtime expression: FivePtsShape1.position = TwoPtsShape2.position;

The five particles move to these positions as soon as the runtime expression executes for the first time: 5 6 5 6 5

0 0 0 0 0

0 0 0 0 0

Using creation expression values in a runtime expression A runtime expression can’t read a variable you’ve defined in a creation expression unless you define the variable as global. However, you can create a custom attribute, assign it a value in a creation expression, then read or write its value in a runtime expression.

174


Particle Expressions Working with particle attributes For example, suppose you assign a particle object’s position to a variable named $oldposition in a creation expression: vector $oldposition = particleShape1.position;

The runtime expression for the same particle shape node can’t read the contents of the $oldposition variable. To solve this problem, you can create an attribute for the object, assign it a value in the creation expression, then use the attribute value in a runtime expression. For example, suppose you create an attribute named oldpos, and assign it the following position in a creation expression: particleShape1.oldpos = particleShape1.position;

You can read the value of particleShape1.oldpos in a runtime expression. Note that you don’t need to create an attribute to hold the object’s initial position. The initial position already exists in its initial state attribute named position0. This attribute doesn’t appear in the Expression Editor’s Attributes List.

Working with position, velocity, and acceleration

Unless you have a solid grasp of physics, avoid setting a combination of the position, velocity, and acceleration attributes. To give a smooth, random motion to particles with a runtime expression, use a random number function such as sphrand to assign random numbers to the particle shape’s acceleration attribute. A change in acceleration always gives smooth motion no matter how abruptly its value changes. To give a jittery random motion to particles with a runtime expression, use a random number function such as sphrand to assign random numbers to the particle shape’s velocity or position attributes. See “Random number functions” on page 239 for details on how to use random number functions. If an expression and a dynamic field control an object’s position, velocity, or acceleration, Maya calculates the expression’s effect first, then adds the field’s effect.


175

Expressions

To create various types of particle motion, you can assign vector values to the position, velocity, or acceleration attribute. See “Writing runtime expressions” on page 153 for examples of working with these attributes.


Example Suppose a particle drops under the influence of a gravity field with default gravity options. Gravity accelerates the particle at 9.8 units per second per second down the Y-axis. In other words, the default acceleration of gravity is <<0,-9.8,0>>. Suppose further you write the following runtime expression for the particle: velocity = velocity + <<1,0,0>>;

As each frame plays, Maya first calculates the particle’s velocity from the expression statement. The velocity increases 1 unit per second in an X-axis direction. Maya then adds the gravitational acceleration to the velocity. Maya uses the combined result to compute the particle’s position. Of course, you won’t see this calculation process. The frame displays the particle in the appropriate position after all computation. Note that the expression adds the constant <<1,0,0>> to the particle’s velocity each frame as the animation plays. This makes the particle move with increasing velocity in an X direction as the time increases. An increasing velocity is the same as acceleration. The ball represents the particle’s position after several frames. The white squares represent the particle’s position as time increases.

Gravity alone

176


Gravity in combination with velocity = velocity + <<1,0,0>>

Particle Expressions Working with particle attributes The acceleration attribute works differently than the position or velocity attributes in an important way. Maya initializes its value to <<0,0,0>> before each frame. If the oversample level is greater than 1, this initialization occurs before each timestep.

Example Suppose you write the following runtime expression for a five-particle object unaffected by gravity: acceleration = acceleration + <<0,1,0>>;

Rather than adding <<0,1,0>> to the acceleration value each frame, acceleration remains a constant <<0,1,0>> for each of the particles. This happens because Maya initializes the value of acceleration to <<0,0,0>> before each frame. Suppose you connect the particle object to gravity with default settings. The acceleration of the particle becomes <<0,1,0>> plus <<0,-9.8,0>>, which equals <<0,-8.8,0>>. The acceleration assigned in the expression slows the downward acceleration of the gravity. Suppose you change the previous expression to this: acceleration = acceleration + sphrand(3);

acceleration = sphrand(3);

As each frame plays, Maya first calculates each particle’s acceleration from the expression statement. Each particle receives the result of the sphrand(3) function. The sphrand(3) function provides a vector whose randomly selected components reside within a spherical region centered at the origin with radius 3. Each particle receives a different vector value. Finally, Maya adds gravity’s acceleration to the expression acceleration resulting from sphrand(3). The frame displays each particle in the resulting position. Because of the random values resulting from the expression, each particle has an acceleration that differs slightly from gravity in direction and magnitude. Because the sphrand(3) function executes for each particle each frame, the acceleration of each particle varies each frame.


177

Expressions

Because Maya sets acceleration to <<0,0,0>> before each frame, the statement has the same result as the following statement:


Gravity in combination with acceleration = sphrand(3)

Position at rewind

Gravity alone (shown for comparison) Position after one second

This example shows that you can take advantage of the additive effect of fields and the acceleration attribute to create custom field effects.

Tip You can turn off the effect of all fields on a particle shape node attribute by setting its dynamicsWeight attribute to 0.

Working with color Coloring particles is a fundamental task for expression writers. As the techniques for coloring particles are easiest to learn by example, we provide the following lesson.

Example Suppose you’ve used the Particle tool to create a randomly positioned collection of particles named Bubbles. You can use a creation expression to give the particles a constant color during animation play.

178



To give the particles a constant color: 1

Select the particle shape node for Bubbles in the Hypergraph or Outliner. To write an expression to color particles, you must dynamically add the attribute that lets you color them.

2

In the Add Dynamic Attributes section of the Attribute Editor, click the Color button.

3

Select Add Per Particle Attribute, then click the Add Attribute button. This adds an rgbPP attribute to the particle shape node for Bubbles. Because you’re adding this attribute as a per particle attribute, you can give each particle a different color.

4

Choose Shading→Smooth Shade All. This step is necessary to make the correct particle color appear when you assign the rgbPP attribute a value in an expression.

5

In the Expression Editor, select Bubble’s particle shape node.

6

Turn on Creation in the Expression Editor. Because you’ll give the particles a color that doesn’t change during the animation, you use a creation expression.

7

Enter this expression: BubblesShape1.rgbPP = <<1,0,0>>;


179

Expressions

A window appears that prompts you to choose whether to add the attribute per object, per particle, or connected to a shader.

Particle Expressions Working with particle attributes When you click the Create button in the Expression Editor, Maya checks the syntax of the expression. Assuming you made no typing errors, the expression executes once for each of the 100 particles. The expression colors all particles in the object red. The double angle brackets << and >> enclose a vector that sets the red, green, and blue components of the rgbPP attribute to 1, 0, and 0. In the RGB color scheme, this gives the object a red color. 8

Play the animation. Because the expression is a creation expression, it executes when you rewind the animation. The particles remain red for entire animation because the red color is never changed by a runtime expression.

Tip See the online version of this documentation for colored illustrations. The following steps show how to give the particles a randomly changing color as the animation plays.

To give the particles a randomly changing color: 1

With BubblesShape1 selected in the Expression Editor, turn on Runtime.

2

Enter this runtime expression: BubblesShape1.rgbPP = sphrand(1);

180

3

Click the Create button to compile the expression.

4

Rewind the animation.


Particle Expressions Working with particle attributes Because the creation expression executes when you rewind, the particles are red. 5

Play the animation. The runtime expression takes control of the rgbPP attribute. Because rgbPP is a per particle attribute, the runtime expression executes for each particle in the object each frame. For each particle, the expression assigns the rgbPP attribute the output from the execution of the sphrand function with an argument of 1. The sphrand function assigns each particle’s rgbPP color a random vector. The vector represents a random point in a spherical region of radius 1. The left, middle, and right rgbPP color components have a value no less than -1 and no greater than 1. Values less than 0 are treated as 0. The sphrand function returns a different random vector each execution. So each particle has a different random rgbPP value, and therefore, a different color. The color of each particle changes each frame.

You can slow the change of colors to create a flashing Christmas light effect. The following steps make the particles change colors every second of animation.

To slow the change of color: 1

Change the runtime expression to this: if ((frame % 24) == 0) BubblesShape1.rgbPP = sphrand(1);


181

Expressions

If you rendered the animation and played it back at 24 frames per second, you would have trouble seeing the color of a particle in any instant because the color changes so quickly.

Particle Expressions Working with particle attributes This expression uses the modulus operator (%) to control when the rgbPP attribute of the particles receives a random color. The modulus operator returns the remainder after division. For example, 24 divided by 24 returns 0, but 25 divided by 24 returns 1. (Dividing 25 by 24 equals 1 with a remainder of 1.) If the value of frame divided by 24 is equal to any number with a remainder of 0, the assignment to BubblesShape1.rgbPP occurs. In other words, the assignment occurs when frame equals 24, 48, 72, and so on. At an animation rate of 24 frames/second, the assignment happens once each second.

Important Avoid using the modulus operator with floating point values. Because of number rounding in floating point division, you won’t likely get a return value of exactly 0 with the modulus operator. Instead use an integer value when possible. 2

Rewind and play the animation. When you rewind the animation, the particles turn red because the creation expression executes. When the animation plays, the particles receive a random color once each second.

1 second

2 seconds

3 seconds

Note that you can change the red rewind color to random colors by changing the creation expression to this: BubblesShape1.rgbPP = sphrand(1);

This is the same expression as the runtime expression.

182



Working with emitted particles If you make an object emit particles, you can write a creation or runtime expression for attributes of the emitted particles. For example, you can assign the emitted particles a value for lifespan, opacity, and color.

To write an expression for emitted particles: 1

Create the emitter.

2

Add the desired dynamic attribute to the emitter shape node.

3

Select the shape node of the emitted particles in the Expression Editor, then write the expression to control the attribute.

Example Suppose you’ve created an emitter and added a per particle lifespanPP attribute to it. The following creation expression gives the emitted particles a lifespan of 2 seconds: particleShape1.lifespanPP = 2;

Each particle disappears two seconds after it’s emitted.

Important Expressions

Avoid assigning a per particle attribute to another object’s per particle attribute if the particles of either object die. As particles die, the order of expression evaluation changes for the object’s particles. This causes unexpected results. You can, though, assign from one attribute to another in the same object with dying particles. The array indexes of the different attributes are in synch with each other. For example, don’t write an expression like this: emittedShape1.lifespanPP = 2; emittedShape1.rgbPP = otherParticleShape2.rgbPP

Working with collisions If you make a particle object collide with an object, you can write an expression to trigger expression statements after the collision. For example, you can change the color, opacity, or lifespan of the colliding particles.


183


To prepare for writing the expression: 1

Select the particle shape node of the particles in the Outliner or Hypergraph.

2

Select Settings→Particle Collision Events from the Dynamics menu bar. The Particle Events window appears.

3

Click Create Event. This adds an event attribute to the selected particle shape node. The Expression Editor displays the added event attribute in the Attributes list. Close the Particle Events window.

To write the expression: 1

Select the particle shape node of the emitted particles.

2

Write the runtime or creation expression using the value of any of these attributes of the emitted particle’s shape node: Long name

Short name

event

Description

Data Type

Contains the number of times each particle in the object has hit something (on a per particle basis).

float array

eventCount

evc

Total number of events that have occurred for all particles of the object.

integer

eventTest

evt

True if an event has occurred since the last time an expression or MEL getAttr command read the eventTest value.

boolean

The eventCount and eventTest are static attributes. A particle shape node has them as soon as you create the particle object. Though they don’t appear in the Expression Editor, you can use their values in an expression. You must first create the event attribute as described previously.

184



Example Suppose you’ve created a five-particle object named Peas that falls with gravity and collides with a plane.

You can make the particles turn red when the first particle hits the plane. Select PeasShape1 in the Outliner or Hypergraph.

2

From the Dynamics menu bar, select Settings→Particle Collision Events.

3

In the Particle Events window, click Create Event, then close the window. This adds an event attribute to PeasShape1.

4

In the Add Dynamic Attributes section of the Attribute Editor, click Color. The Particle Color window appears.

5

Select Add Per Particle Attribute, then click Add Attribute. This adds a per particle attribute named rgbPP. This attribute controls the red, green, and blue color scheme of each particle. The particles turn black after you add the rgbPP attribute. Adding the rgbPP attribute turns off the default coloring of the particles and gives them a value of <<0,0,0>>.

6

Choose Shading→Smooth Shade All. This step is necessary to make the correct particle color appear when you assign the rgbPP attribute a value in an expression.


185

Expressions

1


With PeasShape1 selected in the Expression Editor, create this runtime expression: if (event == 1) rgbPP = <<1,0,0>>; else if (event == 2) rgbPP = <<0,1,0>>; else if (event >= 3) rgbPP = <<0,0,1>>; else rgbPP = <<1,1,1>>;

8

Rewind the animation. Upon rewind, the particles are black. The particles have the default black rgbPP color because no creation expression exists for the object.

9

Play the animation. The particles fall toward the plane. The runtime expression executes as each frame plays. The event attribute is a per particle attribute. This isn’t obvious because its name doesn’t have PP as the last two characters. Because event holds a running count of collisions for each particle, event contains 0 for each particle until the first collision with the plane. Until the first collision occurs, the final else statement executes: else rgbPP = <<1,1,1>>;

This statement executes because event doesn’t equal 1, 2, 3, or a number greater than 3. The vector <<1,1,1>> in the RGB color scheme represents the color white. When the first particle of PeaShape1 hits the plane, Maya sets the event attribute for that particle to 1. This triggers execution of the first assignment, which sets the colliding particle’s rgbPP value to <<1,0,0>>. In the RGB color scheme, this vector value represents red. (When red equals 1, green equals 0, and blue equals 0, the resulting color is red.)

186



Red particle after collision

Tip See the online version of this documentation for colored illustrations. Note that the value of the event attribute reflects the collision count in the frame after each collision. For example, if a particle collides with the plane in frame 10, event is updated in frame 11. Expressions

When the other particles hit the plane for the first time, they also turn red after they collide.

A particle stays red until it collides with the plane for the second time, when event equals 2. After a second collision, the particle turns green.


187

Particle Expressions Working with particle attributes After a third collision, when event is equal to or greater than 3, a particle turns blue. Each particle stays blue for all subsequent collisions as the animation plays.

10 Rewind the animation. The particles turn black again because they receive the default rgbPP value <<0,0,0>>. When you play the animation again, the particles turn white, red, green, and blue in the same sequence as before. You can refine the animation by giving the particles a color other than black for the frame that appears upon rewinding. For example, you can give the particles a white color upon rewinding with two techniques: •

Write this creation rule for PeasShape1: rgbPP = <<1,1,1>>;

This statement executes for each particle in the object, so they all receive the same white color when you rewind the scene. •

Select PeasShape1, rewind the animation, and play one frame.

11 Choose Settings→Initial State→Set for Current. This saves all PeasShape1 attribute values from the current frame for the initial state of the attributes. The current value for rgbPP will be used when you rewind the animation. Because you played the second frame of the animation, this saves the white color of the particles at that frame for use upon rewinding the animation.

188


Particle Expressions Working with particle attributes Note that Set for Current saves all attribute values, including position, velocity, acceleration, and so on. In cases where you have several changing attribute values during playback, Set for Current might save undesired attribute values in addition to the desired ones. In such cases, use a creation expression.

Working with specific particles A per particle attribute holds the attribute values for each of an object’s particles. For example, the rgbPP attribute holds the value for each particle’s rgbPP value. Each particle has a unique numerical particle identifier. A particle’s identifier is stored in a per particle particleId attribute for the particle object. As you create the particles of a particle object, Maya assigns each particle a particleId in sequential order starting at 0. For example, suppose you use the Particle tool to create a five-particle object by clicking positions in the workspace. The first click of the mouse creates a particle with particleId 0, the second click creates a particle with particleId 1, the third click creates a particle with particleId 2, and so on.

You can assign per particle attribute values to specific particles using the particleId attribute.

Example Suppose you’ve used the Particle tool to create a grid of eight particles named ColorGrid. In the Attribute Editor, you’ve set the Render Type of the particles to Spheres. You’ve chosen Shading→Smooth Shade All to display the particles with shading.


189

Expressions

When an emitter emits particles, Maya assigns particleId numbers in sequential order starting with the first particle emitted. The first emitted particle has particleId 0, the second has particleId 1, the third has particleId 2, and so on.


You can give the particles different colors based on their particleId.

To color the particles based on particleId: 1

Select the ColorGrid.

2

In the Add Dynamic Attributes section of the Attribute Editor, click Color. The Particle Color window appears.

3

Select Add Per Particle Attribute, then click Add Attribute. This adds a per particle attribute named rgbPP, which controls the red, green, and blue color scheme of each particle. The particles turn black after you add the rgbPP attribute. Adding the rgbPP attribute turns off the default coloring of the particles and gives them a value of <<0,0,0>>.

4

In the Attribute Editor, choose Numeric from the Render Type menu. The particleId of each particle is displayed instead of spheres:

190



With ColorGridShape1 selected in the Expression Editor, enter this creation expression: if (particleId <= 2) rgbPP = <<1,0,0>>; else if ((particleId > 2) && (particleId < 6)) rgbPP = <<1,1,1>>; else rgbPP = <<0,0,1>>;

The creation expression executes whenever you rewind the animation. The particles don’t show the color assignments yet. The Numeric particle render type ignores color assignments to rgbPP. 6

In the Attribute Editor, set Render Type of the particles to Spheres again. The left, middle, and right columns of particles are red, white, and blue:

Expressions

The expression’s first statement assigns a red color to all particles whose particleId is less than or equal to 2. The value <<1,0,0>> is red in the RGB color scheme. The second statement assigns a white color to all particles whose particleId is greater than 2 and less than 6. The value <<1,1,1>> is white in the RGB color scheme. The third statement assigns a blue color to all particles that don’t meet the conditions in the prior two statements. In other words, all particles whose particleId is greater than or equal to 6 become blue. The value <<0,0,1>> is blue in the RGB color scheme. The following steps show another common way to control an attribute based on the particleId attribute.


191


To color half the particles red, and half the particles blue: 1

Enter the following runtime expression: if ((particleId % 2) == 0) rgbPP = <<1,0,0>>; else rgbPP = <<0,0,1>>;

2

Play the scene. The runtime expression executes each frame as the animation plays. Half the particles are blue, half are red.

The first statement uses a modulus operator (%) to calculate the remainder of dividing a particleId by 2. It then compares the remainder to 0. If the remainder equals 0, the statement assigns the particle a red color. The value <<1,0,0>> is red. The second statement assigns a particle a blue color if the remainder of the modulus operation doesn’t equal 0. The value <<0,0,1>> is blue. For example, dividing particleId 0 by 2 equals 0 with remainder 0. Because the remainder is 0, the particle having particleId 0 receives a red color. Dividing particleId 1 by 2 equals 0 with remainder 1. Because the remainder is 1, the particle having particleId 1 receives a blue color. Dividing particleId 2 by 2 equals 1 with remainder 0. With remainder 0, the particle having particleId 1 receives a blue color. The expression executes for each particle in the object. The result is that even-numbered particleIds become red, odd numbered particles become blue. 3

192

Rewind the animation.


Particle Expressions Assigning to vectors and vector arrays The creation expression executes. The particles become red, white, and blue as described for the previous expression. 4

Play the animation. The runtime expression executes each frame. The particles are red and blue as the animation plays.

Note to programmers You cannot assign values to individual particles with the array index notation commonly used in programming languages. For example, suppose you’ve created an opacityPP attribute for an object made of three particles. You can’t assign values as in this example: opacityPP[0] = 0.3; opacityPP[1] = 0.5; opacityPP[2] = 1;

Assigning to vectors and vector arrays

Expressions

Previous topics in this chapter show general techniques for working with vector array attributes. Vector array attributes are also called per particle attributes. Subtle details of assigning to vector and vector array attributes and variables follow.

Assigning to a vector variable You can assign a literal vector value or another vector variable to a vector variable. Enclose a literal vector value in double angle brackets.

Examples vector $top_velocity = <<2,2,5>>;

This assigns the vector $top_velocity the value <<2,2,5>>. vector $temp; vector $temp = $top_velocity;

This assigns the value of vector variable $top_velocity to the vector variable $temp.


193

Particle Expressions Assigning to vectors and vector arrays

Using the vector component operator with variables You can use a vector component operator (.) to read a component of a vector variable or vector array variable. Format

Meaning

$variable.x

left component

$variable.y

middle component

$variable.z

right component

Examples float $temp; vector $myvector = <<1,2,3>>; float $temp = $myvector.z;

This assigns the right component of $myvector, 3, to the floating point variable $temp. Suppose you have a vector initialized as follows: vector $myvector = <<1,2,3>>;

To replace the right component of $myvector, 3, with a new value such as 7, use this technique to preserve the other two components: $myvector = <<$myvector.x,$myvector.y,7>>;

This statement is incorrect: $myvector.z = 3;

An error occurs. A statement can read, but not directly assign, a component of a vector variable.

Assigning to a vector array attribute component An expression can neither read nor assign a component of a vector or vector array attribute. The following example shows a technique for working around this limitation. For details on working with color attributes, see “Working with color” on page 178.

194


Particle Expressions Assigning to vectors and vector arrays

Example Suppose you have 100-particle Cloud of randomly positioned particles. CloudShape1.position = sphrand(1); vector $pos = CloudShape1.position; CloudShape1.rgbPP = <<0,$pos.y,0>>;

The three statements execute once for each particle in Cloud. The first statement gives a particle a random position within a spherical region of radius 1. The sphrand(1) function gives the X, Y, and Z position components a value no less than -1 and no greater than 1. The second statement assigns a particle’s position to a vector variable $pos. The third statement assigns an RGB color to a particle’s rgbPP attribute. The left, middle, and right vector components of CloudShape1.rgbPP represent red, green, and blue components of the RGB color scheme. The third statement therefore assigns 0 (no color) to the red and blue components of a particle’s colorRGB. It gives a particle’s green component the value of its Y coordinate position.

This colors the particles from black to green, depending on the position.

Increasingly green


195

Expressions

Because a value of 0 or less results in a 0 green value, a particle is black if it’s below the XZ plane. If a particle’s Y coordinate position is above the XZ plane, it has a green component varying from nearly 0 to a fully saturated green.

Particle Expressions List of particle shape attributes

Example particleShape1.rgbPP = <<1,0,CloudShape1.position.z>>;

This causes an error. Maya interprets CloudShape1.position.z as being an attribute named z of an object named CloudShape1.position. You can get the intended result with these statements: vector $temp = CloudShape1.position; particleShape1.rgbPP = <<1,0,$temp.z>>;

The first statement reads all three components of vector attribute CloudShape1.position and assigns it to the vector variable $temp. The second statement reads the value of the right component of $temp, which contains the right component of CloudShape1.position. It then assigns this component to the right component of particleShape1.rgbPP.

Example particleShape1.rgbPP.y = 1;

This also causes an error. You can’t assign a value to a vector array attribute component.

List of particle shape attributes The following table describes the static and dynamic attributes that affect particle shape nodes on a per object or per particle basis. Note the following: •

Attributes you can read but not write are marked with an asterisk (*).

•

Empty boxes in the Render Type column indicate the render type is irrelevant to the attribute’s usage.

•

You can read and write per particle attributes only in particle expressions. See Using Maya: Dynamics for more details on the attributes.

196



Description

Per particle

Render Type

acceleration (acc)

Sets acceleration.

yes

vector array

age* (ag)

Contains number of seconds each particle has existed in scene.

yes

float array

attributeName

Specifies name of attribute whose value is displayed at particle positions.

Numeric

string

betterIllumination

Toggles increased self shadowing.

Cloud

boolean

colorAccum

Toggles additive display effect for RGB and opacity of overlapping particles for this object.

MultiPoint MultiStreak Points Streak

boolean

colorBlue

Sets blue component of RGB color.

float

colorGreen

Sets green component of RGB color.

colorRed

Sets red component of RGB color.

Blobby Surface Cloud MultiPoint MultiStreak Points Sphere Sprite Streak

conserve (con)

Sets amount of momentum conservation.

float

count* (cnt)

Contains number of particles in object.

integer

depthSort (ds)

Toggles depth sorting of particles for rendering.

MultiPoint MultiStreak Points Streak Sprite

Data Type

Expressions

Attribute long name (and short name)

float float

boolean


197



Description

dynamicsWeight (dw)

Scales effect of dynamic fields and collisions on particle object.

event*

Contains number of times each particle in the object has hit something.

eventCount* (evc)

Contains total number of events that have occurred for all particles of the object.

integer

eventTest* (evt)

Contains 1 if an event has occurred on the object since last time an expression or MEL getAttr command read the eventTest value.

boolean

goalPP

Sets how much the particles try to follow goal on a per particle basis.

goalWeight (gw)

Sets how much the particles try to follow goal.

incandescence

Sets glow color.

incandescencePP

Sets glow color.

inheritFactor (inh)

Sets fraction of velocity inherited from the emitter of this particle object.

float

isDynamic (isd)

Toggles dynamics for object.

boolean

lifespan

Sets when all particles die.

float

198


Per particle

Render Type

Data Type

float

yes

float array

yes

float array

float

yes

Cloud

vector

Cloud

vector array



Description

Per particle

Render Type

Data Type

lifespanPP

Sets when particles die on a per particle basis.

yes

levelOfDetail (lod)

Scales number of particles that can be emitted into the object.

lineWidth

Sets width of particle.

mass

Specifies physical mass of particles. As mass increases, the effect of dynamic forces change.

maxCount (mxc)

Sets maximum number of particles that can be emitted into this object.

multiCount

Sets number of points you want displayed for each particle. This number applies to each particle in the object.

MultiPoint Point

float

multiRadius

Sets radius of spherical region in which particles are randomly distributed.

MultiPoint MultiStreak

float

normalDir

Sets direction of normal for particles. Used with useLighting.

MultiPoint MultiStreak Points Streak

integer (1-3)

float array float

MultiStreak Streak yes

float float array

integer

Expressions


199



Description

opacity

Sets amount of transparency.

opacityPP

Sets amount of transparency.

particleId* (id)

Contains id number of each particle.

pointSize

Sets size of particle points.

position (pos)

Sets position.

radius

Sets radius size of all particles.

Blobby Surface Cloud Sphere

float

radius0

Sets starting point radius for tube render type.

Tube

float

radius1

Sets ending point radius for tube render type.

Tube

float

radiusPP

Sets radius size on a per particle basis.

Blobby Surface Cloud Sphere

float array

200


Per particle

Render Type

Data Type

MultiPoint MultiStreak Points Streak Sphere Blobby Surface Cloud Sprite

float

yes

MultiPoint MultiStreak Points Streak Sphere Cloud Sprite

float array

yes

Numeric

float array

MultiPoint, Numeric Points

float

yes

yes

vector array



Description

primaryVisibility (rea)

Per particle

Data Type

Toggles whether surface will be rendered by software renderer.

Cloud Blobby Surface Tube

boolean

visibleInReflections (rrl)

Toggles whether object is visible in reflections.


boolean

visibleInRefractions (rrr)

Toggles whether object is visible in refractions.


boolean

castsShadows (rsh)

Toggles whether object casts shadows.


boolean

rgbPP

Sets color.

MultiPoint MultiStreak Points Sphere Sprite Streak

vector array

selectedOnly

Toggles display of id numbers for selected particles.

Numeric

boolean

spriteNum

Sets image number index for image sequence.

Sprite

integer

spriteNumPP

Sets image number index for image sequence.

Sprite

integer array

spriteScaleX

Sets X-axis image scale.

Sprite

float

spriteScaleXPP

Sets X-axis image scale.

Sprite

float array

spriteScaleY

Sets Y-axis image scale.

Sprite

float

spriteScaleYPP

Sets Y-axis image scale.

Sprite

float array

spriteTwist

Sets image’s rotation angle.

Sprite

float

yes

yes

yes

yes


Expressions

Render Type

201



Description

Per particle

Render Type

Data Type

spriteTwistPP

Sets image’s rotation angle.

yes

Sprite

float array

surfaceShading

Sets level of shading.

Cloud

float

tailFade

Sets opacity of tail fade.

MultiStreak Streak

float

tailSize

Sets length of tail.

MultiStreak Streak Tube

float

threshold

Sets distance between particles at which lofting occurs.

Blobby Surface Cloud

float

useLighting

Toggles whether scene lighting lights up particles.

MultiPoint MultiStreak Points Sprite Streak

boolean

velocity (vel)

Sets velocity.

202


yes

vector array

9

Functions In expressions, you can use built-in operations called functions to create motion, particle positioning, and other effects. This chapter explains the most commonly used functions and a few useful MEL commands. A hermite function shaped the lightning’s soft body geometry, dnoise animated its turbulence, and sphrand enhanced its randomness.

Expressions

Matt Baer

For easy reference, this chapter groups functions by their purpose. For example, all math functions are grouped in the same section. To find out about:

See page:

Understanding functions

205

Function syntax

206

Limit functions abs ceil floor clamp min max sign trunc

209 210 210 210 211 212 212 212 213 Using Maya: Hypergraph, Sets & Expressions

203

Functions

204

To find out about:

See page:

Exponential functions exp log log10 pow sqrt

214 214 214 214 215 215

Trigonometric functions cos cosd sin sind tan tand acos acosd asin asind atan atand atan2 atan2d hypot

216 216 218 219 224 224 225 225 226 226 226 227 227 227 228 228

Vector functions angle cross dot mag rot unit

229 230 230 231 231 232 233

Conversion functions hsv_to_rgb rgb_to_hsv deg_to_rad rad_to_deg

234 235 235 234 234


Functions Understanding functions

See page:

Array functions clear size sort

236 236 237 237

Random number functions gauss noise dnoise rand sphrand seed

239 239 241 242 243 244 246

Curve functions linstep smoothstep hermite

249 249 252 254

General commands eval print system

259 259 261 263

Other functions and commands

264

Expressions

To find out about:

Understanding functions A function generates a value where it occurs in an expression statement. It takes action based on parameters called arguments that you enclose in parentheses next to the function name.

Example Suppose you have an object named Star whose translateX attribute is set with this expression statement: Star.translateX = rand(10);

In this statement, the rand function has the argument 10. With this argument, the function generates a randomly selected floating point number between 0 and 10 each time the statement executes. For example, translateX Using Maya: Hypergraph, Sets & Expressions

205

Functions Function syntax might be assigned 6.5409 the first time the statement executes, 3.2974 the second time, 8.7389 the third time, and so on. This causes Star to jump to random points from 0 to 10 units away from the X-axis as the scene plays. Though functions can be more or less complicated than this example, they all have at least one argument and generate one value. Note that a function is part of an expression statement. They don’t stand alone in an expression. Many functions do mathematical operations. For example, the sin function generates the sine of a specified angle. Though we often provide explanatory figures and details, we assume you’re familiar with the mathematical purpose of such functions. For details on the math behind such functions, see a more elementary reference. Note that the following functions are important to learn if you want to go beyond the basics of expression writing: •

sin or sind

•

linstep

•

smoothstep

•

hermite

•

noise

•

dnoise

•

rand

•

sphrand

•

print

Function syntax To help you quickly reference different functions, this chapter includes a syntax format statement for each function. The format follows: datatype

function(datatype argument)

function is the name of the function. datatype to the left of an argument indicates the data type of the argument. argument is a parameter you type with the function. datatype to the left of the function name indicates the data type returned when the function executes.

206


Functions Function syntax Note that for either datatype, you don’t type a value. The datatype tells you the type of data you must enter or the data type returned by the executed function. The data types are in small type size for easy identification in the syntax definitions.

Example 1 int

abs(int number)

float

abs(float number)

vector abs(vector number)

The function name is abs, which returns the absolute value of the number of your choice. The absolute value of a number is the number without its positive or negative sign. This example shows the abs function has three formats. Each version requires an argument with a different data type and returns a value with a different data type. The first version indicates that you can type an integer argument, and the function returns an integer result. For example, abs(-3) returns 3.

The third version indicates you can type a vector, and the function returns a vector. For example abs(<<3, -6.3, -2>>) returns <<3, 6.3, 2>>.

Example 2 Many functions have only one format, for example, the deg_to_rad function: float

deg_to_rad( float degrees )

This function returns the radian equivalent of to a degree value. It expects a floating point argument and returns a floating point number. Note that Maya ignores spaces between components of functions. For example, the functions in each of these expressions work the same: rotateY = deg_to_rad (45); rotateY = deg_to_rad(45); rotateY = deg_to_rad( 45 );


207

Expressions

The second version indicates that you can type a floating point argument, and the function returns a floating point result. For instance, abs(-7.54) returns 7.54.

Functions Function syntax

Data types In many cases, entering a data type other than the type expected by a function causes an error and prevents the expression from executing. For example, if you enter a vector argument where a floating point number is expected, an error occurs. For a function argument that expects a floating point number, however, you can instead type an integer—a number without a decimal point. Maya converts an integer to a floating point number in arithmetic operations. If an error occurs when you create an expression, check that you’re using the appropriate data types for all arguments.

Notes In this book, examples of floating point return values show no more than three digits to the right of the decimal point. If you display the contents of an attribute or variable in the Script Editor, you’ll see as many as 10 digits to the right of the decimal point. For instance, an example might show a return value as 3.539 rather than the precise value 3.538654390. The examples round up such numbers for ease of reading. Note also that converting radians to degrees and vice versa results in rounding errors. For example, converting a radian value might result in 89.99999996 degrees rather than 90.0 degrees.

Understanding function examples in this chapter Most function examples in this chapter show the use of the function outside of an expression statement. This makes the examples less cluttered and easier to read. For instance, the first example for the abs function on page 209 is: abs(-1)

This returns the value 1 where it occurs in an expression statement. For a function to have effect, you must use it within an expression that assigns a value to an attribute or variable.

208


Functions Limit functions

Examples Ball.scaleY = abs(-1);

This statement assigns Ball.scaleY the value returned by the abs(-1) function. If you use a function in an expression statement and do not assign the returned value to an attribute, the statement has no effect. abs(-1);

Returns 1, but doesn’t assign it to an attribute. This has the same result as the following meaningless statement: 1;

In some function examples in this chapter, the function’s purpose is easier to understand in the context of an expression. In such cases, we show examples of the function in an expression.

Limit functions The limit functions are math functions that impose limits on numbers.

abs

int

Expressions

Returns the absolute value of number. The absolute value of an integer or floating point number is the number without its positive or negative sign. The absolute value of a vector is a vector with components stripped of negative signs. abs(int number)

float

abs(float number)

vector

abs(vector number)

number is the number for which you want the absolute value.

Examples abs(-1)

Returns the value 1. abs(1)

Returns the value 1.


209

Functions Limit functions abs(<<-1,-2.43,555>>)

Returns <<1, 2.43, 555>>. abs(Ball.translateY)

If Ball.translateY contains -20, this returns 20.

ceil Returns a number rounded to the smallest integer value greater than or equal to a floating point number. float

ceil(float number)

number is the number you want to round.

Examples ceil(2.344)

Returns 3. ceil(3.0)

Returns 3. ceil(Rock.scaleY)

If Rock.scaleY contains -2.82, this returns -2.

floor Returns a number rounded to the largest integer less than or equal to a floating point number. float

floor(float number)

number is the number you want to round.

Examples floor(2.344)

Returns 2. floor(3.0)

Returns 3.

210


Functions Limit functions floor(Head.height)

If Head.height is -2.8, this returns -3.

clamp Returns a number within a range. You can use the clamp function to confine an increasing, decreasing, or randomly changing number to a range of values. float

clamp(float minnumber, float maxnumber, float parameter)

minnumber and maxnumber specify the range of the returned value. parameter is an attribute or variable whose value you want to clamp within the range. If parameter is within the numerical range of minnumber and maxnumber, the function returns the value of parameter. If parameter is greater than the range, the function returns the maxnumber. If parameter is less than the range, the function returns the minnumber.

Examples clamp(4,6,22)

clamp(4,6,2)

Returns 4, because 2 is less than 4, the minimum number of the range. clamp(4,6,5)

Returns 5, because it’s within the range. Ball.scaleY = clamp(0,3,time);

Returns a value between 0 and 3 each time the expression executes. When you rewind the animation to frame 1, the above expression executes and Ball’s scaleY attribute receives the value of time—a number slightly above 0. The clamp function returns the value of time because time is within the range 0 to 3. When you play the animation, time increments slightly with each frame. The expression executes with each frame and Ball’s scaleY attribute receives the value of time until time exceeds 3. When time exceeds 3, the clamp function returns the value 3. Using Maya: Hypergraph, Sets & Expressions

211

Expressions

Returns 6, because 22 is greater than 6, the maximum number of the range.


min Returns the lesser of two floating point numbers. float

min( float number, float number)

number is a number you want to compare.

Examples min(7.2,-3.2)

Returns -3.2. Desk.height = -2; Lamp.height = 9; $Mylight = min(Desk.height,Lamp.height);

Sets $Mylight to -2.

max Returns the larger of two floating point numbers. float

max(float number, float number)

number is a number you want to compare.

Examples max(7.2,-3.2)

Returns 7.2. Desk.height = -2; Lamp.height = 9; $Mylight = max(Desk.height,Lamp.height);

Sets $Mylight to 9.

sign Returns one of three values representing the sign of a number. Returns -1 if the number is negative, 1 if positive, 0 if 0. float

sign( float number )

number is the number whose sign you want to determine.

212



Examples sign(-9.63)

Returns -1. sign(0)

Returns 0. sign(10)

Returns 1. sign(Ball.translateX)

If Ball.translateX is 5, this returns 1.

trunc Returns the whole number part of a floating point number. float

trunc(float number)

number is the number you want to truncate.

Examples trunc(2.344)

Expressions

Returns 2. trunc(0.3)

Returns 0. trunc(-2.82)

Returns -2. trunc(time)

If time equals 3.1234, this returns 3.


213

Functions Exponential functions

Exponential functions The following functions work with exponential values.

exp Returns e raised to the power of a number, enumber. The predefined variable e is the base of the natural logarithm, which is 2.718. float

exp(float number)

number is the exponent to which you want to raise e.

Examples exp(1)

Returns 2.718, the value of e. exp(2)

Returns 7.389, the value of e2.

log Returns the natural logarithm of a number, logenumber. The natural logarithm uses the constant e, which is 2.718. float

log(float number)

number is the positive number for which you want the natural logarithm.

Examples log(10)

Returns 2.303. log(2.718282845904)

Returns 1.000.

log10 Returns the log base 10 of a number. float

log10(float number)

number is the positive number for which you want the log base 10.

214


Functions Exponential functions

Examples log10(100)

Returns 2. log10(10)

Returns 1.

pow Returns a base number raised to an exponent. float

pow(float base, float exponent )

base is the base number you want to raise to the exponent. A negative base number with a decimal component causes an error message. exponent is the exponent.

Examples pow(2,3)

Returns 8. pow(-2,3)

Expressions

Returns -8. pow(2,-3)

Returns 0.125.

sqrt Returns the square root of a positive number. float

sqrt(float number)

number is the positive number of which you want the square root. A negative number displays an error message.

Examples sqrt(16)

Returns 4.


215

Functions Trigonometric functions sqrt($side)

If $side is 25, this returns 5.

Trigonometric functions The following functions return trigonometric values. Each function has two formats that let you choose the type of angular unit you work with: degrees or radians. For example, the cos function expects an argument in radians, while cosd expects an argument in degrees. A radian equals 180 degrees divided by pi, or roughly 57.3 degrees. Note that pi equals 3.1415927, which is also 180 degrees.

cos Returns the cosine of an angle specified in radians. float

cos(float number)

number is the angle, in radians, whose cosine you want. For any right triangle, the cosine of an angle is the following ratio:

adjacent B cos θ = ------------------------------ = ---hypotenuse C Y

Y

C

θ

A

θ

B B

X

X

A C

If θ is less than 1/2 pi radians and more than 3/2 pi radians (from 270 to 90 degrees), cos θ is a value between 0 and 1.

216


If θ is between 1/2 pi radians and 3/2 pi radians (90 to 270 degrees), cos θ is a value between 0 and -1.

Functions Trigonometric functions The cosine ratio depends only on the size of the angle and not on the size of the triangle. This constant ratio is called the cosine of the measure of the angle. The cosine ratio is a value between -1 and 1. With a steadily increasing or decreasing argument, the cos function returns steadily increasing or decreasing values between 1 and -1. This is useful for creating rhythmic, oscillating changes in attribute values. The cos function works like the sin function except its return values are 90 degrees, or pi/2, out of phase. See page 219 for ideas on how to use the cyclical characteristics of the sin and cos functions.

Example 1 cos(1)

Returns 0.5403, the cosine of 1 radian.

Example 2 To animate the motion of Ball in a cosine wave pattern, use this expression:

Ball starts at the origin and moves in the X direction at a rate set by the incrementing animation time. Its Y translation moves cyclically up and down according to the return values of the cos function. The cos function uses translateX, and therefore indirectly, time, as its argument. As time increases from 0 to 6.283 seconds, the cos function returns values that change in fine increments from 1 to -1 and back to 1. The value 6.283 is 2 times the value of pi. As time increases beyond 6.283 seconds, the same cycle repeats for each span of 6.283 seconds.


217

Expressions

Ball.translateX = time; Ball.translateY = cos(Ball.translateX);

Functions Trigonometric functions

Ball.translateY = cos(Ball.translateX);

time = 6.283 (2 * pi seconds)

Compare the same expression using the sin function: Ball.translateY = sin(Ball.translateX);


The cosine curve is 1.571 (pi/2) seconds ahead of (or behind) the sine curve, and vice versa.

cosd Returns the cosine of an angle specified in degrees. float

cosd(float number)

number is the angle, in degrees, whose cosine you want. For more details on the cosd function, see the cos function in the preceding topic. The cosd and cos functions do the same operation, but cosd requires its argument in degree measurement units.

218



Example cosd(45)

Returns 0.707, the cosine of 45 degrees.

sin Returns the sine of an angle specified in radians. float

sin(float number)

number is the angle, in radians, whose sine you want. For any right triangle, the sine of an angle is the following ratio:

opposite A sin θ = ------------------------------ = ---hypotenuse C Y

Y

C A

θ

θ

B

X

A C

If θ is from 0 to pi radians (0 to 180 degrees), sin θ is a value between 0 and 1.

If θ is from pi to 2 pi radians (180 to 360 degrees), sin θ is a value between 0 and -1.

The sine ratio depends only on the size of the angle and not on the size of the triangle. This constant ratio is called the sine of the measure of the angle. The sine ratio is a value between -1 and 1. With a steadily increasing or decreasing argument, the sin function returns steadily increasing or decreasing values between -1 and 1. This is useful for creating rhythmic, oscillating changes in attribute values. Using Maya: Hypergraph, Sets & Expressions

219

Expressions

X

B

Functions Trigonometric functions For example, you can use the sin function to manipulate: •

an object’s translate attributes to create snake-like motion

•

a body’s scale attributes to create a breathing cycle

•

a particle object’s opacity or color attributes to cycle a color or opacity pattern

Example 1 float $pi = 3.1415927; sin($pi/2)

Returns 1, the sine of pi/2 radians.

Example 2 Ball.translateY = sin(Ball.translateX);

This statement sets Ball’s translateY attribute equal to the sine of its translateX attribute. If you drag Ball along the X-axis, Ball’s translateY position moves up and down in a cyclical pattern:

Example 3 To animate Ball to the path of the preceding example, use this expression: Ball.translateX = time; Ball.translateY = sin(Ball.translateX);

220


Functions Trigonometric functions Ball starts at the origin and moves in the X direction at a rate set by the incrementing animation time. Its Y translation moves cyclically up and down according to the return values of the sin function. The sin function uses translateX, and therefore indirectly, time, as its argument. As time increases from 0 to 6.283 seconds, the sin function returns values that change in fine increments from 0 to 1 to -1 to 0. The value 6.283 is 2 times the value of pi. The resulting motion resembles a horizontal S-shape:


As time increases beyond 6.283 seconds, the same S-shaped cycle repeats for each span of 6.283 seconds.

This expression animates Ball with larger up and down swings: Ball.translateX = time; Ball.translateY = sin(Ball.translateX) * 2;

By multiplying sin(Ball.translateX) by a number greater than 1, you increase the amplitude of the sine wave pattern. The amplitude is half the distance between the minium and maximum values of the wave.

Amplitude


221

Expressions

Example 4

Functions Trigonometric functions You can decrease the amplitude of the sine wave by multiplying by a number less than 1, for example, 0.5.

Example 5 This expression increases how often the sine wave completes a cycle: Ball.translateX = time; Ball.translateY = sin(Ball.translateX * 2);

By multiplying Ball.translateX by a number greater than 1, you increase the frequency of the sine wave pattern. The frequency is how long it takes the wave to make a complete cycle.

Frequency

You can decrease the frequency of the sine wave by multiplying by a number less than 1, for example, 0.5. This number is known as a frequency multiplier because it multiplies (or divides) the frequency of the sine pattern.

Example 6 This expression offsets the wave pattern higher up the Y-axis: Ball.translateX = time; Ball.translateY = sin(Ball.translateX) + 2;

By adding 2 to sin(Ball.translateX), the wave pattern starts further up the Yaxis. You can, of course, also subtract a number to offset the wave pattern lower on the Y-axis.

222



Offset of 2

Example 7 The following expression sets a frequency multiplier, amplitude, and offset of a sine pattern in a single statement: Ball.translateX = time; Ball.translateY = (sin(Ball.translateX * 2) * 2) + 2;

Expressions

The following diagram shows which values set the frequency multiplier, amplitude, and offset. Frequency multiplier Amplitude Offset Ball.translateY = (sin(Ball.translateX * 2) * 2) + 2;

A general equation showing the factors you can use to create a sine wave pattern follows: attribute = (sin(frequency * frequency multiplier) * amplitude) + offset;


223


sind Returns the sine of an angle specified in degrees. float

sind(float number)

number is the angle, in degrees, whose sine you want. For more details on how to use the sind function, see the sin function in the preceding topic. The sind and sin functions do the same operation, but sind requires its argument in degree measurement units.

Example sind(90)

Returns 1, the sine of 90 degrees.

tan Returns the tangent of an angle specified in radians. float

tan(float number)

number is the angle, in radians, whose tangent you want. For any right triangle, the tangent of an acute angle is the following ratio:

opposite A tan θ = ----------------------- = --adjacent B Y

Y

C

θ

A

θ

B B

X

X

A C

224


Functions Trigonometric functions The ratio depends only on the size of the angle and not on the size of the triangle. This constant ratio is called the tangent of the measure of the angle.

Example tan(1)

Returns 1.557.

tand Returns the tangent of an angle specified in degrees. float

tand(float number)

number is the angle, in degrees, whose tangent you want. For more details on the tand function, see the tan function in the preceding topic. The tand and tan functions do the same operation, but tand requires its argument in degree measurement units.

Example tand(45)

Returns roughly 1, the tangent of 45 degrees.

Returns the radian value of the arc cosine of a number. The arc cosine is the angle whose cosine is the specified number. The returned value is from 0 to pi. float

acos(float number)

number is the cosine of the angle, and must be from -1 to 1.

Example acos(1)

Returns 0. acos(-0.5)

Returns 2.0944 radians.


225

Expressions

acos


acosd Returns the degree value of the arc cosine of a number. The arc cosine is the angle whose cosine is the specified number. The returned value is from 0 to 180. float

acosd(float number)

number is the cosine of the angle, and must be from -1 to 1.

Example acosd(1)

Returns 0 degrees. acosd(-0.5)

Returns 120 degrees.

asin Returns the radian value of the arc sine of a number. The arc sine is the angle whose sine is the specified number. The returned value is from -pi/2 to pi/2. float

asin(float number)

number is the sine of the angle, and must be from -1 to 1.

Example asin(0.5)


asind Returns the degree value of the arc sine of a number. The arc sine is the angle whose sine is the specified number. The returned value is from -90 to 90. float

asind(float number)

number is the sine of the angle, and must be from -1 to 1.

226



Example asind(0.5))

Returns 30 degrees.

atan Returns the radian value of the arc tangent of a number. The arc tangent is the angle whose tangent is the specified number. The returned value is from -pi/2 to pi/2. float

atan(float number)

number is the tangent of the angle and can be any value.

Example atan(1)

Returns 0.785.

atand

float

atand(float number)

number is the tangent of the angle and can be any value.

Example atand(1)

Returns 45 degrees.

atan2 Returns the radian value of the arc tangent of specified X and Y coordinates. The arc tangent is the angle from the X-axis to a line passing through the origin and a point with coordinates X,Y. The returned angle is in radians, from -pi to pi, excluding -pi. float

atan2(float Y, float X )

X is the X coordinate of the point. Using Maya: Hypergraph, Sets & Expressions

227

Expressions

Returns the degree value of the arc tangent of a number. The arc tangent is the angle whose tangent is the specified number. The returned value is from -90 to 90.

Functions Trigonometric functions Y is the Y coordinate of the point.

Example atan2(1,1)


atan2d Returns the degree value of the arc tangent of specified X and Y coordinates. The arc tangent is the angle from the X-axis to a line passing through the origin and a point with coordinates X,Y. The returned angle is in degrees, from -180 to 180, excluding -180. float

atan2d(float Y, float X )

X is the X coordinate of the point. Y is the Y coordinate of the point.

Example atan2d(1,1)

Returns 45 degrees.

hypot Returns the magnitude of two-dimensional vector from the origin to a point with coordinates X, Y. Y

hypot

(X,Y) X

228


Functions Vector functions As shown in the preceding figure, the hypot function returns the radius of a circle whose center is at one end of a right triangle’s hypotenuse and perimeter is at the other end of the hypotenuse. The following equation gives the magnitude of the vector: 2

x +y float

2

hypot(float x, float y)

X is the X coordinate of the point. Y is the Y coordinate of the point.

Example hypot(3,4)

Returns 5.

Vector functions Expressions

The following functions do operations with vectors. The functions take vector arguments and return floating point numbers or vectors.

angle Returns the radian angle between two vectors. Vector1

Angle

Vector2

float

angle( vector vector1, vector vector2)

vector1 is one of the vectors.


229

Functions Vector functions vector2 is the other vector. The returned angle is the shortest angle between the two vectors. The measurement is always less than 180 degrees.

Example angle(<<2,-1,1>>,<<1,1,2>>)

Returns 1.0472 radians, which equals 60 degrees.

cross Returns the cross product of two vectors. For two vectors, the cross product returns the vector that’s normal to the plane defined by the two vectors. Vector1

Vector2

Cross product

vector

cross(vector vector1, vector vector2)

If the cross product is 0, the two vectors are parallel or colinear. If one or both vectors are <<0,0,0>>, the cross product returns <<0,0,0>>. vector1 is one of the vectors. vector2 is the other vector.

Example cross(<<1,2,-2>>,<<3,0,1>>)

Returns <<2, -7, -6>>.

230


Functions Vector functions

dot Returns the floating point dot product of two vectors. The dot product takes two vectors as arguments and returns a scalar value. float

dot(vector vector1, vector vector2)

If the dot product returns 0, the two vectors are perpendicular. vector1 is one of the vectors. vector2 is the other vector.

Example dot(<<1,2,-2>>,<<3,0,1>>)

Returns 1. The dot product of this example is (1 * 3) + (2*0) + (-2*1), which equals 1.

mag Returns the magnitude of a vector. This is the length of the vector. Y-axis

X

Expressions

Z

<> Y X-axis

Z-axis Magnitude float

mag(vector vector)

vector is the vector whose magnitude you want.


231

Functions Vector functions The mag function converts a vector into a floating point number using the following formula. 2

2

x +y +z

2

Example mag(<<7,8,9>>)

Returns 13.928. 2

2

2

7 + 8 + 9 = 13.928

rot Returns a vector that represents the position of a point after it’s rotated a specified number of radians about a specified axis. Rotation is counterclockwise as viewed downward from the axis end position. Position of point before rotation Angle Position of point after rotation

Axis

vector

rot(vector point, vector axis, float angle )

point is the position of a point in the world coordinate system. axis is the axis around which the point rotates. The axis is a line that passes through the origin and the specified axis position. angle is the number of radians the point rotates.

232


Functions Vector functions

Example 1 rot(<<3,3,0>>,<<1,0,0>>,0.5)

Returns <<3, 2.633, 1.438>>. This is a vector representing the position of point <<3,3,0>> after rotating it 0.5 radians around the axis represented by <<1,0,0>>.

Example 2 particleShape1.position = rot(position,<<0,1,0>>,0.1);

Suppose your scene has a single-particle object at position <<4,6,0>>, and you wrote the above runtime expression for its particle shape node. When you play the scene, the particle rotates in a circular pattern around the Y-axis (the axis represented by <<0,1,0>>). In each frame, the particle’s position rotates 0.1 radian, roughly 5.7 degrees. Motion Particle

Expressions

unit Returns the unit vector corresponding to a vector. The unit vector has the same direction as the specified vector, but with a magnitude of 1. vector

unit( vector vector)

vector is the vector whose unit vector you want.

Example unit(<<1,1,1>>)

Returns <<0.577, 0.577, 0.577>>. Using Maya: Hypergraph, Sets & Expressions

233

Functions Conversion functions

Y <<1, 1, 1>> <<0.577, 0.577, 0.577>>

X Z Unit vector (magnitude = 1)

Conversion functions The following functions convert color scheme values or angle measurements.

deg_to_rad Returns the radian equivalent of a degree value. One radian equals roughly 57.29578 degrees. float

deg_to_rad( float degrees )

degrees is the degree angle you want to convert to radians.

Example deg_to_rad(90)

Returns 1.571, which is the same as pi/2.

rad_to_deg Returns the degree equivalent of a radian value. One radian equals roughly 57.29578 degrees. float

rad_to_deg(float radians)

radians is the radian angle you want to convert to degrees.

234


Functions Conversion functions

Examples rad_to_deg(1)

Returns 57.296. float $pi = 3.1415927; rad_to_deg($pi)

Returns 180.

hsv_to_rgb Converts an HSV vector to an RGB vector. vector

hsv_to_rgb(vector hsv)

hsv is a vector representing the hue, saturation, and value components.

Example hsv_to_rgb(<<1,0.5,0.6>>)

Returns <<0.6, 0.3, 0.3>>.

Tip

In the window’s hexagonal color wheel, drag the pointer to a color of interest. The edit boxes in the window list the color’s values for hue, saturation, and value—and their counterpart red, green, and blue values. Note, however, that the Hue value in the Color Chooser has a range of 0 to 360, while the H component of an HSV vector has a corresponding proportional range of 0 to 1. When you launch the Color Chooser by entering colorEditor, it’s useful only for learning about color. You can’t use it to change the color of objects in your scene.

rgb_to_hsv Converts an RGB vector to an HSV vector.


235

Expressions

To see the relationship between HSV and RGB color components, enter the MEL command colorEditor at the Command Line. This displays the Color Chooser window.

Functions Array functions vector

rgb_to_hsv(vector rgb)

rgb is a vector representing the red, green, and blue components.

Example rgb_to_hsv(<<0.6,0.6, 0.6>>)

Returns <<0, 0, 0.6>>.

Array functions The following functions work with integer, floating point, and vector arrays. If you need more information, see a reference book on the C programming language.

clear Empties the array’s contents, freeing all memory reserved for the array. After you clear an array, its size is 0. When you no longer need to use an array, use the clear function to free memory. int

clear(array array)

array is the name of the array you want to clear. The clear function returns 1 if the function succeeds, 0 if it fails. The return value is not typically used in expressions.

Example int $myInts[] = {1,2,3,4,5,6}; print("size of $myInts is: "+size($myInts)+"\n"); clear($myInts); print("size of $myInts is: "+size($myInts)+"\n");

The third statement above clears the array $myInts. The second and fourth statements display the following text in the Script Editor: size of $myInts is: 6 size of $myInts is: 0

236


Functions Array functions

size Returns the number of elements in an array or the number of characters in a string. int

size(array array)

int

size(string string)

array is the name of the array whose size you want. string is the string whose number of characters you want.

Example 1 string $s = "Hello"; $stringlen = size($s);

The size($s) function returns 5, then the statement assigns 5 to $stringlen.

Example 2 int $myInts[] = {1,2,3,4,5,6}; $numInts = size($myInts);

The size($myInts) function returns 6, then the statement assigns 6 to $numInts.

Returns an array sorted in alphabetical or ascending numerical order. The returned array has the same number and type of elements as the original array. array sort(array array)

array is the name of the array to be sorted.

Example 1 int $myInts[] = {3,6,1,4,2,5}; int $afterSorting[] = sort($myInts); print("After sorting, the array contains:\n"); for ($i = 0; $i < 6; $i = $i + 1) { print($afterSorting[$i]+"\n"); }


237

Expressions

sort

Functions Array functions The sort function sorts the elements of $myInts in ascending order. The following appears in the Script Editor: After sorting, the array contains: 1 2 3 4 5 6

Example 2 string $myName[] = {"Peewee","Michael","Kennedy"}; string $afterSorting[] = sort($myName); print("After sorting, the array contains:\n"); for ($i = 0; $i < 3; $i = $i + 1) { print($afterSorting[$i]+"\n"); }

The sort function sorts the elements of $myName in alphabetical order. The following appears in the Script Editor: After sorting, the array contains: Kennedy Michael Peewee

238


Functions Random number functions

Random number functions The following functions generate random numbers. Random numbers are useful when you want the position, motion, or color of an object’s particles or vertices to have a random appearance.

gauss Returns a random floating point number or vector. The number returned falls within a Gaussian (bell curve) distribution with mean value 0. float

gauss(float stdDev)

vector

gauss(float XstdDev, float YstdDev)

vector

gauss(vector stdDevVector)

stdDev specifies the value at which one standard deviation occurs along the distribution. This gives a one-dimensional Gaussian distribution. XstdDev and YstdDev specify the values for one standard deviation. This gives a two-dimensional Gaussian distribution in the XY plane. The right component of the vector returned is 0. stdDevVector specifies the vector component values for one standard deviation. This gives a three-dimensional distribution.

Example gauss(5)

Returns a random floating point value such as 0.239.


239

Expressions

To control the random values returned by this function, see “seed” on page 246.

Functions Random number functions If you were to execute gauss(5) repeatedly and chart the values returned, they would occur roughly with this frequency: Mean One standard deviation

About 2/3 of returned values will be within one standard deviation.

Number of occurrences

0

-5

5

Value returned

If you were to execute gauss(2) repeatedly, return values would occur with this frequency: Mean One standard deviation

About 2/3 of returned values will be within one standard deviation.

Number of occurrences

-2

0

Value returned

240


2


noise Returns a random number from -1 to 1 according to a Perlin noise field generator. float

noise(float number)

float

noise(float xnum, float ynum)

float

noise(vector vector)

number specifies a number that generates a random number. This gives a one-dimensional distribution of return values. xnum and ynum specify numbers for generating a random number. This gives a two-dimensional distribution of return values. vector specifies a vector for generating a random number. This gives a threedimensional distribution of return values. If you execute this function with the same argument value repeatedly, the function returns the same random value each time it executes. If you execute this function with an argument value that steadily increases or decreases in fine increments over time, the function returns random values that increase and decrease over time.

noise(time)

Returns a value between -1 and 1 each time the expression executes as an animation plays. Because time increases in fine increments, the values returned increase and decrease in smooth, yet random, patterns. If you were to chart the values returned over a period of time, they might occur as in this figure:

1 Return value 0 -1 noise(time) as animation plays


241

Expressions

Example 1


Example 2 noise(frame)

Returns a value between -1 and 1 each time the expression executes as an animation plays. Because frame increases in larger increments, the values returned increase and decrease in rougher patterns. If you were to chart the values returned over a period of time, they might occur as in this figure:

1 Return value 0 -1 noise(frame) as animation plays

The value returned by noise(frame) and noise(time) is the same when frame contains the same number as time. For example, when frame equals 10, noise(frame) returns the same value that noise(time) returns when time is 10.

dnoise Returns a vector with each component containing a random number from -1 to 1. It works like the noise function except it expects and returns a vector argument. The returned vector represents the gradient of the noise field in three dimensions. vector

dnoise(vector argument)

argument specifies a vector for generating a random number. This gives a three-dimensional distribution of return values. See the noise function for more details on dnoise operation.

Example dnoise(<<10,20,-30>>)

Returns <<-0.185, 0.441, 0.686>>.

242



rand Returns a random floating point number or vector within a range of your choice. float

rand(float maxnumber)

float

rand(float minnumber, float maxnumber)

vector

rand(vector maxvector)

vector

rand(vector minvector, vector maxvector)

maxnumber specifies the maximum number returned (in the first syntax format listed above). The minimum number returned is 0. In other words, the returned value will be a random number between 0 and maxnumber. minnumber and maxnumber specify the minimum and maximum numbers returned. maxvector specifies the maximum value for each component of the vector returned. The minimum value is 0. Each component returned is a different random number. minvector and maxvector specify the minimum and maximum value for each component of the vector returned.

Example 1 rand(5)

Returns a random floating point number between 0 and 5, for example, 3.539.

Example 2 rand(-1,1)

Returns a random floating point number between -1 and 1, for example, 0.452.


243

Expressions

To control the random values returned by this function, see “seed” on page 246.

Functions Random number functions If you were to execute rand(-1,1) repeatedly as an animation plays, its return values might occur as in this figure:

1 Return value 0 -1 rand(1,-1) as animation plays

Example 3 rand(<<1,1,1>>)

Returns a random vector in which each component is between 0 and 1, for example, <<0.532, 0.984, 0.399>>.

Example 4 rand(<<1,1,1>>,<<100,200,300>>)

Returns a random vector in which the left component is between 1 and 100, the middle component is between 1 and 200, and the right component is between 1 and 300. An example is <<81.234, 49.095, 166.048>>.

sphrand Returns a random vector value that exists within a spherical or ellipsoidal region of your choice. An ellipsoid is a sphere scaled along its X-, Y- or Zaxes. vector

sphrand(float radius)

vector

sphrand(vector vector)

radius is the radius of a sphere in which the returned vector exists. vector is the radius of an ellipsoid along the X-, Y-, and Z-axis. To control the random values returned by this function, see “seed” on page 246.

244



Example 1 sphrand(1)

Returns a vector whose randomly selected coordinates reside within an imaginary sphere centered at the origin and with a radius of 1. An example returned vector is <<0.444, -0.427, 0.764>>.

Outer boundary of returned value Origin

1

radius

Example 2 sphrand(<<2,1,1>>)

Radius in Z 1

2

Radius in X

1

Outer bound of returned value Radius in Y

To create a particle ellipsoid: You can use the sphrand function, for example, to create a cluster of 500 particles randomly positioned within an ellipsoid having a radius of 2 in the X-axis, 1 in the Y-axis, and 1 in the Z-axis. Using Maya: Hypergraph, Sets & Expressions

245

Expressions

Returns a vector whose coordinates reside within an ellipsoid centered at the origin and with a radius of 2 along the X-axis, 1 along the Y-axis, and 1 along the Z-axis.

Functions Random number functions 1

Select Particles→Particle Tool-❒.

2

Enter 500 for Number of Particles, and 1 for Maximum Radius.

3

Click the mouse somewhere in the workspace to position the particles.

4

Select the particle shape node of the particle object in the Expression Editor.

5

Turn on Creation.

6

Enter this expression: position = sphrand(<<2,1,1>>);

Maya executes the expression once for each particle. It gives each particle a different random position around the origin within the ellipsoid specified by <<2,1,1>>.

seed Sets a seed value the gauss, rand, and sphrand functions use to generate random numbers. If you assign a value to the seed then execute the gauss, rand, or sphrand function repeatedly, an identical sequence of random numbers is generated. For clarification, see the example below and “Reproducing randomness” on page 123. int

seed(int number)

number sets an arbitrary number to be used as the seed value.

Example Suppose you create a NURBS sphere named Ball then enter this expression: Ball.translateX = rand(5);

246


Functions Random number functions When you rewind the animation, Ball’s translateX attribute receives a random value between 0 and 5, for example, 1.392. When you play the animation, the translateX attribute receives a different random value between 0 and 5 each frame. When you rewind the animation again, the translateX attribute receives a value that’s different from the value it received the first time you rewound, for example, 3.223. When you play the animation again, the translateX attribute receives a value each frame that’s different from the values it received the first time you played the animation. In short, every time the rand(5) executes, it gives a different random value. Suppose you change the expression to this: if (frame == 1) seed(1); Ball.translateX = rand(5);

Rewinding the scene to frame 1 executes the seed(1) function. It then assigns translateX a random value between 0 and 5, for example, 4.501. When you play the animation, the rand(5) function executes each frame and returns a different value. Example returned values follow: Value

1

4.501

2

3.863

3

3.202

4

3.735

5

2.726

6

0.101

Expressions

Frame

Each time you rewind and play the animation, translateX receives the same sequence of random values. For different seed values, the sequence of numbers returned will differ. You can’t predict the values in the number sequence based on the value of the seed.


247

Functions Random number functions Suppose you change the expression to this: if (frame == 1) seed(500); Ball.translateX = rand(5);

The rand(5) function returns these values as you rewind and play the animation: Frame

Value

1

4.725

2

2.628

3

0.189

4

0.004

5

4.834

6

0.775

By changing the seed function’s value, you change the sequence of random numbers generated. A common mistake while using the seed function follows: seed(1); Ball.translateX = rand(5);

When you rewind the animation, Ball’s translateX attribute receives the value 4.501. When you play the animation, the translateX attribute receives 4.501 each time the expression executes. Because you assign a value (1) to the seed before each execution of rand(5), you initialize the random number sequence. The rand(5) function therefore returns the first value of the number sequence each time it executes.

Important When you set a seed value in an expression or MEL script, the seed value affects the rand, sphrand, and gauss functions in other expressions and MEL scripts. Such functions are affected by this seed value in all scenes you open subsequently in the current work session.

248


Functions Curve functions

Curve functions The step functions let you make smooth, incrementing transitions between values.

linstep Returns a value from 0 to 1 that represents a parameter’s proportional distance between a minimum and maximum value. This function lets you increase an attribute such as opacity from 0 to 1 linearly over a time range. float

linstep(float start, float end, float parameter)

start and end specifies the minimum and maximum values. parameter is the value you want to use to generate the proportional number. If parameter is less than start, linstep returns 0. If parameter is greater than end, linstep returns 1.

Example Suppose you’ve used the Particle Tool to create a collection of particles named Cloud: Expressions

Suppose further you’ve added a dyamic per object opacity attribute to Cloud (see “Working with particle attributes” in Chapter 8). You then write this runtime expression for Cloud’s particle shape node: CloudShape1.opacity = linstep(0,5,age);


249

Functions Curve functions This expression increases the per object opacity attribute of CloudShape1 in equal steps from 0 to 1 for the first 5 seconds of the object’s existence. Because you created the object with the Particle Tool, the particles existence begins in the first frame of the animation. All particles in the object fade in from transparent to opaque for the first 5 seconds of animation. At the first frame that plays, the age of the particles is 0, so the linstep function returns 0 for the opacity. An opacity of 0 is transparent. In each subsequent frame, the linstep function returns a proportionally larger opacity value. When the age of the object reaches 5, the linstep function returns 1 for the opacity. An opacity of 1 is 100% opaque. When the age exceeds 5, the linstep function returns 1. The opacity stays 100% opaque. Here are some values returned for the object’s opacity:

250

Age

Opacity

0.0417

0.0083

0.0833

0.0166

0.125

0.025

0.1667

0.0333

0.2083

0.0417

2.5

0.5

1.0

0.2

3.75

0.75

5

1

5.041

1

5.083

1

10

1


Functions Curve functions As the table shows, the opacity increases in linear increments for the first 5 seconds of the object’s age. At the midpoint of the specified 0 to 5 second age range, the opacity is 0.5. At 3/4 of the way between 0 and 5 seconds, the opacity is 0.75. At 5 seconds of the object’s age, opacity is 1. After 5 seconds, the opacity stays at 1. 1

opacity

0

5 age (in seconds)

Suppose you edit the runtime expression as follows: CloudShape1.opacity = linstep(5,10,age);

This increases the opacity attribute linearly from 0 to 1 as the object’s age increases from 5 to 10 seconds. Expressions

1

opacity

0

5 age (in seconds)

10

Suppose you edit the runtime expression as follows: particleShape1.opacity = 1-linstep(0,5,age);

This decreases the opacity attribute linearly from 1 to 0 for the first 5 seconds of the object’s age. Subtracting linstep(0,5,age) from 1 causes the opacity to fade out rather than fade in.


251


1

opacity

0

5 age (in seconds)

smoothstep Returns a value from 0 to 1 that represents a parameter’s proportional distance between a minimum and maximum value. The smoothstep function lets you increase an attribute such as opacity from 0 to 1 gradually, but nonlinearly, over a time range. The smoothstep function works like the linstep function, except it increases values more quickly near the middle values between the minimum and maximum value. The function uses hermite interpolation between minimum and maximum values. float

smoothstep(float start, float end, float parameter)

start and end specifies the minimum and maximum values. parameter is the value you want to use to generate the smoothstep number. If parameter is less than start, linstep returns 0. If parameter is greater than end, linstep returns 1.

252


Functions Curve functions The following figure compares values returned by smoothstep and linstep over time: parameter

smoothstep

start

linstep

end

Example Suppose you’ve used the Particle Tool to create a collection of particles named Cloud: Expressions

Suppose also you’ve added a dynamic per object opacity attribute to Cloud (see “Working with particle attributes” in Chapter 8). You then write this runtime expression for Cloud’s particle shape node: CloudShape1.opacity = smoothstep(0,5,age);


253

Functions Curve functions This increases the opacity attribute of CloudShape1 in steps from 0 to 1 for the first 5 seconds of the object’s age. This makes the object fade in from transparent to opaque. The fade in and fade out of the opacity occurs more quickly around 2.5, the midpoint between 0 and 5. 1

opacity

0

5 age (in seconds)

See the linstep function for details on similar examples.

hermite Returns values along a hermite curve. You can use the hermite function, for instance, to move a particle object’s position smoothly along a curve. As the examples in the following pages show, you can create various curve shapes by altering the arguments to the hermite function. vector float

hermite(vector start, vector end, vector tan1, vector tan2, float parameter)

hermite(float start, float end, float tan1, float tan2, float parameter)

start is the start point of the curve. end is the end point of the curve. tan1 is the tangent vector that guides the direction and shape of the curve as it leaves the start point of the curve. The vector’s position starts at the start point of the curve. tan2 is the tangent vector that guides the direction and shape of the curve as it approaches the end point of the curve. The vector’s position starts at the end point of the curve. parameter is an floating point value between 0 and 1, for example, the value returned by a linstep function. In the second format, the arguments and return values work in a single dimension.

254



Example 1 Suppose you create an object named dust made of one particle at the origin. To guide its motion along a short upward-bound curve for the first four seconds of animation, you can write the following runtime expression: dust.position = hermite(<<0,0,0>>,<<2,2,0>>, <<3,0,0>>, <<0,3,0>>, linstep(0,4,time));

When you play the animation, the particle moves from the start point <0,0,0> along a curve to the end point <2,2,0>. The tangent vector <3,0,0> sets the curve’s direction and shape as it leaves the start point. The tangent vector <0,3,0> sets the curve’s direction and shape as it approaches the end point. From zero to four seconds of animation play, the particle moves along the curve as defined by the linstep function. (See page 249 for details on linstep.) The function arguments and resulting path of the object follow: Y

tan2 = <<0,3,0>>

Expressions

end = <<2,2,0>>

Object’s path

start = <<0,0,0>>

tan1 = <<3,0,0>>

X

Example 2 Suppose you change the third argument of the previous example expression to <<6,0,0>>: dust.position = hermite(<<0,0,0>>,<<2,2,0>>, <<6,0,0>>, <<0,3,0>>, linstep(0,4,time));


255

Functions Curve functions The slope of the path curve steepens because of the longer tan1 vector: Y

tan2 = <<0,3,0>>

end = <<2,2,0>>

Object’s path X start = <<0,0,0>>

tan1 = <<6,0,0>>

Example 3 The following expression moves dust in an S pattern: dust.position = hermite(<<0,0,0>>,<<2,0,0>>, <<0,3,0>>, <<0,3,0>>, linstep(0,4,time)); Y

tan1 = <<0,3,0>>

tan2 = <<0,3,0>>

X start = <<0,0,0>>

end = <<2,0,0>>

The tan1 vector <<0,3,0>> sets the direction of the curve from the start point to a positive Y direction. The tan2 vector <<0,3,0>> sets the direction of the curve to a positive Y direction as it approaches the end point. Values between the start and end point curves are interpolated to form an S pattern.

256



Example 4 Suppose you change the fourth argument of the previous example expression to <<0,-3,0>>: dust.position = hermite(<<0,0,0>>,<<2,0,0>>, <<0,3,0>>, <<0,-3,0>>, linstep(0,4,time));

The dust particle moves in a pattern resembling a half-circle: Y

tan1 = <<0,3,0>>

X start = <<0,0,0>>

end = <<2,0,0>>

Expressions

tan2 = <<0,-3,0>>

The tan1 vector <<0,3,0>> sets the direction of the curve from the start point to a positive Y direction. The tan2 vector <<0,-3,0>> sets the direction of the curve to a negative Y direction as it approaches the end point.

Example 5 Suppose you change the third argument of the preceding example to <<0,10,0>>: dust.position = hermite(<<0,0,0>>,<<2,0,0>>, <<0,10,0>>, <<0,-3,0>>, linstep(0,4,time));


257


Y tan1 = <<0,10,0>>

X start = <<0,0,0>>

end = <<2,0,0>>

tan2 = <<0,-3,0>>

Because of the longer tan1 vector, the slope of the path curve steepens as it rises from the start point. Because the tan2 vector has a smaller Y magnitude than the Y magnitude of the tan1 vector, the slope of the path curve is flatter as it approaches the end point. The curve’s rise in the Y direction is greater than the previous example because the magnitude of tan1’s Y component is larger (10 instead of 3).

258


Functions General commands

General commands The following functions do various actions in Maya.

eval Executes a MEL command. string

eval(string command)

command is either a command string enclosed in quote marks or a string variable containing a command. The returned value contains command output returned by the command’s execution.

Example 1 eval("select -cl")

Executes the command select -cl, which deselects all objects in the scene. Though the return value is not used in this example, it contains the command output.

Example 2

The first statement assigns the command string select -cl to the string variable $cmd. The second statement executes the contents of $cmd, which is the command select -cl.

Example 3 string $mycommand = "sphere"; eval($mycommand+"-r 5");

The first statement assigns the string sphere to the variable $mycommand. The second statement appends -r 5 to the string sphere and executes the complete command sphere -r 5. This creates a sphere with a radius of 5 grid units.


259

Expressions

string $cmd = "select -cl"; eval($cmd);


Example 4 string $a[]; $a = eval("ls -lights"); print($a);

The first statement defines an array of strings named $a. The second statement executes the MEL command ls -lights, then assigns the command’s output to array $a. The third statement displays the contents of $a to the Script Editor as follows: ambientLightShape1 directionalLightShape1

Note that each line of command output appears on a new line. Each command output line is an array element. Maya formats array output with each array element on a new line.

Example 5 Suppose you’ve created a MEL script file named bunk.mel in your Maya scripts directory and it contains this procedure: global proc string bunk() { string $fog; if (rand(2) < 1) $fog = "particle"; else $fog = "sphere"; return $fog; }

Further suppose you create this expression: string $name = bunk(); eval($name); print($name);

The first expression statement executes the bunk() procedure in the bunk.mel script file. In the bunk procedure, the if-else statement generates a random floating point value between 0 and 2, then compares its value to 1. If the value is less than 1, the statement assigns the MEL command string particle to $fog. If the value is greater than 1, $fog receives the command string sphere.

260


Functions General commands The procedure finishes executing and passes the value of $fog back to the calling procedure, bunk() in the expression. This assigns the command string to the variable $name. The eval function executes the command string stored in the $name. For example, the statement might execute particle, which creates a particle at the origin of the workspace. The fourth statement displays the contents of $name, for example, particle. The expression executes each frame and creates a new particle or sphere.

print Displays text in the Script Editor. You can use this function to display the contents of attributes and variables. This is helpful for debugging an expression. print(string text) print(vector number) print(float number) print(int number) print(array number)

number is a number without the quote marks. Numerical arguments display as strings. There is no returned value for this function. Note the following display considerations. •

You can format displayed text with standard C language escape characters. For example, you can create a new line with “\n” or a tab character with “\t” in the argument.

•

Displaying a floating point value shows the number with up to 10 digits to the right of the decimal point, for example 0.3333333333.

•

Insignificant 0 digits are truncated from floating point numbers. For example, floating point number 2.0 is displayed as 2.


261

Expressions

text is either a string enclosed in quote marks or an attribute name or string variable containing text.

Functions General commands •

A vector appears with a space separating components and no double angle brackets. Each vector component has a floating point value with up to 10 digits to the right of the decimal point. For example, a vector <<1.518876356, 0, -1.290387446>> appears in the Script Editor as this: 1.518876356 0 -1.290387446

•

Arrays are formatted with each array element on a new line.

•

You can use the + operator to join two strings in an argument: "text1" + "text2"

This is displayed as: text1text2

•

You can also append a number to a string: "text" + 1

This is displayed as: text1

•

You cannot use the + operator with a string array.

•

If you assign a string to a variable that’s not a string data type, the following text appears if you display the variable: Variable data type

String assignment

Data displayed

float

"3.14"

3.14

int

"3.14"

3

vector

"3.14"

3.14 0 0

float

"pi is 3.14"

0, error message

As shown in the last row of the table, if a variable is assigned a string that starts with a nonnumerical character, Maya converts the string to 0. •

262

For a nonparticle expression consisting of only print statements, Always Evaluate must be on in the Expression Editor for the expression to execute.



Examples print(time); print("\n");

The first statement displays the value of time. The second statement displays a new-line character after the value of time, so the time appears on a separate line in the Script Editor. float $f = 3.14159; print($f);

Displays the floating point number 3.14159. string $s = "Hello There"; print($s);

Displays the string Hello There. vector $v; $v = <<1.2,2.3,3.4>>; print($v);

Displays the vector as 1.2 2.3 3.4. string $a[]; $a = eval("ls -lights"); print($a+" are the lights in my scene.\n");

Expressions

The print function causes an error message because you cannot use the + operator with a string array.

system Passes a UNIX command to the shell where you launched Maya. int

system( string command)

command is either a command string enclosed in quote marks or a string variable containing a command. The returned value is the output resulting from the command’s execution.

Example string $cmdout; $cmdout = system("date"); print($cmdout+"\n");


263

Functions Other functions and commands Executes the UNIX date command, which outputs your workstation’s date and time to the $cmdout variable. The final statement displays the date in the Script Editor.

Other functions and commands In addition to the functions described in this chapter, you might find the following less commonly used functions and administration commands helpful. For details on usage, see the MEL online documentation. General

Math

Curve

String

File

alias

acosh

besselj0

gmatch

fopen

catch

asinh

besselj1

match

fclose

chdir

atanh

besseljn

size

fflush

env

constrainValue

besselyn

strcmp

popen

error

erf

substitute

pclose

exists

erfc

substring

fprint

getenv

expm1

tokenize

frewind

getpid

fmod

tolower

feof

gmatch

gamma

toupper

fgetline

putenv

log1p

pwd

fwrite

source

fread

trace

filetest

warning whatIs

264

fgetword


Index Symbols

abbreviating attribute names 112, 116 abs function 209 absolute value 209 acceleration 197 acceleration attribute assigning constant value to 156 assigning with runtime expression 155 changing value randomly 156 field’s effect on 175 initialization to zero 177 working with 175 acos function 225, 226 Add Attribute window 50, 120, 169 Add Dynamic Attributes 49, 161 Add Initial State Attribute checkbox 163, 170 adding custom attributes 119, 169 age 197 age of particles at rewind 150 how to examine 150 runtime expression execution and 152 when created with Particle tool 168 alias UNIX command avoiding use with text editor 109 Always Evaluate 130 Always Evaluate checkbox 118 amplitude of sin function 221 angle function 229

angular units conversion of 128 degrees 29, 127 radians 29, 127 arc cosine 225, 226 arc sine 226 arc tangent 227, 228 arguments in functions 205 arithmetic operators 63 array (per particle) attributes 163 assigning to array of different length 172 array functions 236 array indexes invalid assigment to 193 Array option for per particle attributes 170 arrays 93 clearing contents of 236 display format 262 element assignment 95 example initialization and usage 93 exceeding memory capacity of 93 expansion of 93 invalid assignment to indexes 193 obtaining size of 237 sorting 237 asin function 226 assigning to attributes 55 to int or float variables 60 to specific particles 189 to vector attributes 56 to vector components 193, 194 to vector variables 61 vector to three scalar attributes 56 assignment operator 47


265

Index

! 87 - 63 -- 92 != 65, 66 $ 59, 73 % 63, 182 %= 92 && 67 * 63 *= 92 + 63 ++ 92 += 92 / 63 // 75 /= 92 < 65 << >> 53, 73, 180, 193 <= 65 -= 92 = 47, 55 == 65, 66, 74 > 65 >= 65 ?: 86 \n 261 { } 39, 70, 72, 73 | 140 || 67

A

Index

atan function 227 atan2 function 227 atan2d function 228 atand function 227 attribute names renaming as short names 137 attributeName 197

266

attributes abbreviating names 112, 116 assigning conditionally 32 assigning to 55 assigning to multiple 13, 23 assigning to multiple objects 13, 28 connecting to symbolic placeholders 135 custom 50 data types 51 deleting from expressions 131 disconnecting from expressions 132 displayed in Attributes list 18 displaying contents of 123 displaying disconnected 132 dynamic 49 eliminating expression control of 122 full name 51 initial state 158, 162 linking 11, 15, 26 long names 114 name syntax 47 not selecting for particle shape node 112 particle shape node 159 per object 12, 160 per particle 12, 160 reading in expressions 133 removing from expressions 131 seeing abbreviations of 113, 115 static 49 unexpected values 141 Attributes list 103


B base number raised to exponent 215 bell curve function 239 betterIllumination 197 blank lines in expressions 74 Booleans 52 handling as floating point 65 symbolic constants 95 braces 72 in statements 39, 40 matching pairs of 73 brackets double angle 53, 73, 180, 193 break instruction 79

C C language escape characters 261 syntax in expressions 75 case sensitivity in variable names 59 castsShadows 201 ceil function 210 centimeters 127 Channel Box displaying attribute values in 30 choice command 132 circular motion of NURBS sphere 119 clamp function 211 clear function 236 clearing an expression 106 array contents 236

Index

conversion of angular units only 128 of data types 143, 144 of user selected units 127 conversion functions 234 converting degrees to radians 129 measurement units 128 statements to comments 131 copying text in expressions 105 cos function 216 comparison with sin function 218 cosd function 218 cosine 216, 217, 218 cosine wave pattern animating a ball 217 count 197 Create button 22 Create Event 184 creating new expressions 111 creation expressions 148 assigning to rgbPP 179 dynamics start frame 149 example assignment to lifespan 165, 167 example assignment to lifespanPP 164 execution for emitted particles 149 how often execution occurs 148 using values in runtime expressions 174 when to use 150 cross function 230 cross product of two vectors 230 curve functions 249

custom attributes 50 adding to an object 119 adding to particle shape node 169 assigning to 169 examples of assignment 171 when to use 118 custom variables 59 declaring 59 using globally 61 cyclical pattern with sin function 220

D data types attribute 53 Boolean 52 conversion during assignment 143, 144 conversion of displayed strings 262 conversion with arithmetic operators 145 data entry limitations 54 float array 52 floating point 52 functions 208 integers 52 matrix 55 vector array 52 debugging expressions with print function 261 decimal deletion in data type conversion 144 decimal precision in display 261 declaring variables 59 default object in Expression Editor 104 making an object the 115 defining variables 26 deg_to_rad function 234


267

Index

collision example of controlling color resulting from 185 working with particles 183 color Christmas light effect with particles 181 giving object constant color 179 giving particles randomly changing color 180 understanding RGB and HSV 235 working with 178 colorAccum 197 colorBlue 197 colorEditor 235 colorGreen 197 colorRed 197 comments converting statements to 131 in expressions 75 compiling an expression 19, 62, 118 conditional assignment to attributes 32 conditional statements 69 else-if 71 if 34, 69 if-else 70 confining numerical range 211 connectAttr command 135 connecting an attribute 135 Connection Editor 132, 135 conserve 197 constants 62 Boolean 95 continue instruction 80 controlling flow in statements 77

Index

degrees 29 converting to radians 129, 234 deleting attribute names 131 expressions 112, 134 text from expressions 105 depthSort 197 discarded remainders in data type conversions 146 disconnectAttr command 132 disconnecting an attribute 132 displaying attribute contents 123 disconnected attributes 132 text 261 variable contents 123 dnoise function 242 do loop 78 dollar sign ($) in variable names 59, 73 dot function 231 dot product 231 dot product operator 64 double angle brackets 53, 73, 180, 193 dynamic attributes 49 adding to object 49, 50, 161 dynamic per object attribute example assignment to lifespan 167 dynamic per particle attribute example assignment to lifespanPP 164 dynamics changing start frame 149 how often Maya evaluates 149, 152 Dynamics Controller 149, 152 dynamicsWeight 198 dynamicsWeight attribute 178

268

E e raised to power 214 Edit button 22 editing expressions in text field 99, 105 else keyword 38 else-if statements 71 emitted particles age of 149 assigning lifespanPP for 183 creation expression execution and 149 working with 183 English common names for attributes 114 equal to (==) operator 36 errors common expression 95 comparing floats with the == operator 89 from wrong data types in functions 208 in flow control statements 88 logic 95 message format of 95 syntax 36, 73, 95 where they appear 96 eval function 259 event 198 event attribute 184 when collision count increases 187 eventCount 198 eventCount attribute 184 eventTest 198 eventTest attribute 184 examining two or more expressions 106


executing MEL commands in expressions 137 MEL commands with eval function 259 MEL procedures in expressions 139 nonparticle expressions 118 UNIX commands in expressions 263 execution slow expression 127 exp function 214 exponential functions 214 Expression Editor starting 13

Index

slow execution of 127 speeding execution of 127 text field 14 tutorials 15 type case sensitivity 18 when unusable 12 Expressions list 100, 104

F fading opacity 249, 253 fields influence on expression 175 turning off effect in an expression 178 filtering attributes by connected attribute 104 from Expression Editor 104 filtering expressions 99 finding expressions by connected attribute 101 by expression name 100 by item type 102 by selected object 101 float 52, 57 float arrays data type 52 floating point 52 floor function 210 flow control errors 88 flow control statements 77 for loop 79 for-in loop 81 forward slashes (//) for comments 75 frame 0 reason for using in examples 43 frame playback rate 16, 58 frame variable 57 frequency multiplier of sin function 222

frequency of sin function 222 full attribute name 51 functions 47 arguments in 47, 205, 206 array 236 as expression elements 47 complete list of 203 conversion 234 curve 249 data type of arguments 206 data type of returned values 206 essential for advanced expression writing 206 format of 206 introduction to 203 limit 209 others in online documentation 264 random number 123, 239 spaces in 207 trigonometric 216 understanding book examples 208 vector 229

G gauss function 239 Gaussian distribution 239 General button 49 general commands 259 global procedures declaring 139 global variables 61 declaring 62 initializing 62 goal attribute 161 Goal button 161 goalPP 198 goalPP attribute 161


Index

expressions advantage of separate 31 advantage of single 31 comments in 75 common errors 95 comparison with MEL scripts 46 compiling 19 copying text 105 creating 13 creating new 111 creation 148 default object 104 deleting 112, 131 deleting text in 105 displaying connected attributes only 104 editing in text field 99 editing with text editor 106 elements of 46, 47 eliminating control of attributes 122 erasing 106 examining two or more 106 execution for nonparticle shapes 118 field’s influence on 175 filtering 99 finding 100, 101, 102 for particles 147 input to 133, 135 keywords 76 names for particle shape node 100 naming conventions 18 output from 134, 136 programming features 75 redundant execution 130, 154 reloading 106 required elements of 48 runtime 148 runtime execution 152 saving to file 107

269

Index

goalWeight 198 gravity field acceleration’s effect on 177

H half-circle creating motion with hermite function 257 hermite function 254 HSV conversion to RGB 235 hsv_to_rgb function 235 hypot function 229

I if statements 32, 34, 69 if-else abbreviation 86 if-else statements 38, 39, 70 incandescence 198 incandescencePP 198 increment operations and unexpected values 142 inheritFactor 198 initial state attributes 158, 162, 163 creation expression execution 150 naming convention 163 saving values for 158 input to expressions 135 integers 52, 57 handling as floating point 65 internal conversion of units 127 isDynamic 198

jot text editor 107

K keyframes eliminating expression to use 122 keywords in expressions 76

L levelOfDetail 199 lifespan 198 lifespan attribute 161 example assignment in creation expression 167 Lifespan button 161 lifespanPP 199 lifespanPP attribute 161, 164 assigning for emitted particles 183 limit functions 209 lineWidth 199 linking attributes 11, 15, 26 linstep function 249 comparison with smoothstep 253 listAttributes MEL command 163 log base 10 214 log function 214 logic errors 95 logical operators 67 && 67 || 67 long attribute names 114 looping errors 88

J joining text in strings 262

270


M mag function 231 magnitude of a vector 67, 231 mass 199 matrix data type 55 max function 212 maxCount 199 measurement units 127 MEL commands 45, 46 executing with eval function 259 using alone in statements 137 using with eval function 138 using within single quotes 138 MEL procedures using in expressions 138 MEL scripts 46 millimeters 128 min function 212 mixed data types using with arithmetic operators 145 modulus operator (%) 63, 182, 192 risk of using with floats 182 motion creating jittery 175 creating smooth, random 175 multiCount 199 multiRadius 199

N natural logarithm 214 new line characters in print statement 261

Index

noise function 241 returned values with frame argument 242 returned values with time argument 241 normalDir 199 not (!) operator 87 number sequences generating consistently random 125 numeric render type 190

O object names omitting in expressions 115 path of 140 Objects list 103 offset with sin function 222 omitting object names in expressions 115 online function documentation 264 opacity 200 opacity attribute 161 fading over time 249, 253 Opacity button 161 opacityPP 200 opacityPP attribute 161

operators arithmetic 48, 63 assigning values to 36 dot product 64 equal to 36, 65 greater than 65 greater than or equal to 65 less than 35, 65 less than or equal to 65 logical 48, 67 not equal to 65 precedence 68 relational 48, 65 shortcut assignment 91 shortcut increment and decrement 92 order of statements 37 output from expression 134, 136 oversample level 152, 156

P parentheses matching pairs of 73 use in conditionals 68, 73 particle array attributes assigning to different lengths 172 particle attributes list of 196 Particle Collision Events 184 particleId 200 particleId attribute 189


271

Index

particles age of 150 assigning to specific 189 attribute data types 52 expressions for 147 moving position with hermite function 254 selecting shape node 148 shape node attributes 159 transform node attributes 159 using sphrand to create ellipsoid of 245 working with collisions 183 path names of objects 140 per object attributes 12, 160 keyframing 160 naming conventions 160 scalar option 170 per particle attributes 12, 53, 160 Array option 170 assigning to individual particles 189 how to distinguish 160 naming conventions 160 Perlin noise field 241 playback rate 58 pointSize 200 position 200 position attribute assigning with creation expression 158 assigning with runtime expression 157 field’s effect on 175 working with 175 pow function 215 precedence of operators 68 precision of float display 261 predefined variables 57 frame 57 time 57 primaryVisibility 201

Index

print function 261 programming features 75

R radians 29, 127 angle between two vectors 229 converting to degrees 234 radius 200 radius0 200 radius1 200 radiusPP 200 rand function 140, 243 rand functions 123 random lifespan of particles 165 random number functions 123, 239 random numbers making return values consistent 124 redundant expressions 130 relational operators 65 reloading expressions 106 removing an attribute 131 renaming an object 136 render type numeric 190 rewinding effect on creation expressions 149 unexpected values 141 RGB conversion to HSV 235 rgb_to_hsv function 235 rgbPP 201 rgbPP attribute example use of 179 rotate function 232

272

rotating object around its axis 28 point’s position 232 rounding errors from converting radians to degrees 208 rounding numbers 210 rules of syntax 73 runtime expressions 148, 153 assigning rgbPP in 181 how often execution occurs 148, 152

S saving an expression 107 saving attribute values for initial state 158, 188 Scalar option for per object attributes 170 scale multiplying by percentage 41 slowing increase of 21, 26 Script Editor error display 96 scripting with MEL 45 scripts directory 139 seed function 246 making consistent random values 125 selectedOnly 201 Selection list 103, 104 semicolon terminator 18, 48, 73 Set for All Dynamic 162 Set For Current 150 Set for Current 158, 162, 188 shaded spheres how rendered in examples 154


short attribute names renaming as long names 137 shortcut operators assignment 91 increment and decrement 92 sign function 212 sin function 219 equation for various uses of 223 sind function 224 sine 219, 224 size function 237 Smooth Shade All 154 smooth shading setting all objects to 16 smoothly increasing opacity 250 smoothstep function 252 comparison with linstep 253 soft body attributes in common with particles 50 spaces in expressions 74 in functions 207 specific particles assigning to 189 speeding expression execution 127, 128 spheres how shaded in examples 154 sphrand function 123, 140, 156, 177, 244 use with random color 181 spriteNum 201 spriteNumPP 201 spriteScaleX 201 spriteScaleXPP 201 spriteScaleY 201 spriteScaleYPP 201 spriteTwist 201 spriteTwistPP 202

Index

sqrt function 215 square root 215 S-shaped cycle sin function and 221 S-shaped motion creating with hermite function 256 standard deviation with Gaussian values 239 starting the Expression Editor 13 statements between { } 70 order of 37 static attributes 49 strings 57, 90 assigning to a vector 91 concatenating with + 90 data type conversion 91, 262 joining 262 syntax rules 90 surfaceShading 202 switch instruction 83 symbolic placeholders 132, 134 syntax errors 36, 73, 95 rules 73 system function 263

T

U unexpected values after incrementing 142 after rewinding 141 in mixed data type division 146 of attributes 141, 143

unit function 233 unit vector 233 units internal conversion of 127, 128 UNIX commands executing from expressions 263 useLifspanPP attribute 161 useLighting 202

V variables 56 as expression element 48 assigning to vector 61 data type of 57 declaring 59 defining 26 displaying contents 123 predefined 57 unexpected values 143 vector functions 229 vectors 57 assigning to component of array attribute 194 assigning to variable 193 component operator 194 data type 52 definition 52 dot product 231 format in print function output 262 formula for magnitude 67, 231 magnitude of 2D 228 random vectors with sphrand 244 velocity 202


273

Index

tab characters in expressions 74 tailFade 202 tailSize 202 tan function 224 tand function 225 tangent 224, 225 terminator statement 73

text editor changing operation settings 110 quitting 107 selecting 107 selecting default startup 110 using on expression 106 using unlisted 109 valid options 109 threshold 202 time changing 152 default use of seconds 33 definition 57 dividing by 27 multiplying by 27 negative value of 59 predefined variable 19 relationship to frame 59 value at different frames 19 Time Slider setting start and end range 16 timesteps 156 transform nodes not used for particle expressions 151 trigonometric functions 216 trunc function 213 truncating insignificant numbers 213, 261 tutorials for expressions 15

Index

velocity attribute assigning with creation expression 150 assigning with runtime expression 153, 154 field’s effect on 175 working with 175 vi text editor 107 vim text editor 107 visibleInReflections 201 visibleInRefractions 201

W while loop 77 white space in expressions 74 WINEDITOR setting 109, 110

X xemacs text editor 107

274


Maya Tutorial - Expressions

Recommend Documents