2009

Beyond the lesson
In this lesson you learned how to:
■ Create and edit simple expressions using the Expression Editor.
■ Use variables, for example time, to control the specific attributes within
expressions.
When you write expressions, it is common to assign the built-in time
variable to an attribute so the attribute value predictably increases as the
animation plays. It’s also useful to link an attribute’s value to the value of
another attribute, such as setting the ball’s scale Y and Z attributes to equal
the scale X value.
■ Control multiple attributes with a single expression.
Your expressions can control multiple attributes of the same object or of
multiple objects. Alternatively, you can write a single expression for each
attribute or object.
For more information on the various mathematical operators that are
possible with expressions, refer to the Maya Help.
Behind the lesson
Although not shown in the lessons, you can decrease an attribute value during
playback by subtracting time from some number. Example:
Ball.scaleY = 3 - time;
This decreases the value of Ball’s scaleY attribute for the first three seconds of
playback.
When you use the predefined time variable, note the animation start frame
value. The lessons in this chapter use a start time of 0. In your work, you might
create an animation with a start time of 1. With Maya’s default frame rate of
24 frames per second, time is 0.0417 at frame 1.
Because of this small time offset from 0, the prior lesson would have required
more steps and instructions to work with frame 1 as the start time. For instance,
suppose you use following expression with the start time at 1.
Ball.scaleY = time + 1;
If you go to the start time, the expression 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, going to the start time sets scaleY to a
value 0.0417 larger than its initial value.
Beyond the lesson | 573