User Guide

CHAPTER 10
314
Creating Expressions
6 Preview the animation. Notice that the Opacity property of Light is animated, yet there
are no opacity keyframes.
Note: Make sure to use Rotation values that are compatible with the Opacity property. For
example, Opacity values range from 0 to 100, so rotating the Dial layer backward using a
negative value results in an Opacity value of 0.
Editing a pick-whip expression
Once you use the pick whip to create an expression, you can perform simple edits to alter
the expression’s effect. For example, you can add a scale factor to the expression to increase
or decrease the effect. To edit a pick-whip expression, type directly in the expression field
and then press Enter on the numeric keypad or click outside of the field to activate it.
Modify a pick-whip expression by using simple math operations, such as the following:
+ (add)
- (subtract)
/ (divide)
* (multiply)
*-1 (perform opposite of original, such as counterclockwise instead of clockwise)
Using math functions that are a little more complex provide a high degree of flexibility.
For example, adding “/360*100” to an expression converts a parameter’s values that range
from 0 to 360, such as a dial, to values that range from 0 to 100, such as a slider.
Examples of edited pick-whip expressions
The following are examples of pick-whip expressions that have been altered with simple
functions. These expressions are easy to create and result in sophisticated effects that
would otherwise require dozens of keyframes.
Animating a clock Start with two layers: an hour hand and a minute hand. Set the anchor
points for the hands at the center of the clock (see “Setting and animating an anchor
point” on page 182). Set Rotation keyframes for the hour hand; select the Rotation
property for the minute hand and choose Animation > Add Expression. Drag the pick
whip to the hour hand’s Rotation property. The following expression appears:
this_comp.layer("hour hand").rotation
UG.book Page 314 Wednesday, February 21, 2001 12:05 PM