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Chapter 18: Expressions
Creating and modifying expressions
About expressions
Whenyouwanttocreateandlinkcomplexanimations,suchasmultiplecarwheelsspinning,butwouldliketoavoid
creating tens or hundreds of keyframes by hand, try using expressions instead. With expressions, you can create
relationships between layer properties and use one property’s keyframes to dynamically animate another layer. For
example, if you set rotation keyframes for a layer and then apply the Drop Shadow effect, you can use an expression
to link the Rotation property’s values with the Drop Shadow effect’s Direction values; that way, the drop shadow
changes accordingly as the layer rotates.
Expressions are based on the standard JavaScript language, but you do not need to know JavaScript to use expres-
sions. You can create expressions by using simple examples and modifying them to suit your needs or by chaining
objects and methods together.
All of your work with expressions occurs in the Timeline panel. You can use the pick whip to create expressions, or
you can enter and edit expressions manually in the expression field,atextfieldinthetimegraphundertheproperty.
Youcanwriteanexpressioninatexteditorandthencopyitintotheexpressionfield.Whenyouaddanexpression
to a layer property, a default expression appears in the expression field.
In Graph Editor view, the expression field appears as a resizable text box at the bottom of the Graph Editor. Only the
active expression for the selected property appears in the Graph Editor expression field.
Note: If an expression cannot be processed, After Effects displays a message explaining the error and automatically
disables the expression. A yellow warning icon appears next to the expression; click the warning icon to view the error
message again.
If you have used Motion Math scripts in past versions of After Effects, you can now create the same results using
expressions.
Some expressions rely on the names of layers or properties in your project; if you change the name of a layer or
property that is involved in an expression, After Effects will attempt to update the expression to use the new name.
However, in some complex cases After Effects will be unable to automatically update the expression, in which case
the expression may produce an error message, and you must update the expression yourself.
Note: The parameters for some After Effects effects have changed. If you have existing expressions that use parameter
index references rather than parameter names, you may need to make adjustments to the expressions to use parameter
names.
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