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Creating Expressions
Using expressions
Expressions can create a relationship between two properties. For example, if a layer
rotates and has the Drop Shadow effect applied, you can add an expression to the
Rotation property to link the rotation values with the Drop Shadow's Direction values so
that as the layer rotates, the drop shadow changes accordingly. See "Creating expressions
with the pick whip" on page 246.
Note: The After Effects Production Bundle also includes Motion Math scripts (see "Using
Motion Math (PB only)" on page 317). Some scripts may be easier to use than expressions;
for example, to create effects that synchronize all the audio layers in a composition, use
the cmpaud.mm script, or to scale and reposition all layers in a composition, use the
scaleall.mm script.
Expressions are based on the standard JavaScript language—but you don't need to know
JavaScript to use expressions. Instead, you can create expressions by using the pick whip,
by using simple examples and modifying them to suit your needs, or by chaining objects
and methods together using the expression language guide (see "After Effects expression
language guide" on page 254). If you have even a basic understanding of JavaScript, you
can write expressions that create sophisticated relationships between layer properties.
To learn about the more powerful aspects of JavaScript, consult a JavaScript reference
manual, such as JavaScript: The Definitive Guide, by David Flanagan.
Note: After Effects uses only the core standard JavaScript language, not the Web browser-
specific extensions. Instead of Web browser extensions, After Effects contains its own set of
extension objects such as Layer, Comp, Footage, and Camera that you can use to access
most of the values in an After Effects project.
Creating expressions
All of your work adding, editing, and writing expressions occurs in the Timeline window.
When you add an expression to a layer property, a default expression appears in a text
field under the property. Use this field to type new expressions or edit expression
variables. Some expressions rely on the names of layers in your project; if you change the
name of a layer that is involved in an expression, the expression will not work.
To create an expression:
1 Select a property in the Timeline window and do one of the following:
Choose Animation > Add Expression.
•
Alt-click (Windows) or Option-click (Mac OS) the property's stopwatch icon.
•
Note: After Effects automatically fills the expression field with a default expression that
copies the fixed value of the property, or the value of the keyframes, if any.
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Creating Expressions
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