Adobe After Effects CS4 - Manual (Page 659)

653 USING AFTER EFFECTS CS4
Expressions
Last updated 12/21/2009
function average(a, b)
{
return (a + b) / 2;
}
average(position, thisComp.layer(1).position);
Note: You must define each function fully within each expression in which it is used. There is no global library of functions
that you can add to.
If you want to save an expression for use in another project, you should add comments to the expression. (See “ Add
comments to an expression ” on page 652.) You should also use variables so that you can change a value in one place
rather than having to change it in several places.
For example, this expression has a multiline comment at the beginning that explains what the expression does and a
short comment after a variable is declared and initialized that tells what the variable is for:
/* This expression on a Source Text property reports the name
of a layer and the value of its Opacity property. */
var myLayerIndex = 1; // layer to inspect, initialized to 1, for top layer
thisComp.layer(myLayerIndex).name + ": \rOpacity = " +
thisComp.layer(myLayerIndex).opacity.value
You can save an animation preset that includes an expression and reuse it in other projects, as long as the expression
does not refer to properties that don’t exist in the other projects. When you save a preset in which a property has an
expression but no keyframes, only the expression is saved. If the property has one or more keyframes, the saved preset
contains the expression along with all keyframe values.
You can copy an expression from a layer property, with or without the keyframes for the property.
• To copy an expression and keyframes from one property to other properties, select the source layer property in the
Timeline panel, copy the layer property, select the target layer properties, and paste.
• To copy an expression from one property to other properties without copying keyframes, select the source
property, choose Edit > Copy Expression Only, select the target properties, and paste.
Copying an expression without keyframes is useful when you want to simultaneously copy multiple expressions and
paste them onto one or more new layers, or when you want to copy one expression and paste it onto more than one
layer.
Note: As with pasting keyframes and other items, you can often paste the item into the target layer and rely on After
Effects to determine which property should be the target of the paste operation. For example, this works for copying
Position properties from one layer to another, but you must select the target property yourself if you are pasting an
expression from a Position property into a Scale property.
More Help topics
“ Save an animation preset ” on page 388
“ Layer properties in the Timeline panel ” on page 159
Expression Controls effects
Use an Expression Controls effect to add a control that you can use to manipulate the values of one or many properties
by linking the property to the control using expressions. A single control can affect several properties at once.