Adobe After Effects - Manual (Page 674)

Note:
menu is helpful for determining valid elements and their correct syntax; use it as a reference for available elements. When you choose any object,
attribute, or method from the menu, After Effects automatically inserts it in the expression field at the insertion point. If text is selected in the
expression field, the new expression text replaces the selected text. If the insertion point is not in the expression field, the new expression text
replaces all text in the field.
The Expression Language menu lists arguments and default values. This convention makes it easy to remember which elements you can control
when you write an expression. For example, in the language menu, the wiggle method in the Property category appears as wiggle(freq, amp,
octaves=1, amp_mult=.5, t=time). Five arguments are listed in the parentheses following wiggle. The = in the last three arguments indicates that
using those arguments is optional. If you specify no values for them, they default to 1, .5, and the current time, respectively.
You must replace the argument names written by the Expression Language menu with actual values.
Paul Tuersley provides a script on the AE Enhancers forum that automatically adds wiggle, smooth, and loop expressions to selected
properties. Using this script is a good way for a beginner to experiment with expressions.
Jeff Almasol provides a script that automatically processes expressions, making specified changes. For example, the script can be used to
automatically remove or enable disabled expressions. For more information, see Jeff Almasol’s
redefinery website .
Add, disable, or remove an expression
To add an expression to a property, select the property in the Timeline panel and choose Animation > Add Expression or press Alt+Shift+=
(Windows) or Option+Shift+= (Mac OS); or Alt-click (Windows) or Option-click (Mac OS) the stopwatch button next to the property name in
the Timeline panel or Effect Controls panel.
To temporarily disable an expression, click the Enable Expression switch . When an expression is disabled, a slash appears through the
switch .
To remove an expression from a property, select the property in the Timeline panel and choose Animation > Remove Expression, or Alt-click
(Windows) or Option-click (Mac OS) the stopwatch button next to the property name in the Timeline panel or Effect Controls panel.
Edit an expression with the pick whip
If you are not familiar with JavaScript or the After Effects expression language, you can still take advantage of the power of expressions by using
the pick whip. You simply drag the pick whip from one property to another to link the properties with an expression, and the expression text is
entered in the expression field at the insertion point. If text is selected in the expression field, the new expression text replaces the selected text. If
the insertion point is not in the expression field, the new expression text replaces all text in the field.
Drag the pick whip to a property to create a link to the value of the property.
You can drag the pick whip to the name or value of a property. If you drag to the name of a property, the resulting expression displays all the
values as one. For example, if you drag the pick whip to the name of the Position property, an expression like the following appears:
thisComp.layer("Layer 1").transform.position
If you drag the pick whip to one of the component values of the Position property (such as the y value), an expression like the following appears, in
which both the x and y coordinates of the property are linked to the y value of the Position property:
temp = thisComp.layer("Layer 1").transform.position[1];
[temp, temp]
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