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After Effects Nomenclature

expression

An expression in After Effects is computer code, written in JavaScript 1.2, which is applied to a layer property to control that property in some way. Only part of JavaScript is used in After Effects with the addition of some specialized code designed specifically for After Effects. An expression can range from something as simple as a single number to a massive set of instructions.

Example of expression as it appears in the Position Property in the timeline.

This section of the timeline shows where the expression code is written. To activate the expression section, simply command-click (Windows: control-click) on the property stopwatch.

In the example above, the position property is highlighted. Notice the position coordinates are red which indicates that these coordinates are in some way controlled by the expression to the right. Expressions can take complete control of a property, turn control over to an expression control (such as a slider), or partially control the property (such as the clamp expression, where you can manually enter in data, but the expression controls the range allowed by the clamp expression’s parameters).

There is an expression filter on donQmedia’s Word of the Moment directory where you’ll find detailed explanations of a few useful After Effects expressions, as well as the  JavaScript Primmer Tour  which will get you started on the road to learning how to write expressions. There are many off-site resources, and a good start is Adobe’s Expression Language Page which has a list of expressions in JavaScript that are used only in After Effects. Also for a few other off-site expression references see donQmedia’s Off-Site Resources page.

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