About animation, keyframes, and expressions



Animation is change over time. You animate a layer or an effect on a layer by making one or more of its properties change over time. For example, you can animate the Opacity property of a layer from 0% at time zero to 100% at time 1 second to make the layer fade in. Any property with a stopwatch button to the left of its name in the Timeline panel or Effect Controls panel can be animated.

Stopwatch icons

A.
Active stopwatch

B.
Inactive stopwatch

You animate layer properties using keyframes, expressions, or both.

Many animation presets include keyframes and expressions so that you can simply apply the animation preset to the layer to achieve a complex animated result.

You work with keyframes and expressions in After Effects in one of two modes: layer bar mode or Graph Editor mode. Layer bar mode is the default, which shows layers as duration bars, with keyframes and expressions aligned vertically with their properties in the Timeline panel. Graph Editor mode does not show layer bars, and shows keyframes and expression results in value graphs or speed graphs. (See The Graph Editor.)

Keyframes

Keyframes are used to set parameters for motion, effects, audio, and many other properties, usually changing them over time. A keyframe marks the point in time where you specify a value for a layer property, such as spatial position, opacity, or audio volume. Values between keyframes are interpolated. When you use keyframes to create a change over time, you typically use at least two keyframes—one for the state at the beginning of the change, and one for the new state at the end of the change. (See Set or add keyframes.)

When the stopwatch is active for a specific property, After Effects automatically sets or changes a keyframe for the property at the current time whenever you change the property value. When the stopwatch is inactive for a property, the property has no keyframes. If you change the value for a layer property while the stopwatch is inactive, that value remains the same for the duration of the layer.

If you deselect the stopwatch, all keyframes for that layer property are deleted, and the constant value for the property becomes the value at the current time. Don’t deselect the stopwatch unless you’re sure that you want to permanently delete all of the keyframes for that property.

Change the keyframe icons in layer bar mode to numbers by choosing Use Keyframe Indices in the Timeline panel menu.
Keyframes as icons compared to keyframes as numbers

Note: When a layer property that contains keyframes is collapsed, gray dots (summary keyframe indicators) for the property group show that there are keyframes contained within it.

Some tools, such as Motion Sketch and the Puppet tools, automatically set keyframes for you to match motion that you sketch.

Expressions

Expressions use a scripting language based on JavaScript to specify the values of a property and to relate properties to one another. You can create simple expressions by connecting properties with the pick whip. (See About expressions.)

Online animation resources

Aharon Rabinowitz provides some video tutorials that introduce animation as part of the Multimedia 101 series, including “How Does Computer Animation Work?” and “What is interpolation?”

For a video tutorial on animating using keyframes, go to the Adobe website at www.adobe.com/go/vid0227.

Shaun Freeman's website provides links to information about the theory and practice of animation, especially character animation.

For a step-by-step tutorial that demonstrates the animation of individual layers from a Photoshop (PSD) file, see the “Animating Layers in After Effects“ chapter of the After Effects Classroom in a Book on the Peachpit Press website.