About spatial and temporal keyframe interpolation



Interpolation is the process of filling in the unknown data between two known values. You set keyframes to specify a property’s values at certain key times. After Effects interpolates values for the property for all times between keyframes.

Because interpolation generates the property values between keyframes, interpolation is sometimes called tweening. Interpolation between keyframes can be used to animate movement, effects, audio levels, image adjustments, transparency, color changes, and many other visual and audio elements.

After you create keyframes and motion paths to change values over time, you may want to make more precise adjustments to the way that change occurs. After Effects provides several interpolation methods that affect how the in-between values are calculated.

Temporal interpolation is the interpolation of values in time; spatial interpolation is the interpolation of values in space. Some properties—such as Opacity—have only a temporal component. Other properties—such as Position—also have spatial components.

Temporal interpolation and the value graph

Using the value graph in the Graph Editor, you can make precise adjustments to the temporal property keyframes you’ve created for your animation. The value graph displays x values as red, y values as green, and z values (3D only) as blue. The value graph provides complete information about the value of keyframes at any point in time in a composition and allows you to control it. In addition, the Info panel displays the temporal interpolation method of a selected keyframe.

Spatial interpolation and the motion path

When you apply or change spatial interpolation for a property such as Position, you adjust the motion path in the Composition panel. The different keyframes on the motion path provide information about the type of interpolation at any point in time. The Info panel displays the spatial interpolation method of a selected keyframe.

When you create spatial changes in a layer, After Effects uses Auto Bezier as the default spatial interpolation.

To change the default to linear interpolation, choose Edit > Preferences > General (Windows) or After Effects > Preferences > General (Mac OS), and select Default Spatial Interpolation To Linear. Changing the preference setting does not affect keyframes that already exist or new keyframes on properties for which keyframes already exist.
Motion path interpolation 

A.
Linear

B.
Auto Bezier

C.
Continuous Bezier

D.
Bezier

E.
Hold

In some cases, the Auto Bezier spatial interpolation for Position keyframes can cause undesired back-and-forth (boomerang) motion between two keyframes with equal values. In such a case, you can change the earlier keyframe to use Hold interpolation or change both keyframes to use Linear interpolation.

Online resources about keyframe interpolation

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

Aharon Rabinowitz provides a pair of video tutorials on the Creative COW website that describe the issue and solution for the boomerang motion problem that arises from unintentionally having Auto Bezier spatial interpolation set for keyframes of equal value:

Antony Bolante provides information and illustrations about keyframe interpolation in an article on the Peachpit Press website.