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Keyframe Interpolation in Adobe After Effects CS4

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This chapter explains how you can assign various interpolation methods to keyframes to impart nuance and variation to your animations using the Timeline panel’s Graph Editor. You’ll not only learn to decipher how After Effects depicts the ineffable qualities of motion, speed, and acceleration, but you’ll also see how it harnesses them. In the process, you’ll begin to realize that there’s a big difference between animating something and bringing it to life.
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In Chapter 7, “Properties and Keyframes,” you learned to animate layer properties over time by setting keyframes. By defining only the most important, or key, frames, you assume the role of head animator. After Effects fills the role of assistant animator, providing all the in-between frames, or tweens, using what’s known as an interpolation method to determine their values.

Fortunately, you can instruct your assistant to use a range of interpolation methods. Some methods create steady changes from one keyframe to the next; others vary the rate of change. Movement can take a direct path or a curved route; an action can glide in for a soft landing or blast off in a burst of speed.

Without a choice of interpolation methods, your loyal assistant’s abilities would be severely limited. If animated values always proceeded directly and mechanically from one keyframe to another, all but the most basic animations would seem lifeless and robotic. To create a curved movement would require so many keyframes you’d begin to wonder why you had an assistant at all. Calculating acceleration or deceleration in speed would present an even thornier problem.

This chapter explains how you can assign various interpolation methods to keyframes to impart nuance and variation to your animations using the Timeline panel’s Graph Editor. You’ll not only learn to decipher how After Effects depicts the ineffable qualities of motion, speed, and acceleration, but you’ll also see how it harnesses them. In the process, you’ll begin to realize that there’s a big difference between animating something and bringing it to life.

Understanding Interpolation

The beauty of keyframes is that they save you work. If you set keyframes, After Effects calculates the values for the frames in between, a process known as interpolation. But to truly control animation, you’ll need some power over the interpolated values as well. You can gain this control—without significantly increasing your work—by taking advantage of a type of calculation known as a Bézier curve.

As you’ll see, mask and shape paths, spatial interpolation, and temporal interpolation are just different manifestations of the same Bézier principles. In fact, the ability to copy a mask path to a motion path attests to their shared Bézier heritage (see the sidebar, “Bézier Curves and the Motion Path,” later in this chapter). And as you’ll see in the following sections, most interpolation types (many of which include “Bézier in their names), calculate values in terms of both space and time: in other words, spatially and temporally.

Spatial interpolation

Spatial interpolation refers to how After Effects calculates changes in position, how a layer or its anchor point moves in the space of the composition. Does it proceed directly from one keyframe to the next, or does it take a curved route (Figures 9.1 and 9.2)?

As you’ve seen, spatial interpolation is represented as a motion path—a dotted line connecting keyframes. Changes in a layer’s position value appear as a motion path in the Composition panel; changes in a layer’s anchor-point value appear in its Layer panel. Effect point paths can appear in both panels. So far, you’ve learned how to set a layer’s position at a keyframe by dragging in the appropriate panel; in this chapter, you’ll learn how to adjust the path between keyframes, or the interpolated values.

Temporal interpolation

Temporal interpolation refers to any property value’s rate of change between keyframes. Does the value change at a constant rate from one keyframe to the next, or does it accelerate or decelerate?

For example, Figure 9.3 shows two rabbits. They both travel the same distance in the same amount of time. However, one proceeds from the first keyframe to the last at a constant rate. The other gradually accelerates, starting slowly and then speeding up. As a result, the second rabbit falls behind at first and then gradually catches up. Both reach their destination simultaneously.

So far, you’ve viewed keyframes by expanding a layer’s property values in the Timeline. The keyframes’ relative timing and values give you some control of the overall speed of changes. But to see and manipulate the values between keyframes—the interpolated values—you must toggle the view under the time ruler to the Graph Editor. The Graph Editor represents the temporal interpolation as graphs that reflect a property’s rate of change and also lets you control it (Figure 9.4).

Incoming and outgoing interpolation

Although interpolation refers to values between keyframes, it’s important to understand that you assign an interpolation type to keyframes themselves. The interpolation type, in turn, determines how values are calculated before the keyframe and after the keyframe—the incoming and outgoing interpolation. Therefore, the values between any two keyframes (the interpolated values) are determined by the first keyframe’s outgoing interpolation type and the next keyframe’s incoming interpolation type. The concept is most easily understood in spatial terms. A motion path consists of at least two keyframes. A tangent extending from the first keyframe determines the outgoing interpolation, while a tangent extending from the second keyframe determines the incoming interpolation. The motion path between keyframes results from their relative positions, as well as the length and angles of their tangents (Figure 9.5).

Temporal interpolation also affects a property value’s rate of change before and after the keyframe. In a speed or value graph, ease handles work a lot like tangents in a motion path. But because the graph lines don’t trace a spatial path, they can be a little more difficult to understand and adjust (Figure 9.6).