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Adjusting Temporal Interpolation Manually

As you’ve seen, many of the principles of adjusting a motion path apply to adjusting a property graph. Both are described by Bézier curves, although the terminology can differ. And whereas a motion path traces a literal course through space, the line of a graph corresponds to a property’s value or speed. But although the techniques you use to edit Bézier curves resemble one another in principle, they differ in practice. The main difference lies in how you adjust the curves manually: dragging direction lines in a value graph or ease handles in a speed graph.

In a value graph, you can drag direction lines 180 degrees to influence the graph’s curve—and, hence, its incoming and outgoing interpolation. Bézier curves closely resemble their counterparts in a motion path.

In a speed graph, ease handles influence the shape of the curve and, thereby, the interpolation. However, ease handles always extend horizontally from a keyframe; their length but not their angle helps shape the curve. Whereas a sudden change in a value plots a cusp in a value graph, a sudden change in speed splits the keyframe so that it occupies two different vertical positions on the speed graph.

To adjust a value graph manually:

  1. Expand the layer outline to view the value graph for an animated layer property.
  2. Select the keyframes you want to adjust (Figure 9.67).
  3. Figure 9.67 Select the keyframe you want to adjust in the Graph Editor.

  4. Do any of the following to the incoming or outgoing direction lines:
    • Drag a keyframe up to increase the value or down to decrease the value (Figures 9.68 and 9.69).
    • Figure 9.68 Drag a keyframe up to increase...

      Figure 9.69 ...or down to decrease the value (shown here) or speed.

    • To convert auto Bézier to continuous Bézier, drag one direction line so that the direction lines are unequal but retain their continuous relationship (Figure 9.70).
    • Figure 9.70 In the value graph, extend a direction line manually to use continuous Bézier interpolation.

    • To toggle between continuous Bézier and Bézier, Option-drag (Alt-drag) a direction handle (Figure 9.71).
    • Figure 9.71 In the value graph, Option (Alt)-drag a direction handle.

    The Selection tool becomes the Convert Vertex tool when you position it over a direction handle. Dragging a direction handle of a Bézier keyframe converts it to continuous Bézier with two related direction handles; dragging a direction handle of a continuous Bézier keyframe splits the direction handles, converting it to Bézier (Figure 9.72).

    Figure 9.72 Dragging a direction handle of a continuous Bézier keyframe splits the direction handles, converting it to Bézier.

To adjust a speed graph manually:

  1. Expand the layer outline to view the speed graph for an animated layer property.
  2. Select the keyframes you want to adjust (Figure 9.73).
  3. Figure 9.73 Select the keyframes you want to adjust.

  4. Do any of the following to the incoming or outgoing ease handles:
    • Drag an ease handle up to increase the incoming or outgoing speed at a keyframe (Figure 9.74).
    • Figure 9.74 Drag an ease handle up to increase the incoming or outgoing speed at a keyframe.

    • Drag an ease handle down to decrease the incoming or outgoing speed at a keyframe.
    • Drag the left ease handle to change its length and influence on the preceding curve (Figure 9.75).
    • Figure 9.75 Here, dragging an incoming ease handle to the left increases the influence of the previous keyframe’s value.

    • Drag the right ease handle to adjust |its length and influence on the following curve.
    • Option-click (Alt-click) a keyframe to toggle it between linear and auto Bézier.

The shape of the graph and the corresponding property’s speed change according to your adjustments (Figures 9.76 and 9.77). When the incoming and outgoing speeds differ, a keyframe’s icon splits, occupying two different vertical positions on the graph.

Figure 9.76 An abrupt shift from acceleration to deceleration, or bounce, looks like this in a speed graph.

Figure 9.77 A gradual deceleration followed by a gradual acceleration (as when a rising object slows at its apex) looks like this in a speed graph.

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