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Interpolation Controlling change with interpolation 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 change occurs through and between keyframes. For example, if you are setting up motion, you can choose to make a layer change direction abruptly or smoothly through a curve. After Effects interpolates values for a change using the values of the keyframes on both ends of the change.
Interpolation is the process of filling in the unknown data between two known values. This usually means generating new values between two keyframes. For example, if you want a graphic element (such as a title) to move fifty pixels across the screen to the left, and you want it to do so in 15 frames, you’d set the position of the graphic in the first and 15th frames, and mark them both as keyframes. Then the software would complete the work of interpolating the frames in between to make the movement appear smooth. Because interpolation generates all the frames between the two 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.
You can control interpolation between keyframe values over time for all layer properties. For spatial properties— which are layer properties that involve movement, such as Position, Anchor Point, and effect point properties—you can also control interpolation between motion-path keyframes through space.
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.
Note: 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.
See also “About changes in speed” on page 210 “Work with the Graph Editor” on page 184 “Shortcuts for keyframes” on page 650 “Shortcuts for showing properties in the Timeline panel” on page 647 Keyframe interpolation methods In layer bar mode, the appearance of a keyframe icon depends on the interpolation method you choose for the interval between keyframes. When half of the icon is dark gray, the dark half indicates that there is no keyframe adjacent to that side, or that its interpolation is overridden by the Hold interpolation applied to the preceding keyframe.
By default, a keyframe uses one interpolation method, but you can apply two methods: the incoming method applies to the property value as the current time approaches a keyframe, and the outgoing method applies to the property value as the current time leaves a keyframe. When you set different incoming and outgoing interpolation methods, the keyframe icon in layer bar mode changes accordingly. It displays the left half of the incoming interpolation icon and the right half of the outgoing interpolation icon.
To toggle between keyframe icons and keyframe numbers, select Use Keyframe Icons or Use Keyframe Indices from the Timeline panel menu.
All interpolation methods used by After Effects are based on the Bezier interpolation method, which provides direction handles so that you can control the transitions between keyframes. Interpolation methods that don’t use direction handles are constrained versions of Bezier interpolation and are convenient for certain tasks.
To learn more about how different interpolation methods affect temporal properties, experiment by setting up at least three keyframes with different values for a temporal layer property—such as Opacity—and change the interpolation methods as you view the value graph in Graph Editor mode in the Timeline panel.
To learn more about how different interpolation methods affect a motion path, experiment by setting up three keyframes for a spatial property—such as Position—with different values on a motion path, and change the interpolation methods as you preview the motion in the Composition panel.
Note: To change interpolation methods, right-click a keyframe, select Keyframe Interpolation from the menu that appears, and then select an option from the Temporal Interpolation drop-down menu.
In the following descriptions of interpolation methods, the result of each method is described as if you had applied it to all of the keyframes for a layer property. This is done to clarify the examples. In practice, you can apply any available interpolation method to any keyframe.
See also “About animation, keyframes, and expressions” on page 183 “Shortcuts for keyframes” on page 650 “Shortcuts for showing properties in the Timeline panel” on page 647 “Work with the Graph Editor” on page 184 No interpolation No interpolation is the state in which there are no keyframes for a layer property—when the stopwatch is turned off and the I-beam icon appears in the Timeline panel under the current-time indicator. In this state, when you set the value of a layer property, it maintains that value for the layer’s duration, unless overridden by an expression. By default, no interpolation is applied to a layer property. If any keyframes are present for a layer property, some kind of interpolation is in use.
Linear interpolation Linear interpolation creates a uniform rate of change between keyframes, which can add a mechanical look to animations. After Effects interpolates the values between two adjacent keyframes as directly as possible without accounting for the values of other keyframes.
If you apply Linear interpolation to all keyframes of a temporal layer property, change begins instantly at the first keyframe and continues to the next keyframe at a constant speed. At the second keyframe, the rate of change switches immediately to the rate between it and the third keyframe. When the layer reaches the final keyframe value, change stops instantly. In the value graph, the segment connecting two keyframes with Linear interpolation appears as a straight line.
If you apply Bezier interpolation to all keyframes of a layer property, After Effects creates a smooth transition between keyframes. The initial position of the direction handles is calculated using the same method used in Auto Bezier interpolation. After Effects maintains existing direction handle positions as you change a Bezier keyframe value.
Unlike other interpolation methods, Bezier interpolation lets you create any combination of curves and straight lines along the motion path. Because the two Bezier direction handles operate independently, a curving motion path can suddenly turn into a sharp corner at a Bezier keyframe. Bezier spatial interpolation is ideal for drawing a motion path that follows a complex shape, such as a map route or the outline of a logo.
Existing direction handle positions persist as you move a motion-path keyframe. The speed of motion along the path is controlled by the temporal interpolation applied at each keyframe.
Auto Bezier interpolation Auto Bezier interpolation creates a smooth rate of change through a keyframe. You might use Auto Bezier spatial interpolation to create the path of a car turning on a curving road.
As you change an Auto Bezier keyframe value, the positions of Auto Bezier direction handles change automatically to maintain a smooth transition between keyframes. The automatic adjustments change the shape of the value graph or motion path segments on either side of the keyframe. If the previous and next keyframes also use Auto Bezier interpolation, the shape of the segments on the far side of the previous or next keyframes also changes. If you adjust an Auto Bezier direction handle manually, you convert it to a Continuous Bezier keyframe.
Auto Bezier is the default spatial interpolation.
Continuous Bezier interpolation Like Auto Bezier interpolation, Continuous Bezier interpolation creates a smooth rate of change through a keyframe.
However, you set the positions of Continuous Bezier direction handles manually. Adjustments you make change the shape of the value graph or motion path segments on either side of the keyframe.
If you apply Continuous Bezier interpolation to all keyframes of a property, After Effects adjusts the values at each keyframe to create smooth transitions. After Effects maintains these smooth transitions as you move a Continuous Bezier keyframe on either the motion path or the value graph.
You can easily freeze the current frame for the duration of the layer using the Freeze Frame command. To freeze a frame, position the current time indicator at the frame you want to freeze. Make sure the layer is selected and then choose Layer Time Freeze Frame. Time-remapping is enabled, and a Hold keyframe is placed at the position of the current time indicator to freeze the frame.
Note: If you previously enabled time-remapping on the layer, any keyframes you created will be deleted when you apply the Freeze Frame command.
You can use Hold interpolation only for outgoing temporal interpolation (for the frames following a keyframe). If you create a new keyframe following a Hold keyframe, the new keyframe will use incoming Hold interpolation.
To apply or remove Hold interpolation as outgoing interpolation for a keyframe, select the keyframe in the Timeline panel, and choose Animation Toggle Hold Keyframe.
Apply and change keyframe interpolation methods You can apply and change the interpolation method for any keyframe. You can apply changes using the Keyframe Interpolation dialog box, or you can apply them directly to a keyframe in layer bar mode, in a motion path, or in the Graph Editor. You can also change the default interpolation After Effects uses for spatial properties.
See also “Select keyframes” on page 188 “About animation, keyframes, and expressions” on page 183 “Shortcuts for keyframes” on page 650 “Shortcuts for showing properties in the Timeline panel” on page 647 Change interpolation method with the Keyframe Interpolation dialog box The Keyframe Interpolation dialog box provides options for setting temporal and spatial interpolation and—for spatial properties only—roving settings.
1 In layer bar mode or in the Graph Editor, select the keyframes you want to change.
2 Choose Animation Keyframe Interpolation.
3 For Temporal Interpolation, choose one of the following options:
Current Settings Preserves the interpolation values already applied to the selected keyframes. Choose this option when multiple or manually adjusted keyframes are selected and you do not want to change the existing settings.
Linear, Bezier, Continuous Bezier, Auto Bezier, and Hold Apply a temporal interpolation method using default values.
4 If you selected keyframes of a spatial layer property, choose one of the following options for Spatial Interpolation:
Current Settings Preserves the interpolation settings already applied to the selected keyframes.
Linear, Bezier, Continuous Bezier, and Auto Bezier Apply a spatial interpolation method using default values.
5 If you selected keyframes of a spatial layer property, use the Roving menu to choose how a keyframe determines
its position in time, and then click OK:
Current Settings Preserves the currently applied method of positioning the selected keyframes in time.
Lock To Time keeps the selected keyframes at their current position in time. They stay in place unless you move them manually.
For more information on smoothing the rate of change through selected keyframes, see “Smooth motion with roving keyframes” on page 216.
Change interpolation method with the Selection tool in layer bar mode ❖ Using the Selection tool, do one of the following:
• If the keyframe uses Linear interpolation, Ctrl-click (Windows) or Command-click (Mac OS) the keyframe to change it to Auto Bezier.
• If the keyframe uses Bezier, Continuous Bezier, or Auto Bezier interpolation, Ctrl-click (Windows) or Commandclick (Mac OS) the keyframe to change it to Linear.
Change interpolation method in the Graph Editor
• Click the keyframe with the Convert Vertex tool to toggle between linear and Auto Bezier interpolation.
• Select one or more keyframes, and then click the Hold, Linear, or Auto Bezier button at the bottom of the screen to change the interpolation method.
A B C Interpolation buttons in the Graph Editor A. Hold B. Linear C. Auto-Bezier Modify Bezier direction handles In the Graph Editor, keyframes that use Bezier interpolation have direction handles attached to them. You can retract, extend, or rotate the direction handles to fine-tune the Bezier interpolation curve.
Retracting a direction handle makes a tighter, sharper Bezier curve. Extending a direction handle makes a larger, smoother Bezier curve. By default, when you retract or extend a direction handle, the opposite handle on the keyframe moves with it. Splitting direction handles makes the two direction handles attached to a keyframe behave independently.
• To retract or extend direction handles, drag the direction handle toward or away from the center of its keyframe with the Selection tool.
• To split direction handles, Alt-drag (Windows) or Option-drag (Mac OS) a keyframe with the Selection tool. You can also Alt-drag (Windows) or Option-drag (Mac OS) outside a keyframe to draw new handles, whether or not there are existing handles.
• To manipulate the direction handles of two neighboring keyframes simultaneously, drag the value graph segment between the keyframes.