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This chapter is from the book

This chapter is from the book

Changing Keyframe Interpolation

When you recorded a keyframe for the cog rotation value of −300 degrees at 5:00, Motion automatically set a keyframe at the start of the layer with the original value, 0 degrees.

Therefore, the layer needs to change from 0 degrees at 0:00 to −300 degrees at 5:00. To get from 0 to −300 over the course of five seconds, Motion calculates, or interpolates, the value over time between the starting and ending values. You can change the type of interpolation in several ways using the Keyframe Editor.

  1. Choose Window > Keyframe Editor. A new window appears below the Timeline. Drag the divider up to make more room.

    The animated parameter for the Cog1 layer, Transform.Rotation.Z, appears on the left side of the window along with the other two Rotation parameters. Next to it is the value of the parameter at the current frame (−145) and a hollow diamond, indicating that keyframes exist for this parameter but not at the playhead position.

    On the right side of the window, a pink line connects two diamonds. The diamonds are the keyframes, and the line is the keyframe curve. The curve is straight because the default interpolation for rotation is linear, meaning that the rotation value changes at a constant rate. You can change the interpolation in the Animation pop-up menu, which appears as a downward facing arrow in the parameter column to the right of the keyframe icon when you place your pointer over it.

  2. Start playback, and from the Animation pop-up menu, choose Interpolation. The submenu includes four interpolation options: Constant, Linear, Bezier, and Continuous.
  3. Choose Bezier.

    Bezier interpolation creates smoother, more real-world animation by imitating the effect of inertia: Objects with mass take some time to get up to speed and to slow to a stop. Instead of a constant rate of change, the rate of change starts low, increases over time, and then decreases again.


    The curve starts out flat, gets steeper in the middle, and then flattens out at the end. The curtains now start opening slowly, speed up, and then slow to a smooth stop, creating more realistic animation.

  4. Experiment with the other interpolation options. Constant maintains the keyframe value until the next keyframe, when the value changes instantaneously.

    Continuous is similar to Bezier in that it creates a smooth ramping into and out of keyframes. It’s easier to create smooth motion with Continuous interpolation; however, the curve cannot be adjusted as with Bezier interpolation.

  5. Return the interpolation type to Bezier. Bezier interpolation gives you the most creative flexibility to fine-tune your animation. You can use it to speed up the time it takes the curtains to open and make them come to an even slower stop.
  6. Click the keyframe at 5:00 to select it, and then drag the Bezier handle to the left to make it longer and flatten out the curve coming into the keyframe.
  7. Click the keyframe at 0:00. Adjust the Bezier handle to create a steeper curve and, therefore, faster initial movement. Play the project to see the results of your changes.

Currently, the animation starts at 0:00 because keyframe recording automatically added a keyframe at the start of the layer. But what if you want the curtains to stay closed for the first second to build a little suspense? Setting keyframes manually gives you the most creative control.

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