Bouncing Ball

Assignment

Use keyframe interpolation to create a bouncing ball.

Procedure

Launch After Effects.
Open a new composition (NTSC DV, 5 seconds)
Download this ball and import it into AE (as footage)
Place the ball offstage in the upper-left corner of the composition window.
Turn on the position property keyframes.
Move the playhead to about the .5 (half) second mark and drag the ball to the bottom of the stage at a slight angle. This will be the impact point for the first bounce.
Advance the playhead about a half-second further; drag the ball up to the top of the next bounce peak.
Continue to add keyframes, moving the ball up and down until it gets offstage to the right (see below).

Hint: Each bounce peak should be a little shorter than the previous one.

Hint: Each keyframe should be a little closer to the previous one
(as the bounces lose height, they occur more frequently; see below)
Select the Convert Vertex tool .
Use this tool to adjust the direction handles going in and out of each key frame. The handles on the lower (impact) keyframes should be bent up into a "rabbit ears" position; this will create a sharp direction change at each impact. The handles on the upper keyframes should be drawn out horizontally, to create a smooth arc at the top of each bounce.
If you preview your movie now the motion will look good, but it will seem odd; this is because the ball is currently traveling at about the same speed throughout. But of course, that's not very realistic on a world with gravity; it should accelerate as it falls and decelerate as it climbs. We will add the acceleration/deceleration with interpolation.
Click on the Graph Editor button to open the graph editor.
Make sure the position property is selected in the timeline. In the graph editor, you should see a graph consisting of a series of horizontal lines ending in small squares. The horizontal lines indicate the speed of the ball between the anchor points (squares). Ideally, the speed of the ball should be the same throughout, but you'll probably see some up and down variation (you can correct this, or at least improve it, by moving the anchor points a little closer or farther apart).
We need to adjust this graph so that the ball accelerates as it falls and decelerates as it climbs. Start with the anchor points that represent the top (apex) of each bounce. First select an upper point (you can select them in the composition viewer window). Next, convert the anchor point to an auto-bezier point by clicking the auto-bezier button on the bottom of the screen: . Then drag the point down to just above the zero line. Repeat these steps with each apex point (every-other anchor point).
Now select the first impact point. Convert it to an auto-bezier point, then drag it up until it forms a sharp point (you may have to zoom out a bit). Repeat this with each impact point. The resulting graph should look like an upside-down bounce pattern:
You'll probably have to do some adjusting—pull out or push in the handles to alter the amount of curve.
Test your movie; does the bouncing seem more realistic?
If the ball seems to be moving too slow, go back to the main timeline. Select all of the keyframes in the position property. Then with the option key held down, click-and-drag the far-right anchor point to the left; all of the point should move in proportion.
For even more realism, try squashing the ball slightly (with the scale property) when it impacts the ground. Make sure it snaps back into shape as soon as it gets airborne though.
Upon completion, render your composition at best quality, full size, Quicktime movie (lossless compression).