Why does a moving object randomly change direction without a keyframe?

Question

I am creating a graphic in After Effects CC, which includes some rectangles zooming onscreen, slowly moving across the screen, and then zoom offscreen. However, after adding the keyframes to make the rectangle zoom offscreen, now, in the middle of the animation (where it is supposed to be slowly moving across the screen), it moves quicker in the opposite direction as before, then changes direction and moves in the direction I intended for it to move. There is no keyframe at the point where it changes, though it behaves just like there is. I've noticed that adding additional keyframes before/after that point causes another direction change. Does anyone know what is going on? I had this same problem in a similar situation in Premiere Pro a few weeks ago, but I never fixed it, just deleted the object and did something else.
Cameron

Answer 1

This is caused by Bezier interpretation of spatial keyframe data. There is a setting in Preferences that you can change. The ping pong motion is caused by the spatial interpretation of the velocity of the move. Your layer is moving at a certain speed between two keyframes and you decide to add another keyframe. The momentum of the layer carries it past the keyframe and then it bounces back before it continues on it's path.
If you set your preferences to display all keyframes and you select the position property you'll see Bezier handles on the path. Open the graph editor and you can clearly see the problem if you display the reference graph.
To fix the problem press the G key and hold down the Alt/Option key to change the pen tool to the convert vertex tool and click on the end of the problem causing handle and drag it toward teh keyframe or simply click the keyframe to remove the Bezier interpretation.
That's all there is to it. Change the default spatial interpretation from Bezier to Linear in the preferences and this will go away for all new keyframes but you may want to introduce Bezier interpretation manually using the pen tool on some animations.