Recorded or plainly drawn motion can look "disappointingly wooden and dead". In 2006 Jue Wang, Steven Drucker, Maneesh Agrawala and Michael Cohen showed one filter that fixes it: take the motion's acceleration, smooth it, subtract it. "The user only needs to set the desired strength of the filter." Out come a wind-up, an overshoot and squash and stretch that nobody keyed. Why? It is a sharpening filter, the kind that makes halos around the edges in a photo. "As with any sharpening filter, it produces a ringing which is often considered an undesirable artifact in images. In our case, the same ringing produces a desired anticipation and follow-through effect." Push it too far and it breaks: John Lasseter wrote that exaggeration "does not mean arbitrarily distorting shapes"; the animator must "go to the heart of anything or any idea and develop its essence". The filter's makers agreed: it "cannot always produce the exact motion a skilled animator would create".

The ball, the picture and the walker are our own, all computed with the same filter; no figures from the paper.

Where have you seen a wind-up that felt wrong?

Sources: Jue Wang, Steven M. Drucker, Maneesh Agrawala and Michael F. Cohen, "The Cartoon Animation Filter", SIGGRAPH 2006; John Lasseter, "Principles of Traditional Animation Applied to 3D Computer Animation", SIGGRAPH 1987.
Sounds: our own, made in code (earshot, CC0).