Slow-in and Slow-out

To make an action accelerate or slow down we use slow-in and slow-out in order to make it look more realistic. If we want to give the illusion of speed in a right way, we use fewer frames for a fast action and more frames for a slow action. If we combine both we get a smoother action.

For example the video below shows in real life a man swimming with dolphin kicks. There we can see the slow-in and slow-out of his legs. When he kicks is slow-in and we he pulls up his legs is slow-out.



This picture shows the frames used for a bouncing ball in a constant speed, slow-out, slow-in and slow-in with slow-out.