It’s rare for something to move at a constant rate: gravity, atmosphere, constraint and a range of other factors make sure of that.
The movement of the human body, and most other objects, needs time to accelerate and slow down. For this reason, animation looks more realistic if it has more drawings near the beginning and end of an action, emphasizing the extreme poses, and fewer in the middle.
This principle goes for characters moving between two extreme poses, such as sitting down and standing up, but also for inanimate, moving objects.
When something starts to move, it will usually build up energy and speed – it won’t instantaneously achieve a terminal velocity. The same is true when it comes to a stop: the built-up energy needs time to disperse before coming to a halt.
Ease In and Ease Out is the way you represent this phenomenon in animation.
They deal with the way you enter and exit key poses (as opposed to key frames). Spacing your in-betweens or modifying your curves can achieve this acceleration and deceleration. More in-betweens or a flatter curve creates a slower movement, while fewer frames or a steeper curve create a faster one.
Your approach to Ease In and Ease Out can be different depending on your medium: In 3D, you have the invaluable tool of a curve editor. However, people often rely too much on the computer to control these curves – as if the software knows what you’re trying to animate! You need to get stuck into the curve editor and take complete control over all of those tangents. Even then, it might be necessary to do frame-by-frame animation to get the exact result you’re after.
Ease In and Ease Out is absolutely essential to creating subtle and realistic movements and giving life to animation. Choosing where to use them – or where not to use them – can greatly enhance your characters’ believability and the life of the animation.
From Frank Thomas and Ollie Johnson: