2D shadows are mattes (colored or not) placed over the animated characters. We can setup two types of shadows:

1. The character’s own shadows, the ones which lie on it.
2. The projected shadows, the ones that are generated by an object or character, but lie on the character’s surrounding environment.

In both cases they can be created based on mask created from the shape of the character. Remember that to achieve the desired effect, these mattes must be setup on a layer higher than the character they are projected on.