Computer assisted animation: 2D or not 2D?

2D animation can only be automated to the extent that the computer acts as an interactive assistant to the animator. The key problem is that the 3D information which is implicit in the animated drawings is unavailable and this has encouraged the view of 2D animation being a subset of 3D animation in its fullest generality. However, introducing more than the absolute minimum of 3D information, essentially that contained in a hierarchy of drawing overlays, has matching advantages and disadvantages because animators, for aesthetic reasons, deliberately break the rules of geometry and physics as they apply to real-world objects. A software environment which only supports 2D functionality and drawing overlays is sufficient to promote cost-effectively the full range of effects that animators use while the state of the 3D animation art is as yet incapable of this. Essentially the same techniques can be used on live-capture images once they have had a structure similar to that required for animation imposed on them.