This page describes core architecture principles related to animation in RainyNite.

Every part of document is considered to be a "value" with a type assigned. Even layer-like objects are just "values" of Renderable type (this type is special in that it doesn't really correspond to any value outside of rendering engine).

Every value is animateable. Furthermore, animation is not limited to one type like curve smoothing or following a spline. Instead, each value can be a result of arbitrary calculation, encapsulated into a processing "node".

Nodes may have their parameters, which may in turn be animated. Node structure is also not limited to strict tree hierarchy - same node can appear multiple times in structure and change in one place will be reflected in others.

With that simple core design, it is important to define such types and nodes that it will be easy to create flexible documents, extend functionality and create effective interface.

For a similar (though less generic) system at work, look at Synfig and http://wiki.synfig.org/Convert.