I am currently working on a 2D scenegraph system. I have a scene node class that contains a Transform structure describing translation, scaling, and rotation. Child nodes of scene nodes inherit their parents' transformation and then transform relative to that. Translation and scaling are relatively easy to do: just addition and multiplication of the vectors. However, inheriting rotation is difficult. Each node has a rotation relative to its own translation. How do I make child node's calculate their local transformation taking into account the parent's rotation and rotation about their local translations?

A child nodes local rotation can be converted into its parents space by multiplying the child and parents rotation matrices together. You will probably want to cache the result upon first access, but otherwise the mechanism is the same as for inheriting translation and scale.