For some specific reasons, i will have to use a graphic engine based on Scene Graphics with a Physic Engine without this concept. I was thinking about a clear way to integrate both representation but ... i did not found something that convinces me.
The problem that always appears is that the scene graph is a hierarchical structure and the Physical engine is not. The position of a node in a scene graph depends of its parents, and when the parents changes its location, the son must be updated, but the son can be physically simulated (its position is always in global coordinates) ...
(there are lots of others situations similar to this ... - hard to calculate relative positions/orientations and keep both worlds in sync)
The only way i found is to have all the physically simulated objects as direct sons of the root node of the scene graph (no relative positions at all) In the physically simulated nodes, we dont propagate relative positions/orientations, each of this nodes only store world positions/orientations, not relatives. But this constraint sounds a little to much and probably will kill (a bit =P) some of beneficts of scene graph like cullings. (after all, most of my scene wont follow any hierarchy ... )
how people usually solve this kind of situations ?