I have now implemented Erin Catto’s Sequential Impulses solver which he presented in his Box2D mini demo. It’s equivalent to the PGS (Projected Gauss Seidel) algorithm when using SI with clamped accumulated impulses. You find more information in his paper.
The flash runs currently very slow because it’s not optimized at all, but so far I’m happy that it works. I really love SI as it overcomes the jitter of objects which also made stacking nearly impossible in the previous version. But I keep my old solver because it’s very fast and great for non-gravity environments or particle-sized objects where jitter was not an issue.
The last (and probably hardest) step is to rewrite all my collision functions to add contact caching capability, which is quite complicated. The Box2D engine only supports (guess what.. boxes) and the collision and contact generation code is far to slow to be enjoyable in flash, so it’s crucial to find a fast solution for this. Once this is done I can concentrate on the API.