[ODE] patch - position-correcting constraints

[Adam D. Moss]

Megan Fox wrote:
> The point is that, at present, bodies are de-penetrated by forces. 
> These forces are in no way separate from others, and stack on
> eachother with each frame, leading to explosions after stack apon
> stack apon stack of depenetrating force has its way.

That's certainly an issue, I think, but the underlying issue is that
once that corrective force has served its purpose it still remains
in effect, which causes both correction-accumulation and the fake-
bounce effect.

> The patch, in theory, removes that problem, de-penetrating objects at
> a set, reasonable velocity. 

In a round-about way, yes. :)  What it does is remember, within an
island's step, how much of a body's corrective force is only being
applied to correct interpenetration, then it removes that much force
again once the force has affected the position for that step.  So in
effect it makes the effects of interpenetration-correction not extend
beyond the current step (any remaining interpenetration will
cause further correction in the next step, of course), which amounts
more or less to a position correction.

 > This should remove any sort of physical
> explosion due to penetrating bodies, and might even make stacking
> behavior a bit better (maybe?).

It doesn't seem to much affect stacking either way, sadly.

> Apologies on my part, I've been too busy to try your patch - looks
> great, but I need to keep my codebase consistent for now

I understand!

--adam