Multiple Contact Point Bug about impulseengine HOT 4 CLOSED

It's a good learning exercise to try and implement an iterative solver that converges to the global minimum. This code intentionally does not implement such a solver, and instead naively applies impulses in isolation. Read more of Erin Catto's resources if you'd like to delve deeper into this topic. A good test would be to convert ImpulseEngine into an iterative solver.

from impulseengine.

Ah yes that seems to be what Box2D is doing. Any pointers in the right direction to resources would be very appreciated :) I have been porting Impulse to C# but wanted to make the solving more accurate.
It may just be my ignorance but why would taking the vectors rA and rB and adding them and applying the impulse to just the sum not work?

from impulseengine.

rA and rB should be pointing to the same location in world space. They are used to transform from rigid body local space, to world space, or vise versa. Since we compute an impulse along a world space direction, and want to apply this impulse in an equal and opposite manner across both bodies, we must rA and rB from each body respectively.

As for tips I'd suggest looking into Box2D Lite (not the full version). Try to implement the iterative solver without warm starting. The difficult part is tracking touching bodies over multiple frames, and tracking different contact points over multiple frames. From here it's a matter of clamping the accumulated impulse.

from impulseengine.

Found some pseudo code from an old email of someone implementing an iterative solver in ImpulseEngine:

// calculate j scalar
float j = calculate( );

// Clamp j to the accumulated impulse
float temp = contact->cache;
contact->cache = Max( temp + j, 0.0f );
j = contact->cache - temp;

// Apply j like normal

And then once this is working make sure to remove the old position projection in ImpulseEngine. You will now see constraint drift if implemented properly. This can be solved with the Baumgarte technique:

const float k_baumgarte = 0.2f;
const float k_slop = 0.05f;

// Calculate bias term
float b = k_baumgarte * (1.0f / dt) * min( 0.0f, penetration + k_slop );

// calculate j scalar
float j = calculate( ) + b;

// Clamp j to the accumulated impulse
float temp = contact->cache;
contact->cache = Max( temp + j, 0.0f );
j = contact->cache - temp;
// Apply j like normal

You can check out my other 3D code example here on github as well as Box2D Lite for additional references.

from impulseengine.