SimulationCallbacks.Phase.PostSolveJacobians - are you using it?

Hello everyone!

I'd like to ask a question that will help us make a decision of what to do with this callback. The question is - are you using it? If so, what are you using it for?

We are thinking of removing it all together for 2 reasons:
1. Havok Physics triggers this callback only after the integration of bodies, so it's already too late to do anything.
2. The core use case for it with Unity Physics was to be able to zero-out velocities of settled bodies so that stable stacking can be achieved. However, with the recent solver stabilization feature we shipped, we see no other uses for this.

If you have any thoughts or ideas, we'd like to hear them!

Thanks,
Petar

 

Just thinking loud, but maybe for implementing soft-constraints? They're described here:

https://box2d.org/files/ErinCatto_SoftConstraints_GDC2011.pdf

The page 42 describes how to compute the effective mass using the constraint Jacobian.

 

As far as I can tell, modifications for soft constraints come before the solve, not after. We would still keep the PostCreateContactJacobians callback for this use case.

 

I tinkered with it to stabilize stacks and to clip velocities according to my needs. For stacking it seems to be the only way to apply clamping selectively, allowing for things like ice cubes. General solver stabilization is faster for sure, but it's a blanket approach.

 

Would a workaround of scheduling something after step and undoing integration (plus zeroing velocities) be ok for you in case we remove this?

 

Sure, I mean I've just experimented a bit with that callback, nothing production-focused.