Compliant/Soft contacts for rigid bodies

[rodchenkov-sn]

Hi!

I am trying to recreate physx compliant contacts, but found no way to do it without patching jolt. My idea of an implementation is straight-forward:

1. Add SpringSettings field to ContactSettings so i can define a spring for a given contact
2. Replace single bias float with SpringPart in ContactConstraintPart (more ol less copy-paste AxisConstraintPart)
3. Calculate contact constraint props with spring settings where needed

This seems to work (not for ccd though, havent figured it out yet)
My questions are:

1. Am i missing something obvious that would behave like soft contacts?
2. If not, does my implementation idea sounds right?
3. Also, if not, have soft contacts been considered as a feature for jolt? (i've seen pr with negative restitution, but im not sure it's about soft contacts)

[jrouwe]

Jolt doesn't use soft constraints to resolve contacts. It uses Baumgarte stabilization with the caveat that the stabilization only affects positions and not velocities. So when a body is pushed out of collision, it stops the moment it leaves collision. You can make contacts 'softer' by reducing PhysicsSettings::mBaumgarte.

If you want soft contacts (as in a spring) then you indeed need to modify Jolt in the way you describe. Note that you also need to remove the code in ContactConstraintManager::sSolvePositionConstraint to disable Baumgarte stabilization.

Can I ask why you want to have soft contacts? What are you trying to achieve?

[rodchenkov-sn]

thanks for the answer, yeah, i forgot to mention that i also put code in ContactConstraintManager::sSolvePositionConstraint under the check for constraint spring activeness.

i'm working on vehicle physics and trying to 'soften' contacts with static geometry on high velocity so that vehicles do not bounce uncontrollably. Somewhat like in #1581, but i want to damp velocities while allowing some penetration in the process, like the vehicle being deformed without actually using deformable bodies, and then gradually put it out of collision. Setting restitution to zero or overriding mass scales doesnt help. I also have some debris for which i want contacts to be 'hard', so i dont want to lower baumgarte globaly