PhysX tip: solver (velocity) iteration count / max depenetration velocity
If you create rigid bodies in an initially overlapping state in PhysX, you can get a violent reaction like the one in this video.
There are two ways to improve this, as demonstrated in the video.
The first way is to increase the number of velocity iterations in the solver. This is done by calling PxRigidDynamic::setSolverIterationCounts(), with a higher ‘minVelocityIters‘ value. The default is just 1 iteration, which is usually enough - but not when objects are overlapping. As you can see in the video, increasing the number of iterations gets rid of the explosion.
However increasing that number has a performance cost. And it is not always a good idea to increase that number for jointed systems.
So there is a second way, which is to setup a “max depenetration velocity” for the rigid body, using PxRigidBody::setMaxDepenetrationVelocity(). By default this is unlimited. In practice, limiting the depenetration velocity (e.g. to “3.0″ in the video) gets rid of the explosion, and it is much cheaper than increasing the solver iteration counts.
Visually this makes objects slowly move away from each-other until the penetration is resolved. This typical behavior can be seen in other engines like Havok.
Note that reducing the timestep does not improve the results here. The opposite happens: reducing the timestep makes the explosion even more violent, which is perhaps counter-intuitive. This is because PhysX tries to resolve the overlap in a single simulation call, and to move the object over the same distance in a shorter amount of time, it has to apply a larger force. So everything being otherwise equal, you get a more violent explosion.
Also, limiting the depenetration velocity can sometimes lead to thin objects passing through each other more easily, when they’re constantly pushed against one-another. It is easy to tweak on a per-object basis though, so generally speaking it is a good idea to limit the depenetration velocity.