Question about setupFrictionConstraint

[muleax]

Hello,

I found subtle difference when comparing Bullet simulation results to my tiny engine for particular scene. I dug into Bullet's internals and found that the difference comes from btSequentialImpulseConstraintSolver::setupFrictionConstraint method. Specifically, there is a place where constraint velocity is calculated:

btScalar vel1Dotn = solverConstraint.m_contactNormal1.dot(body0 ? solverBodyA.m_linearVelocity + solverBodyA.m_externalForceImpulse : btVector3(0, 0, 0)) + solverConstraint.m_relpos1CrossNormal.dot(body0 ? solverBodyA.m_angularVelocity : btVector3(0, 0, 0));

Note, that m_externalTorqueImpulse isn't present here in any way (unlike how it's done for contact constraint). Why is that? Wouldn't it lead to incorrect behavior in some cases? For example, driven wheel would always have a little slip when torque is applied. Thanks.