Code:

btVector3 tbRigidBody::getVelocityInLocalPoint(const btVector3& rel_pos) const{//we also calculate lin/ang velocity for kinematic objectsreturn m_linearVelocity + m_angularVelocity.cross(rel_pos);//for kinematic objects, we could also use use://return (m_worldTransform(rel_pos) - m_interpolationWorldTransform(rel_pos)) / m_kinematicTimeStep;}/////setup the friction constraintsbtVector3 vel1 = rb0 ? rb0->getVelocityInLocalPoint(rel_pos1) : btVector3(0,0,0);btVector3 vel2 = rb1 ? rb1->getVelocityInLocalPoint(rel_pos2) : btVector3(0,0,0);vel  = vel1 - vel2;rel_vel = cp.m_normalWorldOnB.dot(vel);{   btVector3 frictionDir1 = vel - cp.m_normalWorldOnB * rel_vel;   btScalar lat_rel_vel = frictionDir1.length2();   if (lat_rel_vel > SIMD_EPSILON)//0.0f)   {      //use projected normal direction as first friction direction and cross product of normal and frictionDir1 as second friction direction      frictionDir1 /= btSqrt(lat_rel_vel);      addFrictionConstraint(frictionDir1,solverBodyIdA,solverBodyIdB,frictionIndex,cp,rel_pos1,rel_pos2,colObj0,colObj1, relaxation);      btVector3 frictionDir2 = frictionDir1.cross(cp.m_normalWorldOnB);      frictionDir2.normalize();      addFrictionConstraint(frictionDir2,solverBodyIdA,solverBodyIdB,frictionIndex,cp,rel_pos1,rel_pos2,colObj0,colObj1, relaxation);   } else   {      //calculate two orthogonal vectors to normal direction      //re-calculate friction direction every frame, todo: check if this is really needed      btVector3   frictionDir1,frictionDir2;      btPlaneSpace1(cp.m_normalWorldOnB,frictionDir1,frictionDir2);      addFrictionConstraint(frictionDir1,solverBodyIdA,solverBodyIdB,frictionIndex,cp,rel_pos1,rel_pos2,colObj0,colObj1, relaxation);      addFrictionConstraint(frictionDir2,solverBodyIdA,solverBodyIdB,frictionIndex,cp,rel_pos1,rel_pos2,colObj0,colObj1, relaxation);   }}