///User can override this material combiner by implementing gContactAddedCallback and setting body0.m_collisionFlags |= btCollisionObject::customMaterialCallback;
public static double calculateCombinedFriction( btCollisionObject body0, btCollisionObject body1 )
{
double friction = body0.getFriction() * body1.getFriction();
if( friction < -MAX_FRICTION )
friction = -MAX_FRICTION;
if( friction > MAX_FRICTION )
friction = MAX_FRICTION;
return friction;
}