implemented rolling friction, using a contact constraint. Useful to get rolling spheres to rest, even on a slightly sloped plane.

See http://www.youtube.com/watch?v=RV7sBAsKu4M and Bullet/Demos/RollingFrictionDemo
Fixes in FractureDemo (mouse picking constraint needs to be removed, otherwise constraint solver crashes/asserts)

src/BulletCollision/CollisionDispatch/btCollisionObject.cpp

@@ -31,6 +31,7 @@ btCollisionObject::btCollisionObject()
 		m_activationState1(1),
 		m_deactivationTime(btScalar(0.)),
 		m_friction(btScalar(0.5)),
+		m_rollingFriction(0.0f),
 		m_restitution(btScalar(0.)),
 		m_internalType(CO_COLLISION_OBJECT),
 		m_userObjectPointer(0),

src/BulletCollision/CollisionDispatch/btCollisionObject.h

@@ -85,6 +85,7 @@ protected:
 
 	btScalar		m_friction;
 	btScalar		m_restitution;
+	btScalar		m_rollingFriction;
 
 	///m_internalType is reserved to distinguish Bullet's btCollisionObject, btRigidBody, btSoftBody, btGhostObject etc.
 	///do not assign your own m_internalType unless you write a new dynamics object class.
@@ -263,6 +264,16 @@ public:
 		return m_friction;
 	}
 
+	void	setRollingFriction(btScalar frict)
+	{
+		m_rollingFriction = frict;
+	}
+	btScalar	getRollingFriction() const
+	{
+		return m_rollingFriction;
+	}
+
+
 	///reserved for Bullet internal usage
 	int	getInternalType() const
 	{

src/BulletCollision/CollisionDispatch/btManifoldResult.cpp

@@ -22,6 +22,23 @@ subject to the following restrictions:
 ///This is to allow MaterialCombiner/Custom Friction/Restitution values
 ContactAddedCallback		gContactAddedCallback=0;
 
+
+
+///User can override this material combiner by implementing gContactAddedCallback and setting body0->m_collisionFlags |= btCollisionObject::customMaterialCallback;
+inline btScalar	calculateCombinedRollingFriction(const btCollisionObject* body0,const btCollisionObject* body1)
+{
+	btScalar friction = body0->getRollingFriction() * body1->getRollingFriction();
+
+	const btScalar MAX_FRICTION  = btScalar(10.);
+	if (friction < -MAX_FRICTION)
+		friction = -MAX_FRICTION;
+	if (friction > MAX_FRICTION)
+		friction = MAX_FRICTION;
+	return friction;
+
+}
+
+
 ///User can override this material combiner by implementing gContactAddedCallback and setting body0->m_collisionFlags |= btCollisionObject::customMaterialCallback;
 inline btScalar	calculateCombinedFriction(const btCollisionObject* body0,const btCollisionObject* body1)
 {
@@ -91,7 +108,7 @@ void btManifoldResult::addContactPoint(const btVector3& normalOnBInWorld,const b
 
 	newPt.m_combinedFriction = calculateCombinedFriction(m_body0Wrap->getCollisionObject(),m_body1Wrap->getCollisionObject());
 	newPt.m_combinedRestitution = calculateCombinedRestitution(m_body0Wrap->getCollisionObject(),m_body1Wrap->getCollisionObject());
-
+	newPt.m_combinedRollingFriction = calculateCombinedRollingFriction(m_body0Wrap->getCollisionObject(),m_body1Wrap->getCollisionObject());
 	btPlaneSpace1(newPt.m_normalWorldOnB,newPt.m_lateralFrictionDir1,newPt.m_lateralFrictionDir2);
 	
 

src/BulletCollision/NarrowPhaseCollision/btManifoldPoint.h

@@ -64,6 +64,7 @@ class btManifoldPoint
 					m_normalWorldOnB( normal ), 
 					m_distance1( distance ),
 					m_combinedFriction(btScalar(0.)),
+					m_combinedRollingFriction(btScalar(0.)),
 					m_combinedRestitution(btScalar(0.)),
 					m_userPersistentData(0),
 					m_lateralFrictionInitialized(false),
@@ -90,6 +91,7 @@ class btManifoldPoint
 		
 			btScalar	m_distance1;
 			btScalar	m_combinedFriction;
+			btScalar	m_combinedRollingFriction;
 			btScalar	m_combinedRestitution;
 
 			//BP mod, store contact triangles.