improve rolling friction using anisotropic direction, to avoid resting in an instable position

(for implicit capsule, cylinder and cone shape)
See Bullet/Demos/RollingFrictionDemo for an example

src/BulletCollision/CollisionDispatch/btCollisionObject.h

@@ -138,6 +138,13 @@ public:
 		CO_USER_TYPE=32
 	};
 
+	enum AnisotropicFrictionFlags
+	{
+		CF_ANISOTROPIC_FRICTION_DISABLED=0,
+		CF_ANISOTROPIC_FRICTION = 1,
+		CF_ANISOTROPIC_ROLLING_FRICTION = 2
+	};
+
 	SIMD_FORCE_INLINE bool mergesSimulationIslands() const
 	{
 		///static objects, kinematic and object without contact response don't merge islands
@@ -148,14 +155,15 @@ public:
 	{
 		return m_anisotropicFriction;
 	}
-	void	setAnisotropicFriction(const btVector3& anisotropicFriction)
+	void	setAnisotropicFriction(const btVector3& anisotropicFriction, int frictionMode = CF_ANISOTROPIC_FRICTION)
 	{
 		m_anisotropicFriction = anisotropicFriction;
-		m_hasAnisotropicFriction = (anisotropicFriction[0]!=1.f) || (anisotropicFriction[1]!=1.f) || (anisotropicFriction[2]!=1.f);
+		bool isUnity = (anisotropicFriction[0]!=1.f) || (anisotropicFriction[1]!=1.f) || (anisotropicFriction[2]!=1.f);
+		m_hasAnisotropicFriction = isUnity?frictionMode : 0;
 	}
-	bool	hasAnisotropicFriction() const
+	bool	hasAnisotropicFriction(int frictionMode = CF_ANISOTROPIC_FRICTION) const
 	{
-		return m_hasAnisotropicFriction!=0;
+		return (m_hasAnisotropicFriction&frictionMode)!=0;
 	}
 
 	///the constraint solver can discard solving contacts, if the distance is above this threshold. 0 by default.

src/BulletCollision/CollisionShapes/btCapsuleShape.h

@@ -104,6 +104,14 @@ public:
 
 	}
 
+	virtual btVector3	getAnisotropicRollingFrictionDirection() const
+	{
+		btVector3 aniDir(0,0,0);
+		aniDir[getUpAxis()]=1;
+		return aniDir;
+	}
+
+
 	virtual	int	calculateSerializeBufferSize() const;
 
 	///fills the dataBuffer and returns the struct name (and 0 on failure)

src/BulletCollision/CollisionShapes/btCollisionShape.h

@@ -109,6 +109,13 @@ public:
 
 	
 	int		getShapeType() const { return m_shapeType; }
+
+	///the getAnisotropicRollingFrictionDirection can be used in combination with setAnisotropicFriction
+	///See Bullet/Demos/RollingFrictionDemo for an example
+	virtual btVector3	getAnisotropicRollingFrictionDirection() const
+	{
+		return btVector3(1,1,1);
+	}
 	virtual void	setMargin(btScalar margin) = 0;
 	virtual btScalar	getMargin() const = 0;
 

src/BulletCollision/CollisionShapes/btConeShape.h

@@ -84,6 +84,11 @@ public:
 			return m_coneIndices[1];
 		}
 
+	virtual btVector3	getAnisotropicRollingFrictionDirection() const
+	{
+		return btVector3 (0,1,0);
+	}
+
 	virtual void	setLocalScaling(const btVector3& scaling);
 
 };
@@ -93,6 +98,12 @@ class btConeShapeX : public btConeShape
 {
 	public:
 		btConeShapeX(btScalar radius,btScalar height);
+
+	virtual btVector3	getAnisotropicRollingFrictionDirection() const
+	{
+		return btVector3 (1,0,0);
+	}
+
 };
 
 ///btConeShapeZ implements a Cone shape, around the Z axis
@@ -100,6 +111,12 @@ class btConeShapeZ : public btConeShape
 {
 	public:
 		btConeShapeZ(btScalar radius,btScalar height);
+
+	virtual btVector3	getAnisotropicRollingFrictionDirection() const
+	{
+		return btVector3 (0,0,1);
+	}
+
 };
 #endif //BT_CONE_MINKOWSKI_H
 

src/BulletCollision/CollisionShapes/btCylinderShape.h

@@ -97,6 +97,13 @@ BT_DECLARE_ALIGNED_ALLOCATOR();
 		return m_upAxis;
 	}
 
+	virtual btVector3	getAnisotropicRollingFrictionDirection() const
+	{
+		btVector3 aniDir(0,0,0);
+		aniDir[getUpAxis()]=1;
+		return aniDir;
+	}
+
 	virtual btScalar getRadius() const
 	{
 		return getHalfExtentsWithMargin().getX();