switch from global to local damping

include/bounce/dynamics/body.h

@@ -59,18 +59,47 @@ struct b3BodyDef
 		linearVelocity.SetZero();
 		angularVelocity.SetZero();
 		gravityScale = 1.0f;
+		linearDamping = 0.0f;
+		angularDamping = 0.0f;
 	}
 
+	//
 	b3BodyType type;
+	
+	//
 	bool awake;
+	
+	//
 	bool fixedRotationX;
+	
+	//
 	bool fixedRotationY;
+	
+	//
 	bool fixedRotationZ;
+	
+	//
 	void* userData;
+	
+	//
 	b3Vec3 position;
+	
+	//
 	b3Quat orientation;
+	
+	//
 	b3Vec3 linearVelocity;
+	
+	//
 	b3Vec3 angularVelocity;
+	
+	//
+	float32 linearDamping;
+
+	//
+	float32 angularDamping;
+
+	//
 	float32 gravityScale;
 };
 
@@ -210,10 +239,22 @@ public:
 	// Transform a frame to the world space.
 	b3Transform GetWorldFrame(const b3Transform& localFrame) const;
 
-	// Get the gravity scale of the body. One is used by default.
+	// Get the linear damping of the body. 
+	float32 GetLinearDamping() const;
+	
+	// Set the linear damping of the body. Zero is set by default.
+	void SetLinearDamping(float32 damping);
+
+	// Get the angular damping of the body. 
+	float32 GetAngularDamping() const;
+
+	// Set the angular damping of the body. Zero is set by default.
+	void SetAngularDamping(float32 damping);
+
+	// Get the gravity scale of the body. 
 	float32 GetGravityScale() const;
 
-	// Set the gravity scale of the body.
+	// Set the gravity scale of the body. One is set by default.
 	void SetGravityScale(float32 scale);
 
 	// See if the body is awake.
@@ -310,12 +351,15 @@ private:
 	// Inverse inertia about the body world center of mass.
 	b3Mat33 m_worldInvI;
 	
-	float32 m_gravityScale;
 	b3Vec3 m_force;
 	b3Vec3 m_torque;
 	b3Vec3 m_linearVelocity;
 	b3Vec3 m_angularVelocity;
 	
+	float32 m_linearDamping;
+	float32 m_angularDamping;
+	float32 m_gravityScale;
+	
 	// Motion proxy for CCD.
 	b3Sweep m_sweep;
 
@@ -477,6 +521,26 @@ inline void b3Body::SetAwake(bool flag)
 	}
 }
 
+inline float32 b3Body::GetLinearDamping() const
+{
+	return m_linearDamping;
+}
+
+inline void b3Body::SetLinearDamping(float32 damping) 
+{
+	m_linearDamping = damping;
+}
+
+inline float32 b3Body::GetAngularDamping() const
+{
+	return m_angularDamping;
+}
+
+inline void b3Body::SetAngularDamping(float32 damping) 
+{
+	m_angularDamping = damping;
+}
+
 inline float32 b3Body::GetGravityScale() const
 { 
 	return m_gravityScale; 

src/bounce/dynamics/body.cpp

@@ -79,6 +79,8 @@ b3Body::b3Body(const b3BodyDef& def, b3World* world)
 	m_xf.position = m_sweep.worldCenter;
 	m_xf.rotation = b3ConvertQuatToMat(m_sweep.orientation);
 	
+	m_linearDamping = def.linearDamping;
+	m_angularDamping = def.angularDamping;
 	m_gravityScale = def.gravityScale;
 	m_userData = def.userData;
 	m_islandID = -1;

src/bounce/dynamics/island.cpp

@@ -108,14 +108,6 @@ void b3Island::Solve(const b3Vec3& gravity, float32 dt, u32 velocityIterations,
 {
 	float32 h = dt;
 
-	// Apply some damping.
-	// ODE: dv/dt + c * v = 0
-	// Solution: v(t) = v0 * exp(-c * t)
-	// Step: v(t + dt) = v0 * exp(-c * (t + dt)) = v0 * exp(-c * t) * exp(-c * dt) = v * exp(-c * dt)
-	// v2 = exp(-c * dt) * v1
-	const float32 k_d = 0.005f;
-	float32 d = exp(-k_d * h);
-
 	// 1. Integrate velocities
 	for (u32 i = 0; i < m_bodyCount; ++i) 
 	{
@@ -165,9 +157,15 @@ void b3Island::Solve(const b3Vec3& gravity, float32 dt, u32 velocityIterations,
 			// Clear torques
 			b->m_torque.SetZero();
 			
-			// Apply damping
-			v *= d;
-			w *= d;
+			// Apply local damping.
+			// ODE: dv/dt + c * v = 0
+			// Solution: v(t) = v0 * exp(-c * t)
+			// Step: v(t + dt) = v0 * exp(-c * (t + dt)) = v0 * exp(-c * t) * exp(-c * dt) = v * exp(-c * dt)
+			// v2 = exp(-c * dt) * v1
+			// Pad<61> approximation:
+			// 1 / (1 + c * dt) 
+			v *= 1.0f / (1.0f + h * b->m_linearDamping);
+			w *= 1.0f / (1.0f + h * b->m_angularDamping);
 		}
 
 		m_velocities[i].v = v;