Added per triangle stretching force and damping. This gives more realistics results. Also updated the tests.

Saying goodbye to mass-spring system!

include/bounce/bounce.h

@@ -68,6 +68,7 @@
 #include <bounce/cloth/cloth.h>
 #include <bounce/cloth/particle.h>
 #include <bounce/cloth/cloth_triangle.h>
+#include <bounce/cloth/strech_force.h>
 #include <bounce/cloth/spring_force.h>
 
 #include <bounce/cloth/garment/sewing_pattern.h>

include/bounce/cloth/cloth.h

@@ -57,7 +57,8 @@ struct b3ClothDef
 	{
 		mesh = nullptr;
 		density = 0.0f;
-		structural = 0.0f;
+		streching = 0.0f;
+		sewing = 0.0f;
 		bending = 0.0f;
 		damping = 0.0f;
 		thickness = 0.0f;
@@ -70,12 +71,15 @@ struct b3ClothDef
 	// Cloth density in kg/m^2
 	float32 density;
 
-	// Structural stiffness
-	float32 structural;
+	// Streching stiffness
+	float32 streching;
 
 	// Bending stiffness
 	float32 bending;
 
+	// Sewing stiffness
+	float32 sewing;
+	
 	// Damping stiffness
 	float32 damping;
 
@@ -152,6 +156,8 @@ public:
 private:
 	friend class b3Particle;
 	friend class b3ClothTriangle;
+	friend class b3StrechForce;
+	friend class b3SpringForce;
 	friend class b3ClothContactManager;
 
 	// Compute mass of each particle.

include/bounce/cloth/cloth_triangle.h

@@ -42,6 +42,7 @@ public:
 private:
 	friend class b3Cloth;
 	friend class b3Particle;
+	friend class b3StrechForce;
 	friend class b3ClothContactManager;
 	friend class b3ParticleTriangleContact;
 	friend class b3ClothSolver;
@@ -70,6 +71,14 @@ private:
 
 	// Broadphase ID
 	u32 m_broadPhaseId;
+
+	// Alpha
+	float32 m_alpha;
+
+	// Strech matrix
+	float32 m_du1, m_dv1;
+	float32 m_du2, m_dv2;
+	float32 m_inv_det;
 };
 
 inline u32 b3ClothTriangle::GetTriangle() const

include/bounce/cloth/force.h

@@ -29,6 +29,7 @@ class b3Particle;
 // Force types
 enum b3ForceType
 {
+	e_strechForce,
 	e_springForce,
 };
 

include/bounce/cloth/particle.h

@@ -120,6 +120,7 @@ private:
 	friend class b3ParticleTriangleContact;
 	friend class b3ClothContactSolver;
 	friend class b3Force;
+	friend class b3StrechForce;
 	friend class b3SpringForce;
 
 	b3Particle(const b3ParticleDef& def, b3Cloth* cloth);

include/bounce/cloth/strech_force.h

@@ -0,0 +1,126 @@
+/*
+* Copyright (c) 2016-2019 Irlan Robson https://irlanrobson.github.io
+*
+* This software is provided 'as-is', without any express or implied
+* warranty.  In no event will the authors be held liable for any damages
+* arising from the use of this software.
+* Permission is granted to anyone to use this software for any purpose,
+* including commercial applications, and to alter it and redistribute it
+* freely, subject to the following restrictions:
+* 1. The origin of this software must not be misrepresented; you must not
+* claim that you wrote the original software. If you use this software
+* in a product, an acknowledgment in the product documentation would be
+* appreciated but is not required.
+* 2. Altered source versions must be plainly marked as such, and must not be
+* misrepresented as being the original software.
+* 3. This notice may not be removed or altered from any source distribution.
+*/
+
+#ifndef B3_STRECH_FORCE_H
+#define B3_STRECH_FORCE_H
+
+#include <bounce/cloth/force.h>
+
+class b3ClothTriangle;
+
+struct b3StrechForceDef : public b3ForceDef
+{
+	b3StrechForceDef()
+	{
+		type = e_strechForce;
+	}
+
+	// Triangle
+	b3ClothTriangle* triangle;
+
+	// Streching stiffness
+	float32 streching;
+
+	// Damping stiffness
+	float32 damping;
+	
+	// Desired strechiness in u direction
+	float32 bu;
+
+	// Desired strechiness in v direction
+	float32 bv;
+};
+
+// Strech force acting on a cloth triangle.
+class b3StrechForce : public b3Force
+{
+public:
+	bool HasParticle(const b3Particle* particle) const;
+
+	b3ClothTriangle* GetTriangle() const;
+
+	float32 GetStrechingStiffness() const;
+
+	float32 GetDampingStiffness() const;
+	
+	b3Vec3 GetActionForce1() const;
+
+	b3Vec3 GetActionForce2() const;
+	
+	b3Vec3 GetActionForce3() const;
+private:
+	friend class b3Force;
+	friend class b3Cloth;
+
+	b3StrechForce(const b3StrechForceDef* def);
+	~b3StrechForce();
+
+	void Apply(const b3ClothForceSolverData* data);
+
+	// Solver shared
+
+	// Triangle
+	b3ClothTriangle* m_triangle;
+
+	// Streching stiffness
+	float32 m_ks;
+
+	// Damping stiffness
+	float32 m_kd;
+	
+	// bu
+	float32 m_bu;
+
+	// bv
+	float32 m_bv;
+
+	// Action forces
+	b3Vec3 m_f1, m_f2, m_f3;
+};
+
+inline b3ClothTriangle* b3StrechForce::GetTriangle() const
+{
+	return m_triangle;
+}
+
+inline float32 b3StrechForce::GetStrechingStiffness() const
+{
+	return m_ks;
+}
+
+inline float32 b3StrechForce::GetDampingStiffness() const
+{
+	return m_kd;
+}
+
+inline b3Vec3 b3StrechForce::GetActionForce1() const
+{
+	return m_f1;
+}
+
+inline b3Vec3 b3StrechForce::GetActionForce2() const
+{
+	return m_f2;
+}
+
+inline b3Vec3 b3StrechForce::GetActionForce3() const
+{
+	return m_f3;
+}
+
+#endif