Fix for issue 632 (erincatto#642)

- Fixes issue erincatto#632
- Originally the idea was to rename rope to cloth to avoid confusion with the rope joint. However, I realized the distance joint could be extended to cover the rope joint functionality.
- Extended distance joint to have a minimum and maximum limit
- Removed rope joint
- This is API breaking: 0 stiffness now means the spring is turned off rather than making the joint rigid.

include/box2d/b2_distance_joint.h

@@ -26,10 +26,9 @@
 #include "b2_joint.h"
 
 /// Distance joint definition. This requires defining an anchor point on both
-/// bodies and the non-zero length of the distance joint. The definition uses local anchor points
-/// so that the initial configuration can violate the constraint
-/// slightly. This helps when saving and loading a game.
-/// @warning Do not use a zero or short length.
+/// bodies and the non-zero distance of the distance joint. The definition uses
+/// local anchor points so that the initial configuration can violate the
+/// constraint slightly. This helps when saving and loading a game.
 struct b2DistanceJointDef : public b2JointDef
 {
 	b2DistanceJointDef()
@@ -38,12 +37,14 @@ struct b2DistanceJointDef : public b2JointDef
 		localAnchorA.Set(0.0f, 0.0f);
 		localAnchorB.Set(0.0f, 0.0f);
 		length = 1.0f;
+		minLength = 0.0f;
+		maxLength = FLT_MAX;
 		stiffness = 0.0f;
 		damping = 0.0f;
 	}
 
-	/// Initialize the bodies, anchors, and length using the world
-	/// anchors.
+	/// Initialize the bodies, anchors, and rest length using world space anchors.
+	/// The minimum and maximum lengths are set to the rest length.
 	void Initialize(b2Body* bodyA, b2Body* bodyB,
 					const b2Vec2& anchorA, const b2Vec2& anchorB);
 
@@ -53,10 +54,16 @@ struct b2DistanceJointDef : public b2JointDef
 	/// The local anchor point relative to bodyB's origin.
 	b2Vec2 localAnchorB;
 
-	/// The natural length between the anchor points.
+	/// The rest length of this joint. Clamped to a stable minimum value.
 	float length;
 
-	/// The linear stiffness in N/m. A value of 0 disables softness.
+	/// Minimum length. Clamped to a stable minimum value.
+	float minLength;
+
+	/// Maximum length. Must be greater than or equal to the minimum length.
+	float maxLength;
+
+	/// The linear stiffness in N/m.
 	float stiffness;
 
 	/// The linear damping in N*s/m.
@@ -86,11 +93,30 @@ class b2DistanceJoint : public b2Joint
 	/// The local anchor point relative to bodyB's origin.
 	const b2Vec2& GetLocalAnchorB() const  { return m_localAnchorB; }
 
-	/// Set/get the natural length.
-	/// Manipulating the length can lead to non-physical behavior when the frequency is zero.
-	void SetLength(float length) { m_length = length; }
+	/// Get the rest length
 	float GetLength() const { return m_length; }
 
+	/// Set the rest length
+	/// @returns clamped rest length
+	float SetLength(float length);
+
+	/// Get the minimum length
+	float GetMinLength() const { return m_minLength; }
+
+	/// Set the minimum length
+	/// @returns the clamped minimum length
+	float SetMinLength(float minLength);
+
+	/// Get the maximum length
+	float GetMaxLength() const { return m_maxLength; }
+
+	/// Set the maximum length
+	/// @returns the clamped maximum length
+	float SetMaxLength(float maxLength);
+
+	/// Get the current length
+	float GetCurrentLength() const;
+
 	/// Set/get the linear stiffness in N/m
 	void SetStiffness(float stiffness) { m_stiffness = stiffness; }
 	float GetStiffness() const { return m_stiffness; }
@@ -102,6 +128,9 @@ class b2DistanceJoint : public b2Joint
 	/// Dump joint to dmLog
 	void Dump() override;
 
+	///
+	void Draw(b2Draw* draw) const override;
+
 protected:
 
 	friend class b2Joint;
@@ -114,13 +143,17 @@ class b2DistanceJoint : public b2Joint
 	float m_stiffness;
 	float m_damping;
 	float m_bias;
+	float m_length;
+	float m_minLength;
+	float m_maxLength;
 
 	// Solver shared
 	b2Vec2 m_localAnchorA;
 	b2Vec2 m_localAnchorB;
 	float m_gamma;
 	float m_impulse;
-	float m_length;
+	float m_lowerImpulse;
+	float m_upperImpulse;
 
 	// Solver temp
 	int32 m_indexA;
@@ -130,10 +163,12 @@ class b2DistanceJoint : public b2Joint
 	b2Vec2 m_rB;
 	b2Vec2 m_localCenterA;
 	b2Vec2 m_localCenterB;
+	float m_currentLength;
 	float m_invMassA;
 	float m_invMassB;
 	float m_invIA;
 	float m_invIB;
+	float m_softMass;
 	float m_mass;
 };
 

include/box2d/box2d.h

@@ -52,7 +52,6 @@
 #include "b2_prismatic_joint.h"
 #include "b2_pulley_joint.h"
 #include "b2_revolute_joint.h"
-#include "b2_rope_joint.h"
 #include "b2_weld_joint.h"
 #include "b2_wheel_joint.h"
 

src/CMakeLists.txt

@@ -48,7 +48,6 @@ set(BOX2D_SOURCE_FILES
 	dynamics/b2_prismatic_joint.cpp
 	dynamics/b2_pulley_joint.cpp
 	dynamics/b2_revolute_joint.cpp
-	dynamics/b2_rope_joint.cpp
 	dynamics/b2_weld_joint.cpp
 	dynamics/b2_wheel_joint.cpp
 	dynamics/b2_world.cpp
@@ -82,7 +81,6 @@ set(BOX2D_HEADER_FILES
 	../include/box2d/b2_pulley_joint.h
 	../include/box2d/b2_revolute_joint.h
 	../include/box2d/b2_rope.h
-	../include/box2d/b2_rope_joint.h
 	../include/box2d/b2_settings.h
 	../include/box2d/b2_shape.h
 	../include/box2d/b2_stack_allocator.h