Rolling resistance (erincatto#876)

Added rolling resistance. Rolling resistance is a static resistance with
a constraint, so it can work on ramps. The resistance scales based on
the contact normal forces and the radius of the shape.

Added tangent speed to all shapes for conveyor belts, etc.

Added b2SurfaceMaterial to support separate properties for each segment
of a chain shape.

Author: erincatto

benchmark/amd7950x_avx2/joint_grid.csv

@@ -1,9 +1,9 @@
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benchmark/amd7950x_avx2/large_pyramid.csv

@@ -1,9 +1,9 @@
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benchmark/amd7950x_avx2/many_pyramids.csv

@@ -1,9 +1,9 @@
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benchmark/amd7950x_avx2/rain.csv

@@ -1,9 +1,9 @@
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benchmark/amd7950x_avx2/smash.csv

@@ -1,9 +1,9 @@
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benchmark/amd7950x_avx2/spinner.csv

@@ -1,9 +1,9 @@
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benchmark/amd7950x_avx2/tumbler.csv

@@ -1,9 +1,9 @@
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include/box2d/collision.h

@@ -535,14 +535,18 @@ typedef struct b2ManifoldPoint
 /// @note Box2D uses speculative collision so some contact points may be separated.
 typedef struct b2Manifold
 {
-	/// The manifold points, up to two are possible in 2D
-	b2ManifoldPoint points[2];
-
 	/// The unit normal vector in world space, points from shape A to bodyB
 	b2Vec2 normal;
 
+	/// Angular impulse applied for rolling resistance. N * m * s = kg * m^2 / s
+	float rollingImpulse;
+
+	/// The manifold points, up to two are possible in 2D
+	b2ManifoldPoint points[2];
+
 	/// The number of contacts points, will be 0, 1, or 2
 	int pointCount;
+
 } b2Manifold;
 
 /// Compute the contact manifold between two circles

include/box2d/types.h

@@ -346,9 +346,11 @@ typedef struct b2ShapeDef
 	float restitution;
 
 	/// The rolling resistance usually in the range [0,1].
-	/// todo
 	float rollingResistance;
 
+	/// The tangent speed for conveyor belts
+	float tangentSpeed;
+
 	/// User material identifier. This is passed with query results and to friction and restitution
 	/// combining functions. It is not used internally.
 	int material;
@@ -394,6 +396,35 @@ typedef struct b2ShapeDef
 /// @ingroup shape
 B2_API b2ShapeDef b2DefaultShapeDef( void );
 
+/// Surface materials allow chain shapes to have per segment surface properties.
+/// @ingroup shape
+typedef struct b2SurfaceMaterial
+{
+	/// The Coulomb (dry) friction coefficient, usually in the range [0,1].
+	float friction;
+
+	/// The coefficient of restitution (bounce) usually in the range [0,1].
+	/// https://en.wikipedia.org/wiki/Coefficient_of_restitution
+	float restitution;
+
+	/// The rolling resistance usually in the range [0,1].
+	float rollingResistance;
+
+	/// The tangent speed for conveyor belts
+	float tangentSpeed;
+
+	/// User material identifier. This is passed with query results and to friction and restitution
+	/// combining functions. It is not used internally.
+	int material;
+
+	/// Custom debug draw color.
+	uint32_t customColor;
+} b2SurfaceMaterial;
+
+/// Use this to initialize your surface material
+/// @ingroup shape
+B2_API b2SurfaceMaterial b2DefaultSurfaceMaterial( void );
+
 /// Used to create a chain of line segments. This is designed to eliminate ghost collisions with some limitations.
 /// - chains are one-sided
 /// - chains have no mass and should be used on static bodies
@@ -420,28 +451,19 @@ typedef struct b2ChainDef
 	/// The point count, must be 4 or more.
 	int count;
 
-	/// The friction coefficient, usually in the range [0,1].
-	float friction;
-
-	/// The restitution (elasticity) usually in the range [0,1].
-	float restitution;
+	/// Surface materials for each segment. These are cloned.
+	const b2SurfaceMaterial* materials;
 
-	/// User material identifier. This is passed with query results and to friction and restitution
-	/// combining functions. It is not used internally.
-	int material;
+	/// The material count. Must be 1 or count. This allows you to provide one
+	/// material for all segments or a unique material per segment.
+	int materialCount;
 
 	/// Contact filtering data.
 	b2Filter filter;
 
-	/// Custom debug draw color.
-	uint32_t customColor;
-
 	/// Indicates a closed chain formed by connecting the first and last points
 	bool isLoop;
 
-	/// Generate events when a sensor overlaps this chain
-	bool enableSensorEvents;
-
 	/// Used internally to detect a valid definition. DO NOT SET.
 	int internalValue;
 } b2ChainDef;

samples/car.cpp

@@ -53,6 +53,7 @@ void Car::Spawn( b2WorldId worldId, b2Vec2 position, float scale, float hertz, f
 
 	shapeDef.density = 2.0f / scale;
 	shapeDef.friction = 1.5f;
+	shapeDef.rollingResistance = 0.1f;
 
 	bodyDef.position = b2Add( { -1.0f * scale, 0.35f * scale }, position );
 	bodyDef.allowFastRotation = true;