Bullet 学习笔记之 CollisionShape 和 CollisionObject

先初步看一下碰撞相关的对象都有些什么。

（小疑惑，为什么对在 btDiscreteDynamicsWorld 外面单独有一个 CollisionShape，好像是说，CollisionShape 是包含在 CollisionObject 里面的。难道是为了节省内存，对于不同的 CollisionObject，如果他们的碰撞外形相同，那么就可以只建立一个 CollisionShape ，在不同的 CollisionObject 之间共享。如果是这样的话，那也太细了吧）

（TM 果真是这样的。作者在代码中解释道 Re-using the same collision is better for memory usage and performance）

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碰撞外形类 CollisionShape

基类定义在 bullet3/src/BulletCollision/CollisionShapes/btCollisionShape.h 中，为 btCollisionShape ，作者这样描述该类：///The btCollisionShape class provides an interface for collision shapes that can be shared among btCollisionObjects.

在 btCollisionShape 类中，成员变量也很简单：

int m_shapeType;
void* m_userPointer;
int m_userIndex;

从成员/接口函数分析它的主要功能，有

///getAabb returns the axis aligned bounding box in the coordinate frame of the given transform t.
virtual void getAabb(const btTransform& t, btVector3& aabbMin, btVector3& aabbMax) const = 0;

virtual void getBoundingSphere(btVector3 & center, btScalar & radius) const;

///getAngularMotionDisc returns the maximum radius needed for Conservative Advancement to handle time-of-impact with rotations.
virtual btScalar getAngularMotionDisc() const;

virtual btScalar getContactBreakingThreshold(btScalar defaultContactThresholdFactor) const;

///calculateTemporalAabb calculates the enclosing aabb for the moving object over interval [0..timeStep)
///result is conservative
void calculateTemporalAabb(const btTransform& curTrans, const btVector3& linvel, const btVector3& angvel, btScalar timeStep, btVector3& temporalAabbMin, btVector3& temporalAabbMax) const;

感觉，大概就 AABB，碰撞检测相关的一些东西吧。

从 btCollisionShape 的成员变量可知，对于碰撞形状，也分为不同的类型。具体有：（还挺多的 >_<）

/// btDispatcher uses these types
/// IMPORTANT NOTE:The types are ordered polyhedral, implicit convex and concave
/// to facilitate type checking
/// CUSTOM_POLYHEDRAL_SHAPE_TYPE,CUSTOM_CONVEX_SHAPE_TYPE and CUSTOM_CONCAVE_SHAPE_TYPE can be used to extend Bullet without modifying source code
enum BroadphaseNativeTypes
{
	// polyhedral convex shapes
	BOX_SHAPE_PROXYTYPE,
	TRIANGLE_SHAPE_PROXYTYPE,
	TETRAHEDRAL_SHAPE_PROXYTYPE,
	CONVEX_TRIANGLEMESH_SHAPE_PROXYTYPE,
	CONVEX_HULL_SHAPE_PROXYTYPE,
	CONVEX_POINT_CLOUD_SHAPE_PROXYTYPE,
	CUSTOM_POLYHEDRAL_SHAPE_TYPE,
	//implicit convex shapes
	IMPLICIT_CONVEX_SHAPES_START_HERE,
	SPHERE_SHAPE_PROXYTYPE,
	MULTI_SPHERE_SHAPE_PROXYTYPE,
	CAPSULE_SHAPE_PROXYTYPE,
	CONE_SHAPE_PROXYTYPE,
	CONVEX_SHAPE_PROXYTYPE,
	CYLINDER_SHAPE_PROXYTYPE,
	UNIFORM_SCALING_SHAPE_PROXYTYPE,
	MINKOWSKI_SUM_SHAPE_PROXYTYPE,
	MINKOWSKI_DIFFERENCE_SHAPE_PROXYTYPE,
	BOX_2D_SHAPE_PROXYTYPE,
	CONVEX_2D_SHAPE_PROXYTYPE,
	CUSTOM_CONVEX_SHAPE_TYPE,
	//concave shapes
	CONCAVE_SHAPES_START_HERE,
	//keep all the convex shapetype below here, for the check IsConvexShape in broadphase proxy!
	TRIANGLE_MESH_SHAPE_PROXYTYPE,
	SCALED_TRIANGLE_MESH_SHAPE_PROXYTYPE,
	///used for demo integration FAST/Swift collision library and Bullet
	FAST_CONCAVE_MESH_PROXYTYPE,
	//terrain
	TERRAIN_SHAPE_PROXYTYPE,
	///Used for GIMPACT Trimesh integration
	GIMPACT_SHAPE_PROXYTYPE,
	///Multimaterial mesh
	MULTIMATERIAL_TRIANGLE_MESH_PROXYTYPE,

	EMPTY_SHAPE_PROXYTYPE,
	STATIC_PLANE_PROXYTYPE,
	CUSTOM_CONCAVE_SHAPE_TYPE,
	SDF_SHAPE_PROXYTYPE = CUSTOM_CONCAVE_SHAPE_TYPE,
	CONCAVE_SHAPES_END_HERE,

	COMPOUND_SHAPE_PROXYTYPE,

	SOFTBODY_SHAPE_PROXYTYPE,
	HFFLUID_SHAPE_PROXYTYPE,
	HFFLUID_BUOYANT_CONVEX_SHAPE_PROXYTYPE,
	INVALID_SHAPE_PROXYTYPE,

	MAX_BROADPHASE_COLLISION_TYPES

};

感觉其他派生出的形状也都差不多，无非就是 AABB，碰撞间隙，形状信息等等。

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碰撞对象 CollisionObject

该基类定义在 bullet3/src/BulletCollision/CollisionDispatch/btCollisionObject.h
作者对该类的描述为

/// btCollisionObject can be used to manage collision detection objects.
/// btCollisionObject maintains all information that is needed for a collision detection: Shape, Transform and AABB proxy.
/// They can be added to the btCollisionWorld.

CollisionObject 类就复杂多了，针对场景中的每一个对象的信息，包含了形状、位姿、摩擦等多种信息。可以说，每一个 CollisionObject 实例，都代表了仿真场景中的一个实体。

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