先初步看一下碰撞相關的對象都有些什么。

（小疑惑，為什么對在 btDiscreteDynamicsWorld 外面單獨有一個 CollisionShape，好像是說，CollisionShape 是包含在 CollisionObject 里面的。難道是為了節省內存，對于不同的 CollisionObject，如果他們的碰撞外形相同，那么就可以只建立一個 CollisionShape ，在不同的 CollisionObject 之間共享。如果是這樣的話，那也太細了吧）

（TM 果真是這樣的。作者在代碼中解釋道 Re-using the same collision is better for memory usage and performance）

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碰撞外形類 CollisionShape

基類定義在 bullet3/src/BulletCollision/CollisionShapes/btCollisionShape.h 中，為 btCollisionShape ，作者這樣描述該類：///The btCollisionShape class provides an interface for collision shapes that can be shared among btCollisionObjects.

在 btCollisionShape 類中，成員變量也很簡單：

int m_shapeType;
void* m_userPointer;
int m_userIndex;

從成員/接口函數分析它的主要功能，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;

感覺，大概就 AABB，碰撞檢測相關的一些東西吧。

從 btCollisionShape 的成員變量可知，對于碰撞形狀，也分為不同的類型。具體有：（還挺多的 >_<）

/// 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

};

感覺其他派生出的形狀也都差不多，無非就是 AABB，碰撞間隙，形狀信息等等。

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碰撞對象 CollisionObject

該基類定義在 bullet3/src/BulletCollision/CollisionDispatch/btCollisionObject.h
作者對該類的描述為

/// 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 類就復雜多了，針對場景中的每一個對象的信息，包含了形狀、位姿、摩擦等多種信息。可以說，每一個 CollisionObject 實例，都代表了仿真場景中的一個實體。

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