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Box2D是一个用于模拟2D刚体物体的C++引擎。Box2D集成了大量的物理力学和运动学的计算，并将物理模拟过程封装到类对象中，将对物体的操作，以简单友好的接口提供给开发者。我们只需要调用引擎中相应的对象或函数，就可以模拟现实生活中的加速、减速、抛物线运动、万有引力、碰撞反弹等等各种真实的物理运动。
Box2D中的名词：
>>世界(World)
传说世界本是一片混沌，自从盘古开天辟地之后，盘古神斧砍下出了天和地，形成了真正的世界，而后形成才有了山川、河流、花草、树木。Box2D也一样，现在Box2D在我们的脑中也是混沌的一片，我们要创建物体或进行物理模拟之前，首先也是先创建物理世界。
在Box2D中用b2World类来表示世界。它是Box2D的一个核心类之一，集成了Box2D对所有对象的创建、删除、碰撞模拟的相关接口。
>>刚体(b2Body)
生活中我们看到的任何物体都可以用东西来描述，飞的小鸟，马路上行驶的汽车，等等。“东西”这个词在Box2D的字典中叫做“刚体”（b2Body），其实刚体也就是物理世界中的物体。b2Body是Box2D的核心类，是学习Box2D的基础，也是重中之重。b2Body用来模拟现实物理世界中的所有物体。Box2D中的任何碰撞、反弹、运动轨迹等各种物理现象模拟和数据计算都是基于刚体实现的，所以刚体b2Body所包含的信息有很多，如物体的坐标、角度、受力大小、速度、质量等大量的信息。
Box2D引擎中b2Body定义：
1 /// The body type. 2 /// static: zero mass, zero velocity, may be manually moved 3 /// kinematic: zero mass, non-zero velocity set by user, moved by solver 4 /// dynamic: positive mass, non-zero velocity determined by forces, moved by solver 5 enum b2BodyType 6 { 7 b2_staticBody = 0, //静止Body 8 b2_kinematicBody, //浮动Body 9 b2_dynamicBody //动态Body 10 // TODO_ERIN 11 //b2_bulletBody, 12 }; 13 14 /// A body definition holds all the data needed to construct a rigid body. 15 /// You can safely re-use body definitions. Shapes are added to a body after construction. 16 struct b2BodyDef 17 { 18 /// This constructor sets the body definition default values. 19 b2BodyDef() 20 { 21 userData = NULL; 22 position.Set(0.0f, 0.0f); //位置 23 angle = 0.0f; //弧度 24 linearVelocity.Set(0.0f, 0.0f); //直线速度设置 25 angularVelocity = 0.0f; 26 linearDamping = 0.0f; //直线阻尼 27 angularDamping = 0.0f; 28 allowSleep = true; 29 awake = true; 30 fixedRotation = false; //角度 31 bullet = false; 32 type = b2_staticBody; 33 active = true; 34 gravityScale = 1.0f; 35 } 36 37 /// The body type: static, kinematic, or dynamic. 38 /// Note: if a dynamic body would have zero mass, the mass is set to one. 39 40 ///如果一个动态的身体将有零质量，质量被设置为一。 41 b2BodyType type; 42 43 /// The world position of the body. Avoid creating bodies at the origin 44 /// since this can lead to many overlapping shapes. 45 46 /// 刚体在物理世界中的位置 47 b2Vec2 position; 48 49 /// The world angle of the body in radians. 50 float32 angle; 51 52 /// The linear velocity of the body's origin in world co-ordinates. 53 b2Vec2 linearVelocity; 54 55 /// The angular velocity of the body. 56 float32 angularVelocity; 57 58 /// Linear damping is use to reduce the linear velocity. The damping parameter 59 /// can be larger than 1.0f but the damping effect becomes sensitive to the 60 /// time step when the damping parameter is large. 61 float32 linearDamping; 62 63 /// Angular damping is use to reduce the angular velocity. The damping parameter 64 /// can be larger than 1.0f but the damping effect becomes sensitive to the 65 /// time step when the damping parameter is large. 66 float32 angularDamping; 67 68 /// Set this flag to false if this body should never fall asleep. Note that 69 /// this increases CPU usage. 70 bool allowSleep; 71 72 /// Is this body initially awake or sleeping? 73 bool awake; 74 75 /// Should this body be prevented from rotating? Useful for characters. 76 bool fixedRotation; 77 78 /// Is this a fast moving body that should be prevented from tunneling through 79 /// other moving bodies? Note that all bodies are prevented from tunneling through 80 /// kinematic and static bodies. This setting is only considered on dynamic bodies. 81 /// @warning You should use this flag sparingly since it increases processing time. 82 bool bullet; 83 84 /// Does this body start out active? 85 bool active; 86 87 /// Use this to store application specific body data. 88 void* userData; 89 90 /// Scale the gravity applied to this body. 91 float32 gravityScale; 92 };
>>夹具(Fixture)
Fixture在Box2D中是一种夹具，主要作用是用来定义刚体所固有的一些属性，并保存在b2Fixture对象中。现实中通常是物体材料特性相关的一些属性，如刚体的密度、摩擦系数等属性都是由b2FixtureDef保存的。
Box2D中b2FixtureDef结构体定义：
1 /// A fixture definition is used to create a fixture. This class defines an 2 /// abstract fixture definition. You can reuse fixture definitions safely. 3 struct b2FixtureDef 4 { 5 /// The constructor sets the default fixture definition values. 6 b2FixtureDef() 7 { 8 shape = NULL; 9 userData = NULL; 10 friction = 0.2f; 11 restitution = 0.0f; 12 density = 0.0f; 13 isSensor = false; 14 } 15 16 /// The shape, this must be set. The shape will be cloned, so you 17 /// can create the shape on the stack. 18 const b2Shape* shape; 19 20 /// Use this to store application specific fixture data. 21 void* userData; 22 23 /// The friction coefficient, usually in the range [0,1]. 24 float32 friction; 25 26 /// The restitution (elasticity) usually in the range [0,1]. 27 float32 restitution; 28 29 /// The density, usually in kg/m^2. 30 float32 density; 31 32 /// A sensor shape collects contact information but never generates a collision 33 /// response. 34 bool isSensor; 35 36 /// Contact filtering data. 37 b2Filter filter; 38 };
>>形状(Shape)
形状是一个b2Shape类型的对象，实现了刚体的具体形状，Box2D将基于这个形状进行精确的物理碰撞模拟。实际上，b2Shape只是一个抽象的父类，没有实际创建形状的过程。在实际开发过程中，b2FixtureDef.shape的属性值都是b2CircleShape、b2PolygonShape等b2Shape的子类对象。
Box2D中Shape的定义：
1 /// A shape is used for collision detection. You can create a shape however you like. 2 /// Shapes used for simulation in b2World are created automatically when a b2Fixture 3 /// is created. Shapes may encapsulate a one or more child shapes. 4 5 class b2Shape 6 { 7 public: 8 9 enum Type 10 { 11 e_circle = 0, //圆形 12 e_edge = 1, //边界 13 e_polygon = 2, //自定义 14 e_chain = 3, 15 e_typeCount = 4 16 }; 17 //余下部分省略 18 };
