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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 };
时间： 2024-10-08 04:53:30