cocos2d-x box2d 简单物体(二)

物体的形状，大小，材质，密度，弹性等属性由定制器描述(fixture)，有人翻译成夹具。物体碰撞后会根据各自的定制器做出反应。定制器主要属性如下

-形状

-弹性

-摩擦

-密度

物体间的碰撞依赖物体的形状，box2d可以定义圆弧，矩形和多边形。

    {
        // 圆形
        bodyDef.type = b2_dynamicBody;
        bodyDef.position.Set(2.0, 3.0); // body position
        b2Body* circleBody = mWorld->CreateBody(&bodyDef);
        
        b2CircleShape circleShape;
        circleShape.m_p.Set(0, 0);      // 圆心，相对其 body position
        circleShape.m_radius = 0.5;     // 半径
        circleBody->CreateFixture(&circleShape,1);
        
    }
    {
        // 三角形
        bodyDef.type = b2_dynamicBody;
        bodyDef.position.Set(4.0, 3.0); // body position
        b2Body* triangleBody = mWorld->CreateBody(&bodyDef);
        
        b2PolygonShape triangleShape;
        b2Vec2 vertices[3];             // 顶点数组
        vertices[0].Set(-0.5, -0.5);
        vertices[1].Set(0.5, 0.5);
        vertices[2].Set(0, 1);
        triangleShape.Set(vertices, 3);
        triangleBody->CreateFixture(&triangleShape,1);
    }
    {
        // 矩形
        bodyDef.position.Set(8.0, 3.0);
        b2Body* rectangle = mWorld->CreateBody(&bodyDef);
        
        b2PolygonShape rectangleShape;
        rectangleShape.SetAsBox(1, 2);  // 设置矩形半高半宽
        rectangle->CreateFixture(&rectangleShape,1);
    }

其实矩形也是多边形，看矩形的SetAsBox函数的实现

void b2PolygonShape::SetAsBox(float32 hx, float32 hy)
{
    m_vertexCount = 4;
    m_vertices[0].Set(-hx, -hy);
    m_vertices[1].Set( hx, -hy);
    m_vertices[2].Set( hx,  hy);
    m_vertices[3].Set(-hx,  hy);
    m_normals[0].Set(0.0f, -1.0f);
    m_normals[1].Set(1.0f, 0.0f);
    m_normals[2].Set(0.0f, 1.0f);
    m_normals[3].Set(-1.0f, 0.0f);
    m_centroid.SetZero();
}

注意，圆心的坐标是相对于其body position而言的，多边形的顶点坐标也是相对body position而言。多边形顶点在右手坐标系中逆时针旋转。

box2d默认的多边形最大边是8，如果你要更多的顶点多边形，需要改这个值，在Box2D/Common/b2Settings.h里面定义的一个宏

/// The maximum number of vertices on a convex polygon. You cannot increase
/// this too much because b2BlockAllocator has a maximum object size.
#define b2_maxPolygonVertices    8

cocos2d-x box2d 简单物体(二),布布扣,bubuko.com

cocos2d-x box2d 简单物体(二)

原文：http://blog.csdn.net/dawn_moon/article/details/20476091