如何在box2d中创建椭圆形状？

Question

我需要为body创建椭圆形状，并且不知道如何在box2d中创建椭圆形状。

任何想法如何做到这一点通过使用b2PolygonShape有很多顶点的不。

Answer 1

**选项1 **

利用一款机身搭配两场比赛：第一是中央圆形给身体一些质量。第二个是椭圆形，顶点由链状构成。

第一个夹具可以使您的身体像一个有质量的身体一样行动，而第二个夹具可以让您正确处理碰撞。

我最近做了一个轮盘赌的例子。我发布了下面的代码来创建它。将椭圆的常数值（OUTER_RADIUS）替换为椭圆的极坐标形式found here on wikipedia。

void MainScene::CreateBody() 
{ 
    const float32 INNER_RADIUS = 2.50; 
    const float32 OUTER_RADIUS = 3.0; 
    const float32 BALL_RADIUS = 0.1; 
    const uint32 DIVISIONS = 36; 

    Vec2 position(0,0); 

    // Create the body. 
    b2BodyDef bodyDef; 
    bodyDef.position = position; 
    bodyDef.type = b2_dynamicBody; 
    _body = _world->CreateBody(&bodyDef); 
    assert(_body != NULL); 

    // Now attach fixtures to the body. 
    FixtureDef fixtureDef; 
    fixtureDef.density = 1.0; 
    fixtureDef.friction = 1.0; 
    fixtureDef.restitution = 0.9; 
    fixtureDef.isSensor = false; 

    // Inner circle. 
    b2CircleShape circleShape; 
    circleShape.m_radius = INNER_RADIUS; 
    fixtureDef.shape = &circleShape; 
    _body->CreateFixture(&fixtureDef); 

    // Outer shape. 
    b2ChainShape chainShape; 
    vector<Vec2> vertices; 
    const float32 SPIKE_DEGREE = 2*M_PI/180; 
    for(int idx = 0; idx < DIVISIONS; idx++) 
    { 
     float32 angle = ((M_PI*2)/DIVISIONS)*idx; 
     float32 xPos, yPos; 

     xPos = OUTER_RADIUS*cosf(angle); 
     yPos = OUTER_RADIUS*sinf(angle); 
     vertices.push_back(Vec2(xPos,yPos)); 
    } 
    vertices.push_back(vertices[0]); 
    chainShape.CreateChain(&vertices[0], vertices.size()); 
    fixtureDef.shape = &chainShape; 
    _body->CreateFixture(&fixtureDef); 

} 

您还可以检查出this post的轮盘赌（加图片善良）的解决方案。

选项＃2 除了使用链条/边缘形状，您可以创建一个夹具的三角形风扇。一旦你有椭圆的顶点，你可以做类似的事情：

b2Vec2 triVerts[3]; 
triVerts[0] = Vec2(0,0); 
for(int idx = 1; idx < vertices.size(); idx++) 
{ 
    b2PolygonShape triangle; 
    fixtureDef.shape = &triangle; 
    // Assumes the vertices are going around 
    // the ellipse clockwise as angle increases. 
    triVerts[1] = vertices[idx-1]; 
    triVerts[2] = vertices[idx]; 
    triangle.Set(triVerts,3); 
    _body->CreateFixture(&fixtureDef); 
} 

对您有帮助吗？