如何在box2d中創建橢圓形狀？

Question

我需要爲body創建橢圓形狀，並且不知道如何在box2d中創建橢圓形狀。

任何想法如何做到這一點通過使用b2PolygonShape有很多頂點的不。

Answer 1

**選項1 **

利用一款機身搭配兩場比賽：第一是中央圓形給身體一些質量。第二個是橢圓形，頂點由鏈狀構成。

第一個夾具可以使您的身體像一個有質量的身體一樣行動，而第二個夾具可以讓您正確處理碰撞。

我最近做了一個輪盤賭的例子。我發佈了下面的代碼來創建它。將橢圓的常數值（OUTER_RADIUS）替換爲橢圓的極座標形式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); 

} 

您還可以檢查出this post的輪盤賭（加圖片善良）的解決方案。

選項＃2 除了使用鏈條/邊緣形狀，您可以創建一個夾具的三角形風扇。一旦你有橢圓的頂點，你可以做類似的事情：

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); 
} 

對您有幫助嗎？