iphone - box2d 凹体

Question

我正在为我的 iOS 游戏创建一个 box2d 主体，它由 4 个凸形形状构建而成。 问题是调用init方法时失败。 这是我的代码:

@implementation Banan
-(void)createBodyAtLocation:(CGPoint)location
{
    int num;
    float density = 1.0f;

    b2BodyDef bodyDef;
    bodyDef.type = b2_dynamicBody;
    bodyDef.position = b2Vec2(location.x/PTM_RATIO/RETSIZE, location.y/PTM_RATIO/RETSIZE);
    body = world->CreateBody(&bodyDef);
    body -> SetUserData(self);

    //first shape
    b2FixtureDef fixtureDef1;
    b2PolygonShape shape1;
    num = 4;
    b2Vec2 verts1[4];
        verts1[0].Set(58.9f / PTM_RATIO, 46.3f / PTM_RATIO);
         verts1[1].Set(63.3f / PTM_RATIO, 41.5f / PTM_RATIO);
        verts1[2].Set(47.4f / PTM_RATIO, 15.6f / PTM_RATIO);
         verts1[3].Set(43.6f / PTM_RATIO, 24.3f / PTM_RATIO);
    shape1.Set(verts1, num);
    fixtureDef1.shape = &shape1;
    fixtureDef1.density = density;


    //second
    b2FixtureDef fixtureDef2;
    b2PolygonShape shape2;
    num = 5;
    b2Vec2 verts2[5];
    verts2[0].Set(42.1f / PTM_RATIO, 21.7f / PTM_RATIO);
    verts2[1].Set(46.6f / PTM_RATIO, -0.1f / PTM_RATIO);
    verts2[2].Set(29.1f / PTM_RATIO, -32.2f / PTM_RATIO);
    verts2[3].Set(2.5f / PTM_RATIO, -45.2f / PTM_RATIO);
    verts2[4].Set(6.8f / PTM_RATIO, -10.4f / PTM_RATIO);
    shape2.Set(verts2, num);
    fixtureDef2.shape = &shape2;
    fixtureDef2.density = density;

    //third
    b2FixtureDef fixtureDef3;
    b2PolygonShape shape3;
    num = 4;
    b2Vec2 verts3[4];
    verts3[0].Set(5.6f / PTM_RATIO, -9.7f / PTM_RATIO);
    verts3[1].Set(-0.3f / PTM_RATIO, -45.7f / PTM_RATIO);
    verts3[2].Set(-32.7f / PTM_RATIO, -41.2f / PTM_RATIO);
    verts3[3].Set(-28.2f / PTM_RATIO, -15.7f / PTM_RATIO);
    shape3.Set(verts3, num);
    fixtureDef3.shape = &shape3;
    fixtureDef3.density = density;

    //fourth
    b2FixtureDef fixtureDef4;
    b2PolygonShape shape4;
    num = 4;
    b2Vec2 verts4[4];
    verts4[0].Set(-28.7f / PTM_RATIO, -14.8f / PTM_RATIO);
    verts4[1].Set(-40.8f / PTM_RATIO, -36.0f / PTM_RATIO);
    verts4[2].Set(-60.5f / PTM_RATIO, -2.4f / PTM_RATIO);
    verts4[3].Set(-58.0f / PTM_RATIO, 2.9f / PTM_RATIO);
    shape4.Set(verts3, num);
    fixtureDef4.shape = &shape4;
    fixtureDef4.density = density;

    //attach to shape
    body->CreateFixture(&fixtureDef1);
    body->CreateFixture(&fixtureDef2);
    body->CreateFixture(&fixtureDef3);
    body->CreateFixture(&fixtureDef4);
}
- (id)initWithWorld:(b2World *)tworld atLocation:(CGPoint)location {
    if ((self = [super init])) {
        world = tworld;
        [self initWithFile:@"Banana.png"];
        [self createBodyAtLocation:location];
    }
    return self;
}


-(void)dealloc
{
    [super dealloc];
}
@end

“-initWithWorld: atLocation:”方法由 ccTouchBegan 方法调用，然后终止我的应用程序。 错误如下:

断言失败:(区域> 1.19209290e-7F)，函数ComputeCentroid，...

-(void)createBananaAtLocation:(CGPoint)location
{
    Banan *ban = [[[Banan alloc] initWithWorld:world atLocation:location] autorelease];//fails at this line when is invoked
    [self addChild:ban];

}
-(BOOL)ccTouchBegan:(UITouch *)touch withEvent:(UIEvent *)event
{
    CGPoint touchLocation = [touch locationInView:[touch view]];
    touchLocation = [[CCDirector sharedDirector]
                     convertToGL:touchLocation];
    touchLocation = [self convertToNodeSpace:touchLocation];
    b2Vec2 locationWorld =
    b2Vec2(touchLocation.x/PTM_RATIO, touchLocation.y/PTM_RATIO);

    [self createBananaAtLocation:touchLocation]; //error at this line
    return YES;
}

Answer 1

在您的情况下，顶点是顺时针方向，而 b2Polygon 要求是逆时针方向。 它导致计算多边形的负面积。您必须更改顶点的顺序。