在box2d中检测菱形的问题

- (id)init {

if ((self=[super init])) {

    CGSize winSize = [CCDirector sharedDirector].winSize;

    self.isTouchEnabled = YES;

    self.isAccelerometerEnabled=YES;
    // Create a world
    b2Vec2 gravity = b2Vec2(0.0f, 0.0f);
    bool doSleep = true;
    _world = new b2World(gravity, doSleep);

    // Create edges around the entire screen
    b2BodyDef groundBodyDef;
    groundBodyDef.position.Set(0,0);
    _groundBody = _world->CreateBody(&groundBodyDef);
    b2PolygonShape groundBox;
    b2FixtureDef groundBoxDef;
    groundBoxDef.shape = &groundBox;
    groundBox.SetAsEdge(b2Vec2(0,0), b2Vec2(winSize.width/PTM_RATIO, 0));
    _bottomFixture = _groundBody->CreateFixture(&groundBoxDef);
    groundBox.SetAsEdge(b2Vec2(0,0), b2Vec2(0, winSize.height/PTM_RATIO));
    _groundBody->CreateFixture(&groundBoxDef);
    groundBox.SetAsEdge(b2Vec2(0, winSize.height/PTM_RATIO), b2Vec2(winSize.width/PTM_RATIO, winSize.height/PTM_RATIO));
    _groundBody->CreateFixture(&groundBoxDef);
    groundBox.SetAsEdge(b2Vec2(winSize.width/PTM_RATIO, winSize.height/PTM_RATIO), b2Vec2(winSize.width/PTM_RATIO, 0));
    _groundBody->CreateFixture(&groundBoxDef);

    // Create sprite and add it to the layer
    CCSprite *ball = [CCSprite spriteWithFile:@\"ball1.png\" rect:CGRectMake(0, 0, 16,16)];
   // ball.position = ccp(180, 400);
    ball.tag = 1;

// Create ball body 
    b2BodyDef ballBodyDef;
    ballBodyDef.type = b2_dynamicBody;
    ballBodyDef.position.Set(180/PTM_RATIO, 450/PTM_RATIO);
    ballBodyDef.userData = ball;
     ballBody = _world->CreateBody(&ballBodyDef);

    // Create circle shape
    b2CircleShape circle;
    circle.m_radius = 16/PTM_RATIO;
    //circle.m_radius = 50/PTM_RATIO;

    // Create shape definition and add to body
    b2FixtureDef ballShapeDef;
    ballShapeDef.shape = &circle;
    ballShapeDef.density = 1.0f;
    ballShapeDef.friction = 0.0f; // We don\'t want the ball to have friction!
    ballShapeDef.restitution = 0.0f;
    _ballFixture = ballBody->CreateFixture(&ballShapeDef);

    // Give shape initial impulse...
    b2Vec2 force = b2Vec2(0, 0);
    ballBody->ApplyLinearImpulse(force, ballBodyDef.position);

    for(int i = 0; i < 5; i++) 


    {
        static int padding=25;



        CCSprite *block = [CCSprite spriteWithFile:@\"diamond2.png\"];
       int  xOffset = padding+block.contentSize.width/5+((block.contentSize.width+padding)*i);
        block.position = ccp(xOffset, 250);
        block.tag = 2;
        [self addChild:block];


        // Create block body
        b2BodyDef blockBodyDef;
       // blockBodyDef.type = b2_dynamicBody;
        blockBodyDef.position.Set(xOffset/PTM_RATIO, 400/PTM_RATIO);
        blockBodyDef.userData = block;

        b2Body *blockBody = _world->CreateBody(&blockBodyDef);

        // Create block shape
        b2PolygonShape blockShape;
        blockShape.SetAsBox(block.contentSize.width/PTM_RATIO/8,
                            block.contentSize.height/PTM_RATIO/8
                            );

        // Create shape definition and add to body
        b2FixtureDef blockshapeDef;
        blockshapeDef.shape = &blockShape;
        blockshapeDef.density = 0.0;
        blockshapeDef.friction = 10.0;
        blockshapeDef.restitution = 0.1f;
        blockBody->CreateFixture(&blockshapeDef);
    }

    [self addChild:ball]; 

// Create contact listener
    _contactListener = new MyContactListener();
    _world->SetContactListener(_contactListener);

    [[SimpleAudioEngine sharedEngine] playBackgroundMusic:@\"background-music-aac.caf\"];

    [self schedule:@selector(tick:)];

}
return self;

- (void)tick:(ccTime) dt {

   // bool blockFound = false;
_world->Step(dt, 10, 10);    
for(b2Body *b = _world->GetBodyList(); b; b=b->GetNext()) {    
    if (b->GetUserData() != NULL) {
        CCSprite *sprite = (CCSprite *)b->GetUserData();     



        if (sprite.tag == 1) {
            static int maxSpeed = 20;

            b2Vec2 velocity = b->GetLinearVelocity();
            float32 speed = velocity.Length();

            // When the ball is greater than max speed, slow it down by
            // applying linear damping.  This is better for the simulation
            // than raw adjustment of the velocity.
            if (speed > maxSpeed) {
                b->SetLinearDamping(0.2);
            } else if (speed < maxSpeed) {
                b->SetLinearDamping(0.0);
            }

        }

       sprite.position = ccp(b->GetPosition().x * PTM_RATIO,
                                b->GetPosition().y * PTM_RATIO);
        sprite.rotation = 1 * CC_RADIANS_TO_DEGREES(b->GetAngle());

}
}
}