Question

Basically I have 2 polygons for my body. When I add a sprite for userData, the position of the texture isn t where I want it to be. What I want to do is adjust the position of the texture within the body. Here s the code sample of where I am setting this:

CCSpriteSheet *sheet = (CCSpriteSheet*) [self getChildByTag:kTagSpriteSheet];
CCSprite *pigeonSprite = [CCSprite spriteWithSpriteSheet:sheet rect:CGRectMake(0,0,40,32)];
[sheet addChild:pigeonSprite z:0 tag:kPigeonSprite];

pigeonSprite.position = ccp( p.x, p.y);

bodyDef.position.Set(p.x/PTM_RATIO, p.y/PTM_RATIO);
bodyDef.userData = sprite;
b2Body *body = world->CreateBody(&bodyDef);

b2CircleShape dynamicCircle;
dynamicCircle.m_radius = .25f;
dynamicCircle.m_p.Set(0.0f, 1.0f);

        // Define the dynamic body fixture.
b2FixtureDef circleDef;
circleDef.shape = &dynamicCircle;   
circleDef.density = 1.0f;
circleDef.friction = 0.3f;

body->CreateFixture(&circleDef);

b2Vec2 vertices[3];
vertices[0].Set(-0.5f, 0.0f);
vertices[1].Set(0.5f, 0.0f);
vertices[2].Set(0.0f, 1.0f);
b2PolygonShape triangle;
triangle.Set(vertices, 3);

b2FixtureDef triangleDef1;
triangleDef1.shape = &triangle; 
triangleDef1.density = 1.0f;
triangleDef1.friction = 0.3f;

body->CreateFixture(&triangleDef1);

Answer 1

I m not that familiar with objective-c but I ll give it a try.

All I can see is that you are storing a pointer to the sprite object in the body s user data and then leaving it there. If you want the body s position to be transferred to the sprite you need to update it every frame.

In C++ this would look something like this.

// To be called each time physics should be updated.
void physicsStep(float32 timeStep, int32 velocityIterations, int32 positionIterations) {
    // This is the usual update routine.
    world.Step(timeStep, velocityIterations, positionIterations);
    world.ClearForces();

    // SpriteClass can be replaced with any class you favor.
    // Assume there is a known pointer to the b2Body. Otherwise you ll have to get that,
    // or iterate over all bodies in the world.
    SpriteClass *sprite = (SpriteClass*)body->GetUserData();

    // Once you have the pointer you can transfer all the data.
    sprite.position = body->GetPosition();
    sprite.angle = body->GetAngle();
    // ... and so on
}

User data is just an arbitrary storage space in the b2Body and Box2D has no idea about what you decide to store there.

Answer 2

To move sprite with body then You have to set the sprite position accotring to body

in your update method

like this in COCOS2D-X Here

sprite = static_cast<CCSprite*>body->GetUserData();
sprite->setPosition(vec2(body->GetPosition().x*PTM_RATIO, body->GetPosition().y*PTM_RATIO));
sprite->setRotation(-CC_Radian_to_Degree(body->GetAngle));

PTM_RATIO = 32;

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