Why isn't my sprite aligned with b2Body? - iphone

Question

I recently started game programming on the iPhone using Cocos2d and Box2d. So here's my problem:
I've got a Player class which inherits from CCSprite, and within that class, there's this method:
-(void) createBox2dObject:(Player *)sender
forWorld:(b2World*)world {
b2BodyDef playerBodyDef;
playerBodyDef.type = b2_dynamicBody;
playerBodyDef.position.Set(sender.position.x/PTM_RATIO, sender.position.y/PTM_RATIO);
playerBodyDef.userData = sender;
body = world->CreateBody(&playerBodyDef);
b2PolygonShape dynamicBox;
dynamicBox.SetAsBox(sender.contentSize.width/PTM_RATIO,
sender.contentSize.height/PTM_RATIO);
b2FixtureDef polygonShapeDef;
polygonShapeDef.shape = &dynamicBox;
polygonShapeDef.density = 1.0f;
polygonShapeDef.friction = 1.0f;
polygonShapeDef.restitution = 0;
body->CreateFixture(&polygonShapeDef);
}
and here's how I call this:
self.player = [Player spriteWithSpriteFrameName:#"runningrupol-1.png"];
_player.position = ccp(_player.boundingBox.size.width/2 + 32, _player.boundingBox.size.height/2 + 32);
self.walkAction = [CCRepeatForever actionWithAction:
[CCAnimate actionWithAnimation:walkAnim restoreOriginalFrame:NO]];
[_player runAction:_walkAction];
[spriteSheet addChild:_player];
[_player createBox2dObject:_player forWorld:_world];
Obviously, I'm using a spritesheet which is animated.
Here's how I update the world:
- (void)tick:(ccTime) dt {
_world->Step(dt, 8, 10);
for(b2Body *b = _world->GetBodyList(); b; b=b->GetNext()) {
if (b->GetUserData() != NULL) {
CCSprite *playerData = (CCSprite *)b->GetUserData();
playerData.position = ccp(b->GetPosition().x * PTM_RATIO,
b->GetPosition().y * PTM_RATIO);
playerData.rotation = -1 * CC_RADIANS_TO_DEGREES(b->GetAngle());
}
}
}
And here's how I call it in the init method:
[self schedule:#selector(tick:)];
This is what I see:
Please help. And if you need additional info, just tell me.

Answer 1

SetAsBox() uses half-height and half-width (quirky I know), so divide the parameters by 2:
dynamicBox.SetAsBox(sender.contentSize.width/PTM_RATIO/2,
sender.contentSize.height/PTM_RATIO/2);
When you try this, leave the anchor point as is (the default value if you don't explicitly set it should be ccp(0.5,0.5), the center of your sprite, and you want that).

Answer 2

You can change the anchorpoint for the sprite. Here is a good tutorial:
http://www.qcmat.com/understanding-anchorpoint-in-cocos2d/