Box2D Difference Between WorldCenter and Position

Question

So this problem has been brothering for a couple of days now. First off, what is the difference between say Body.getWorldCenter() and Body.getPosition(). I heard that WorldCenter might have to do with the center of gravity or something.

Second, When I create a Box2D Body for a sprite the Body is always at the lower left corner. I check it by printing a Rectangle of 1 pixel around the box.getWorldCenter(). From what I understand the Body should be in the center of the Sprite and its bounding box should wrap around the Sprite, correct?

Here's an image of what I mean (The Sprite is Red, Body Blue):

Here's some code:

Body Creator:

    public static Body createBoxBody( final World pPhysicsWorld, final BodyType pBodyType, 
            final FixtureDef pFixtureDef, Sprite pSprite ) {

        float pRotation = 0;
        float pCenterX = pSprite.getX() + pSprite.getWidth() / 2;
        float pCenterY = pSprite.getY() + pSprite.getHeight() / 2;
        float pWidth = pSprite.getWidth();
        float pHeight = pSprite.getHeight();

        final BodyDef boxBodyDef = new BodyDef();
        boxBodyDef.type = pBodyType;

        //boxBodyDef.position.x = pCenterX / Constants.PIXEL_METER_RATIO;
        //boxBodyDef.position.y = pCenterY / Constants.PIXEL_METER_RATIO;

        boxBodyDef.position.x = pSprite.getX() / Constants.PIXEL_METER_RATIO;
        boxBodyDef.position.y = pSprite.getY() / Constants.PIXEL_METER_RATIO;

        Vector2 v = new Vector2( boxBodyDef.position.x * Constants.PIXEL_METER_RATIO, boxBodyDef.position.y * Constants.PIXEL_METER_RATIO );
        Gdx.app.log("@Physics", "createBoxBody():: Box Position: " + v);

        // Temporary Box shape of the Body
        final PolygonShape boxPoly = new PolygonShape();
        final float halfWidth = pWidth * 0.5f / Constants.PIXEL_METER_RATIO;
        final float halfHeight = pHeight * 0.5f / Constants.PIXEL_METER_RATIO;
        boxPoly.setAsBox( halfWidth, halfHeight );  // set the anchor point to be the center of the sprite

        pFixtureDef.shape = boxPoly;


        final Body boxBody = pPhysicsWorld.createBody(boxBodyDef);
        Gdx.app.log("@Physics", "createBoxBody():: Box Center: " + boxBody.getPosition().mul(Constants.PIXEL_METER_RATIO));
        boxBody.createFixture(pFixtureDef);

        boxBody.setTransform( boxBody.getWorldCenter(), MathUtils.degreesToRadians * pRotation );


        boxPoly.dispose();

        return boxBody;
}

Making the Sprite:

    public Car( Texture texture, float pX, float pY, World world ) {

        super( "Car" ); 

        mSprite = new Sprite( texture );
        mSprite.setSize( mSprite.getWidth() / 6, mSprite.getHeight() / 6 );
        mSprite.setPosition( pX, pY );
        mSprite.setOrigin( mSprite.getWidth()/2, mSprite.getHeight()/2);

        FixtureDef carFixtureDef = new FixtureDef();

        // Set the Fixture's properties, like friction, using the car's shape
        carFixtureDef.restitution = 1f;
        carFixtureDef.friction = 1f;
        carFixtureDef.density = 1f; // needed to rotate body using applyTorque      

        mBody = Physics.createBoxBody( world, BodyDef.BodyType.DynamicBody, carFixtureDef, mSprite );
}

Answer 1

First off, what is the difference between say Body.getWorldCenter() and Body.getPosition().

Body.getWorldCenter() is the center of gravity. Body.getPosition() is the center of the AABB. For a simple box or circle they will be the same however if your body is unbalanced (ex. multiple fixtures /w different densities) they will differ. Also try setting your origin before the location.

So

mSprite.setOrigin( mSprite.getWidth()/2, mSprite.getHeight()/2);
mSprite.setPosition( pX, pY );

instead of

mSprite.setPosition( pX, pY );
mSprite.setOrigin( mSprite.getWidth()/2, mSprite.getHeight()/2);