Box2D objects have their anchor point in the center by default, while for Flash objects, it's in the top left. To position them properly, you need to take this into account
Easy way
Wrap your Bitmaps
/whatever in a Sprite
and center them:
// create the image, center it, and add it to a holder Sprite
var image:Bitmap = new Bitmap( objGraphicsBitmapData );
image.x = -image.width * 0.5;
image.y = -image.height * 0.5;
var holder:Sprite = new Sprite;
holder.addChild( image );
Now just set the position and rotation of holder as you do currently, and it should be fine
Hard way
You need to manually adjust the position offset based on the object's rotation. A simple rotation function:
public function rotate( p:Point, radians:Number, out:Point = null ):Point
{
// formula is:
// x1 = x * cos( r ) - y * sin( r )
// y1 = x * sin( r ) + y * cos( r )
var sin:Number = Math.sin( radians );
var cos:Number = Math.cos( radians );
var ox:Number = p.x * cos - p.y * sin;
var oy:Number = p.x * sin + p.y * cos;
// we use ox and oy in case out is one of our points
if ( out == null )
out = new Point;
out.x = ox;
out.y = oy;
return out;
}
First we need to store the object's offset - this is normally new Point( -obj.width * 0.5, -obj.height * 0.5 )
. You need to stock this while it's rotation is 0
, and rotating the object will change its width
and height
properties, so the following won't work properly.
obj.offset = new Point( -obj.width * 0.5, -obj.height * 0.5 );
When you're updating the position, simply rotate the offset by the rotation and add it:
// get our object's position and rotation
// NOTE: you'll probably need to adjust the position based on your pixels per meter value
var pos:Point = new Point( obj.pBody.GetPosition().x, obj.pBody.GetPosition().y ); // pos in screen coords
var rotR:Number = obj.pBody.GetAngle(); // rotation in radians
var rotD:Number = radiansToDegrees( rotR ); // rotation in degrees
// rotate our offset by our rotation
var offset:Point = rotate( obj.offset, rotR );
// set our position and rotation
obj.x = pos.x + offset.x;
obj.y = pos.y + offset.y;
obj.rotation = rotD;
Other useful functions:
public function degreesToRadians( deg:Number ):Number
{
return deg * ( Math.PI / 180.0 );
}
public function radiansToDegrees( rad:Number ):Number
{
return rad * ( 180.0 / Math.PI );
}