Wait, first you want to center bitmap, now vector data? man, get real!
For every problem, there's solution (algorithmic, or not). In your case I'd consider drawing on scalable vector graphics and then drawing that image on Preview's canvas. This way you can scale stuff, rotate and such and there would be no problem. Another way is dynamic bitmap creation. That way you draw your stuff on bitmap and draw bitmap on preview's canvas just like I said.