Bitmaps blow.
Alright... bitmaps and programming in general is not set up to handle circular objects. While it's a little easier in 3D space, 2D graphics and programming aren't happy with it. See, I'm working on a game, a popular one, and I'm being really-really picky about the physics. While most people won't notice the difference, I will and I always have in every version I've played.
The problem is this: 2D Programming is best left dealing with squares. Computers like straight lines and comparative statements like < > and =. Anything else tends to piss 'em off. Then they just do stupid things like crash. So, when working with a ball in a 2D game the collisions are dermined by a box around the ball, not the ball itself. Look:
The issue is that while the two boxes (the red outline and the big grey thing) intersect each other, they actually do not hit. Further, the reacton of the ball should it hit something with any part of its diameter besides the absolute top, bottom, left or right, will be different than you would get with "Hit-box" collision detection.
So, using that picture, if I use the hit box method it wouldn't be accurate. On the flip-side to be accurate involves calculating vectors and tangents that I really would rather not do...