Collision detection using gltranslatef?

Hi,

I'm slowly working towards using my opengl knowledge to create a pong clone. If I've been reading right, I should move objects (ball in particular) around using gltranslatef.

The problems comes when I'm trying to figure out collision detection. If the coordinates for the ball are set at 0,0 and I use gltranslatef to translate around and eventually collide with the edge of the window, I can't work out if any of the vertices meet the edge because (as i've used gltranslatef) the coordinates for the ball still equal 0,0.

Basically:

Set ball coordinates at 0,0 -> Translate in the positive y direction -> I don't know how to detect a collision if the ball appears at the top, but the coordinates are still set at 0,0 because I've been using gltranslatef...

So I haven't been able to find a place that explains how to do collision detection when using gltranslatef (which is how I've been told to move things) instead of just specifying new coordinates every time.

Thanks in advance =).