z-buffer from anywhere in space

Hi,
after DirectX i want to start with OpenGl, i have the problem to generate the z-buffer-information from anywhere in space in any direction i want- note its not the camera.
For the camera I need the (a) z-buffer too.
If you can help me.
Henning

render the view from the POV of the z-buffer you want. you ARE allowed to move cameras around the place for a given frame, you know =))

cheers,
John