z-buffer from anywhere in space

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.

In OpenGL the z-buffer is used in junction with the frame buffer because z-buffers are designed for use in the rendering stage. So if you want a depth test that is not the same as the current view frustum, you will have to render the scene from that specific orientation. You might be able to not render to the frame buffer and save some time.