I am rendering a simple 3D object (e.g. a cube, vertices only). I need to know what is the z-coordinate value for the points lying on the image plane. OpenGL transforms the projection matrix so that the z-coordinate remains unchanged and is used in the depth buffer. Can I assume that the focal distance/z-coordinate of points lying on image plane after projection is equal to the near clipping plane value?
To sum up, my question is, how do I find out the focal length or equivalently the z-coordinate of points lying on the image plane after projection (using OpenGL projection matrix, that is)?