How is gl_FragCoord calculated. ?

[Vadim]

I need to shift the depth of a fragment a little bit (converting circles to spheres). Can anyone explain me how the original gl_FragCoord.z value is calculatged ?

[Dark Photon]

I need to shift the depth of a fragment a little bit (converting circles to spheres). Can anyone explain me how the original gl_FragCoord.z value is calculatged ?
I'm sure there are lots of places, but here are a few:
* gl_FragCoord.z (OpenGL.org thread) (http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=267320#Post2673 20)
* Comparing Depth in Fragment Shader (OpenGL.org thread) (http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=268892#Post2688 92)

[overlay]

here is another OpenGL.org thread:
http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=255766#Post2557 66