When and how does openGL calculate F_depth(depth value)

[GClements]

@nirla123

But, one thing, during window transformation z value seems to map to [0,1], meaning (x’,y’,z’), where x’,y’,z’ are the window transformed coordinates. meaning (x’,y’,z’) don’t span a plane, they span a space, so while drawing, the z’ coords are ignored?

If you don’t have a depth buffer, the z’ coordinates are redundant. The x’,y’ coordinates determine which pixels are covered by the primitive (point, line, triangle).