value stored by the hardware in the depth buffer is a non-linear mapping of the
actual depth from the camera based on the near and far clipping planes.
is done so that objects closer to the camera have more depth resolution.
mapping is of the form:
~ a - b / z
a = zFar / ( zFar - zNear )
b = zFar * zNear / ( zFar – zNear )
z = distance from camera to object
inversely proportional to distance
Tip: set zNear to a value > 0