OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Depth Test

For each pixel on the screen, the depth buffer keeps track of the distance between the viewpoint and the object occupying that pixel. Then, if the specified depth test passes, the incoming depth value replaces the value already in the depth buffer.

The depth buffer is generally used for hidden-surface elimination. If a new candidate color for that pixel appears, it’s drawn only if the corresponding object is closer than the previous object. In this way, after the entire scene has been rendered, only objects that aren’t obscured by other items remain. Initially, the clearing value for the depth buffer is a value that’s as far from the viewpoint as possible, so the depth of any object is nearer than that value. If this is how you want ...