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

Depth Buffer

The depth buffer stores a depth value for each pixel, and is used for determining the visibility of objects in a three-dimensional scene. Depth is measured in terms of distance to the eye, so pixels with larger depth-buffer values are overwritten by pixels with smaller values. This is just a useful convention, however, and the depth buffer’s behavior can be modified as described in “Depth Test” on Page 163. The depth buffer is sometimes called the z-buffer (the z comes from the fact that x- and y-values measure horizontal and vertical displacement on the screen, and the z-value measures distance perpendicular to the screen).