OpenGL 4 Shading Language Cookbook - Third Edition

Configuring the depth test

GLSL 4 provides the ability to configure how the depth test is performed. This gives us additional control over how and when fragments are tested against the depth buffer.

Many OpenGL implementations automatically provide an optimization known as the early depth test or early fragment test. With this optimization, the depth test is performed before the fragment shader is executed. Since fragments that fail the depth test will not appear on the screen (or the framebuffer), there is no point in executing the fragment shader at all for those fragments and we can save some time by avoiding the execution.

The OpenGL specification, however, states that the depth test must appear to be performed after the fragment shader. ...