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

Conditional Rendering

Advanced

One of the issues with occlusion queries is that they require OpenGL to pause processing geometry and fragments, count the number of affected samples in the depth buffer, and return the value to your application. Stopping modern graphics hardware in this manner usually catastrophically affects performance in performance-sensitive applications. To eliminate the need to pause OpenGL’s operation, conditional rendering allows the graphics server (hardware) to decide if an occlusion query yielded any fragments, and to render the intervening commands. Conditional rendering is enabled by surrounding the rendering operations you would have conditionally executed using the results of glGetQuery*().

void glBeginConditionalRender ...