We just finished a new improved version of this system, and we were ecstatic to see how good the occluder coverage turned out to be. I wrote about this before: Covering the Sun with a finger. Voxels and clipmap scenes make it very easy to perform occlusion tests. Naturally these simple, approximated models must cover as much of the original content as possible. It is necessary to find out simplified models of the scene geometry. If the test is not fast enough, it could still be faster to render everything than to test and then render the visible parts.
MINECRAFT VOXEL MAP VS JOURNEY MAP HOW TO
The challenge is how to do it very quickly. The idea is simple: using a simplified model of the scene we can predict when some portions of the scene become eclipsed by another parts of the scene.
Occlusion testing on the other hand is a dynamic approach to visibility. For an application where the content constantly changes, they are a very poor choice or not practical at all. To make it worse, these visibility structures take long to build. What works for indoors breaks in large open spaces. (The acronyms in this case make it sound simpler.) These approaches perform well for some cases to then fail miserably in other cases. There is a long history of hackery in this topic: BSP trees, PVS, portals etc. While rendering is now a soft problem, finding out what is potentially visible remains difficult. Real time rendering systems (like the ones in game engines) have two big problems to solve: First to determine what is visible from the camera's point of view, then to render what is visible.