两种software occlusion culling实现
1. Practical Occlusion Culling in Killzone 3, Software Occlusion culling on SPU
http://www.slideshare.net/guerrillagames/practical-occlusion-culling-in-killzone-3
2. Umbra: a 3rd party occlusion culling application used in Big World and Unity.
Umbra is not used in UE (additional purchase is needed)