[Graphics]Occlusion Culling

Occlusion Culling是用来裁剪看不见的物体或者说被遮挡的物体。在3D计算机图形学中，通常物体是按照离摄像头的远近来绘制的，由远及近，即overdraw。这与Frustum Culling不同，后者only disables the renderers for objects that are outside the camera’s viewing area.

The occlusion culling process will go through the scene using a virtual camera to build a hierarchy of potentially visible sets of objects. This data is used at runtime by each camera to identify what is visible and what is not. 有效减少了draw call，提高游戏性能。

Occlusion culling的数据由cell组成，更确切的说是cell构成了用于裁剪的binary tree，分为两类View Cells（Static Objects）和Target Cells（Moving Objects）。You can sometimes improve the culling by breaking large objects into smaller pieces. However you can still merge small objects together to reduce draw calls, as long as they all belong to the same cell, occlusion culling won’t be affected.

PVS：potentially visible set, the collection of cells and the visibility information.