Occlusion culling is a process which prevents Unity from performing rendering calculations for GameObjects that are completely hidden from view (occluded) by other GameObjects.
Every frame, Cameras perform culling operations that examine the Renderers in the Scene and exclude (cull) those that do not need to be drawn. By default, Cameras perform frustum culling, which excludes all Renderers that do not fall within the Camera’s view frustum. However, frustum culling does not check whether a Renderer is occluded by other GameObjects, and so Unity can still waste CPU and GPU time on rendering operations for Renderers that are not visible in the final frame. Occlusion culling stops Unity from performing these wasted operations.
To determine whether occlusion culling is likely to improve the runtime performance of your Project, consider the following:
Occlusion culling generates data about your Scene in the Unity Editor, and then uses that data at runtime to determine what a Camera can see. The process of generating data is known as baking.
When you bake occlusion culling data, Unity divides the Scene into cells and generates data that describes the geometry within cells, and the visibility between adjacent cells. Unity then merges cells where possible, to reduce the size of the generated data. To configure the baking process, you can change parameters in the Occlusion Culling window, and use Occlusion Areas in your Scene.
At runtime, Unity loads this baked data into memory, and for each Camera that has its Occlusion Culling property enabled, it performs queries against the data to determine what that Camera can see. Note that when occlusion culling is enabled, Cameras perform both frustum culling and occlusion culling.
Unity uses the Umbra library to perform occlusion culling. For links to articles with more information on Umbra, see the Additional resources page.