【Culling & Depth Testing】

　　Culling is an optimization that does not render polygons facing away from the viewer. All polygons have a front and a back side. Culling makes use of the fact that most objects are closed; if you have a cube, you will never see the sides facing away from you (there is always a side facing you in front of it) so we don‘t need to draw the sides facing away. Hence the term: Backface culling.

　　The other feature that makes rendering looks correct is Depth testing. Depth testing makes sure that only the closest surfaces objects are drawn in a scene.

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参考：file://localhost/Applications/Unity/Unity.app/Contents/Documentation/Documentation/Components/SL-CullAndDepth.html