How is a polygon determined to be front facing or back facing?

Questions & Answers
1

I originally thought that the mesh normals were used to determine whether a face is facing towards or away from the camera. However, this doesn’t seem to be the case.

111570-flippednormalcomparison.png

In this image, there are 2 quads. The one on the left is the original, and the one on the right has normals flipped. When viewed from the other side, both quads are culled.

How does Unity determine whether a polygon is back facing or not?

2

It is based on the order that a triangle’s vertices are presented in.

For a left-handed system like Unity, the vertices are expected to form triangles in a clockwise fashion. Although you can specify normals any way you like, graphics APIs implicitly define sides for you in the same way that normals can be auto-generated.