Culling Technique #2
Detect a change in screen-space orientation.
If all face vertices are ordered in a consistent way, back-facing primitives can be found by
detecting a reversal in this order. One choice is a counterclockwise ordering when viewed from
outside of the manifold. This is consistent with computing face normals (Why?).
If, after projection, we ever see a clockwise face, it must be back facing.
This approach will work for all cases, but it comes even later in the pipe, at triangle setup.
We already do this calculation in our triangle rasterizer. It is equivalent to determining a triangle with negative
area.
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