Back-face Culling

Polygon is back-facing if:

    V.N > 0 

Assuming view is along Z (v = (0, 0, 1))

    V.N = (0 + 0 + Zn)

Simplify further to

    if Zn > 0, then cull

Works for non-overlapping convex polyhedra

    with concave polyhedra, some hidden surfaces
              will not be culled