Back Face Algorithm

Before the Viewing Transformation
1. List the vertices for each polygon clockwise
with respect to the front face.
P1, P4, P3, P2
P5, P6, P7, P8
2. Compute the equation of the plane for each polygon such that the normal vector points out of the front face.
3. If A xe + B ye + C ze + D < 0 The polygon is a backface.

After the Perspective Projection
1. Look at the z-coordinate of the Normal vector