After transformation a polygon may become be
back-faced, and the information of the polygon can not
be seen by viewer. So the polygon should to be culled.
In back-faced culling model [10], the system
determines whether the polygon is back face, and it is a
relatively simple operation. If the polygon is back face,
it will be culled and not to be passed to the following
stage. Then if the polygon is projected outside of the
view volume, it can be culled by clipping operations.
The back-faced culling and the clip culling are both
operated in geometric subsystem.