Removing Polygons Hidden through a Surrounding Polygon:
The key to capable visibility calculation lies actually a polygon is not visible whether it is in back of a surrounding polygon. Thus, it can be eliminated by the PVPL. To assist processing, this list is sorted through zmin, the smallest z coordinate of the polygon inside this area. Furthermore, for all surrounding polygons S, we record also its largest z coordinate, zsmax.
For a polygon P on the list as zpmin >zsmax only for a surrounding polygon S, then P is hidden through S and hence is not visible. As well, all other polygons after P on the list will also be hidden through S, thus we can eliminate these polygons by the PVPL.