Determine the Disjoint of division method
A polygon is disjoint from the viewport if the x- and y-extents of the polygon do not overlap the viewport anywhere. In this case; regardless of the relative z-extents of type polygon and the current background, and regardless of which one is in front of the other, the polygon is not visible.
The concept of disjointness may also be used while comparing different polygons in the same viewport. If, of all the polygons visible within a viewport, the x-y extent of only one polygon P is not intersected by the x-y extents of any of the other polygons, then P can be declared to be visible in the viewport, and no further evaluation will be necessary.