Contained:

A polygon is contained in a viewport if no part of this is outside any of the edges of the viewport. In this case, if the z-extent of the polygon is behind the z-extent of the current background, the polygon is totally invisible. If the z-extent of the polygon is in front of the current background, the polygon is totally visible .

Disjoint

A polygon is disjoint from the viewport if the x- & 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 can also be utilized while comparing different polygons in the same viewport. If, of all the polygons visible in 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 may be declared to be visible in the viewport, and no further evaluation shall be necessary.