Surrounding of sub division method

A polygon surrounds a viewport if it completely encloses or covers the viewport. This happens if none of its sides cuts any edge of the viewport. It is not sufficient if the x-y extent encloses the viewport, because even with the same extent, if the right edge of the viewport were to be shifted a little more to the right as shown dotted in Figure(a), the polygon's right edge would cut the top and right edges of the viewport. If the surrounding polygon is the closest to the viewer, the entire viewport is covered by the colour and other attributes of the polygon.