Clipping a Concave Polygon
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Publisher Summary This chapter discusses how to clip a concave polygon. Polygons are a popular modeling primitive. Polygon clipping against a line or a plane is one of the most common rendering operations. The classic re-entrant method clips a convex polygon against a line, yielding a pair of polygons lying on either side of the line. The algorithm is conceptually elegant and easy to program for convex polygons, but becomes difficult to implement for concave ones. Although it can be patched up to treat these cases, the patching becomes complicated, involving the detection of degenerate edges. This chapter presents a simple yet robust method for clipping concave polygons. In the general case, both polygons are returned, suggesting an algorithm that can accommodate multiple polygons such as those produced when a concave polygon is split into many fragments (which may also be concave).
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