A STRING PATTERN-RECOGNITION APPROACH TO POLYGON CLIPPING

This paper describes a new approach to the clipping of a line segment by a simply-connected polygon. The polygon is not restricted to a convex polygon. The vertices and other key points of the polygon are given labels according to their relative locations to the line segment. When these labeled points are traversed in a predetermined order, a string of symbols is obtained. Different clipping of the line segment results in distinct string patterns of the symbols. By recognizing the different patterns, the nature of the intersection can be identified. We are interested in identifying four different types of clipped line segments: the interior line, the exterior line, the edge line, and the intersecting line. Using the concept of topologically equivalent polygon developed in this paper, a clipped polygon can be reduced to the simplest form without changing the type of the line segment. The string of symbols taken from this simplified polygon is easily classified into different line types. To expedite the recognition of string patterns, a phrase-structured string grammar is constructed. The major application of the clipping procedure is in the area of computer vision. A system capable of recognizing partially occluded planar objects uses the procedure described in this paper to identify the types of line segments connecting any two vertices in an object modeled as polygon. Other uses include hidden line removal in computer graphics and layout design in VLSI. © 1990.