A Parallel Algorithm for the Visibility of a Simple Polygon Using Scan Operations

This paper describes a parallel algorithm for computing the visible portion of a simple planar polygon with N vertices from a given point on or inside the polygon. The algorithm accomplishes this in O(k log N) time using O(N/log N) processors, where k is the link-diameter of the polygon in consideration. The link-diameter of a polygon is the maximum number of straight line segments needed to connect any two points within the polygon, where all line segments lie completely within the polygon. The algorithm can also be used to compute the visible portion of the plane given a point outside of the polygon. Except in this case, the parameter k in the asymptotic bounds would be the link diameter of a different polygon. The algorithm is optimal for sets of polygons that have a constant link diameter. It is a rather simple algorithm, and has a very small run time constant, making it fast and practical to implement. The interprocessor communication needed involves only local neighbor communication and scan operations (i.e., parallel prefix operations). Thus the algorithm can be implemented not only on an EREW PRAM, but also on a variety of other more practical machine architectures, such as hypercubes, trees, butterflies, and shuffle exchange networks. The algorithm was implemented on the Connection Machine as well as the MasPar MP- 1, and various performance tests were conducted.