Determining sector visibility of a polygon

We consider a generalization of notions of external visibility of simple polygons, namely weak external visibility, weak external visibility from a line and monotonicity, that we call sector visibility. Informally, sector visibility addresses the question of external visibility along rays (or sight lines) whose angles are restricted to a sector (wedge) of specified width &sgr;. This provides an interesting measure of the degree of external visibility of a polygon. Our framework also permits a unification and extension of a number of previously unrelated results. Finally, our results uncover a curious complexity discontinuity in this family of problems; algorithms are &THgr;(n) when &sgr; ≤ π or &sgr; = 2π, but require &OHgr;(n log n) time (at least), when π < &sgr; < 2π.

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