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**Covering orthogonal polygons with star polygons: The perfect graph approach.**
*(English)*
Zbl 0705.68082

Summary: This paper studies the combinatorial structure of visibility in orthogonal polygons. We show that the visibility graph for the problem of minimally covering simple orthogonal polygons with star polygons is perfect. A star polygon contains a point p, such that for every point q in the star polygon, there is an orthogonally convex polygon containing p and q. This perfectness property implies a polynomial algorithm for the above polygon covering problem. It further provides us with an interesting duality relationship. We first establish that a minimum clique cover of the visibility graph of a simple orthogonal polygon corresponds exactly to a minimum star cover of the polygon. In general, simple orthogonal polygons can have concavities (dents) with four possible orientations. We show that the visibility graph is weakly triangulated. We thus obtain an \(O(n^ 8)\) algorithm. Since weakly triangulated graphs are perfect, we also obtain an interesting duality relationship. In the case where the polygon has at most three dent orientations, we show that the visibility graph is triangulated or chordal. This gives us an \(O(n^ 3)\) algorithm.

### MSC:

68R10 | Graph theory (including graph drawing) in computer science |

68U05 | Computer graphics; computational geometry (digital and algorithmic aspects) |

68Q25 | Analysis of algorithms and problem complexity |

### Keywords:

star polygons; perfect graph; combinatorial structure of visibility; orthogonal polygons; polygon covering problem
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\textit{R. Motwani} et al., J. Comput. Syst. Sci. 40, No. 1, 19--48 (1990; Zbl 0705.68082)

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