Partitioning a polygonal region into trapezoids

  • Authors:
  • Takao Asano;Tetsuo Asano;Hiroshi Imai

  • Affiliations:
  • Univ. of Tokyo, Tokyo, Japan;Osaka Electro-Communication Univ., Osaka, Japan;Univ. of Tokyo, Tokyo, Japan

  • Venue:
  • Journal of the ACM (JACM)
  • Year:
  • 1986

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Abstract

The problem of partitioning a polygonal region into a minimum number of trapezoids with two horizontal sides is discussed. A triangle with a horizontal side is considered to be a trapezoid with two horizontal sides one of which is degenerate. First, a method of achieving a minimum partition is presented. The number M* of the trapezoids in the minimum partition of a polygonal region P is shown to be M* = n + w - h - d - 1, where n, w, and h are the number of vertices, windows (holes), and horizontal edges of P, respectively, and d is the cardinality of a maximum independent set of the straight-lines-in-the-plane graph associated with P. Next, this problem is shown to be polynomially equivalent to the problem of finding a maximum independent set of a straight-lines-in-the-plane graph, and consequently, it is shown to be NP-complete. However, for a polygonal region without windows, an O(n2)-time algorithm for partitioning it into a minimum number of trapezoids is presented. Finally, an O(n log n)-time approximation algorithm with the performance bound 3 is presented.