Computing the visibility polygon using few variables

  • Authors:
  • Luis Barba;Matias Korman;Stefan Langerman;Rodrigo I. Silveira

  • Affiliations:
  • Universidad Nacional Autónoma de México (UNAM), Mexico D.F., Mexico;Université Libre de Bruxelles (ULB), Brussels, Belgium;Université Libre de Bruxelles (ULB), Brussels, Belgium;Universitat Politècnica de Catalunya (UPC), Barcelona, Spain

  • Venue:
  • ISAAC'11 Proceedings of the 22nd international conference on Algorithms and Computation
  • Year:
  • 2011

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Abstract

We present several algorithms for computing the visibility polygon of a simple polygon $\ensuremath{\mathcal{P}}$ from a viewpoint inside the polygon, when the polygon resides in read-only memory and only few working variables can be used. The first algorithm uses a constant number of variables, and outputs the vertices of the visibility polygon in $O(n\ensuremath{\bar{r}})$ time, where $\ensuremath{\bar{r}}$ denotes the number of reflex vertices of $\ensuremath{\mathcal{P}}$ that are part of the output. The next two algorithms use O(logr) variables, and output the visibility polygon in O(nlogr) randomized expected time or O(nlog2r) deterministic time, where r is the number of reflex vertices of $\ensuremath{\mathcal{P}}$.