Constructing strongly convex hulls using exact or rounded arithmetic

  • Authors:
  • Zhenyu Li;Victor Milenkovic

  • Affiliations:
  • Harvard University;Harvard University

  • Venue:
  • SCG '90 Proceedings of the sixth annual symposium on Computational geometry
  • Year:
  • 1990

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Abstract

One useful generalization of the convex hull of a set S of points is the &egr;-strongly convex &dgr;-hull. It is defined to be a convex polygon Rgr; with vertices taken from S such that no point in S lies farther than &dgr; outside Rgr; and such that even if the vertices of Rgr; are perturbed by as much as &egr;, Rgr; remains convex. It was an open question as to whether an &egr;-strongly convex &Ogr;(&egr;)-hull existed for all positive &egr;. We give here an &Ogr;(n log n) algorithm for constructing it (which thus proves its existence). This algorithm uses exact rational arithmetic. We also show how to construct an &egr;-strongly convex &Ogr;(&egr; + &mgr;)-hull in &Ogr;(n log n) time using rounded arithmetic with rounding unit &mgr;. This is the first rounded arithmetic convex hull algorithm which guarantees a convex output and which has error independent of n.