Computer Laboratory

Technical reports

Towards robust inexact geometric computation

Julian M. Smith

December 2009, 186 pages

This technical report is based on a dissertation submitted July 2009 by the author for the degree of Doctor of Philosophy to the University of Cambridge, St. Edmund’s College.

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Geometric algorithms implemented using rounded arithmetic are prone to robustness problems. Geometric algorithms are often a mix of arithmetic and combinatorial computations, arising from the need to create geometric data structures that are themselves a complex mix of numerical and combinatorial data. Decisions that influence the topology of a geometric structure are made on the basis of certain arithmetic calculations, but the inexactness of these calculations may lead to inconsistent decisions, causing the algorithm to produce a topologically invalid result or to fail catastrophically. The research reported here investigates ways to produce robust algorithms with inexact computation.

I present two algorithms for operations on piecewise linear (polygonal/polyhedral) shapes. Both algorithms are topologically robust, meaning that they are guaranteed to generate a topologically valid result from a topologically valid input, irrespective of numerical errors in the computations. The first algorithm performs the Boolean operation in 3D, and also in 2D. The main part of this algorithm is a series of interdependent operations. The relationship between these operations ensures a consistency in these operations, which, I prove, guarantees the generation of a shape representation with valid topology. The basic algorithm may generate geometric artifacts such as gaps and slivers, which generally can be removed by a data-smoothing post-process. The second algorithm presented performs simplification in 2D, converting a geometrically invalid (but topologically valid) shape representation into one that is fully valid. This algorithm is based on a variant of the Bentley-Ottmann sweep line algorithm, but with additional rules to handle situations not possible under an exact implementation.

Both algorithms are presented in the context of what is required of an algorithm in order for it to be classed as robust in some sense. I explain why the formulaic approach used for the Boolean algorithm cannot readily be used for the simplification process. I also give essential code details for a C++ implementation of the 2D simplification algorithm, and discuss the results of extreme tests designed to show up any problems. Finally, I discuss floating-point arithmetic, present error analysis for the floating-point computation of the intersection point between two segments in 2D, and discuss how such errors affect both the simplification algorithm and the basic Boolean algorithm in 2D.

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BibTeX record

  author =	 {Smith, Julian M.},
  title = 	 {{Towards robust inexact geometric computation}},
  year = 	 2009,
  month = 	 dec,
  url = 	 {},
  institution =  {University of Cambridge, Computer Laboratory},
  number = 	 {UCAM-CL-TR-766}