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Correct Adaptive Tests

  The correctness of the incremental and divide-and-conquer algorithms depends on reliable orientation and incircle tests. The orientation test determines whether a point lies to the left of, to the right of, or on a line; it is used in many (perhaps most) geometric algorithms. The incircle test determines whether a point lies inside, outside, or on a circle. Inexact versions of these tests are vulnerable to roundoff error, and the wrong answers they produce can cause geometric algorithms to hang, crash, or produce incorrect output. Figure 17 demonstrates a real example of the failure of Triangle's divide-and-conquer algorithm.


Figure 17: Left: A Delaunay triangulation (two of the guitar's tuning screws). Right: An invalid triangulation created by Triangle with exact arithmetic disabled.

The easiest solution to many of these robustness problems is to use software implementations of exact arithmetic, albeit often at great expense. It is common to hear reports of implementations being slowed by factors of ten or more as a consequence. The goal of improving the speed of correct geometric calculations has received much recent attention [4, 8, 1], but the most promising proposals take integer or rational inputs, often of limited precision. These methods do not appear to be usable if it is convenient or necessary to use ordinary floating-point inputs.

Triangle includes fast correct implementations of the orientation and incircle tests that take floating-point inputs. They owe their speed to two features. First, they employ new fast algorithms for arbitrary precision arithmetic that have a strong advantage over other software techniques in computations that manipulate values of extended but small precision. Second, they are adaptive; their running time depends on the degree of uncertainty of the result, and is usually small. For instance, the adaptive orientation test is slow only if the points being tested are nearly or exactly collinear.

The orientation and incircle tests both work by computing the sign of a determinant. Fortune and Van Wyk [8] take advantage of the fact that only the sign is needed by using a floating-point filter: the determinant is first evaluated approximately, and only if forward error analysis indicates that the sign of the approximate result cannot be trusted does one use an exact test. Triangle's adaptive implementations carry this suggestion to its logical extreme by computing a sequence of successively more accurate approximations to the determinant, stopping only when the accuracy of the sign is assured. To reduce computation time, some of these approximations can reuse previous, less accurate computations. Shewchuk [16] presents details of the arbitrary precision arithmetic algorithms and the adaptivity scheme, and provides empirical evidence that multiple-stage adaptivity can significantly improve on two-stage adaptivity when difficult point sets are triangulated.

Using the adaptive tests, Triangle computes Delaunay triangulations, constrained Delaunay triangulations, and convex hulls exactly, roundoff error notwithstanding. Table 1 shows that the robust tests usually incur only a 10% to 30% overhead, though more time may be needed for points sets with many near-degeneracies. One exception is the divide-and-conquer algorithm with vertical cuts. Because this algorithm repeatedly merges tall, thinly separated triangulations, it performs many orientation tests on nearly-collinear points, and hence the robust version is much slower than the non-robust version. The variant that uses alternating cuts encounters nearly-collinear points less often; hence, its robust version suffers a smaller speed handicap, and its non-robust version is less likely to fail.

Of course, adaptive tests do not solve all robustness problems. Geometric computations that produce new vertices, including circumcenters and segment intersections, could be performed exactly in principle, but the results would have large bit complexity and would be inconvenient to manipulate and expensive to store. Worse, vertices of arbitrarily large bit complexity could eventually be produced in a cascading effect when the Delaunay refinement algorithm inserts circumcenters of triangles whose vertices were themselves circumcenters. Hence, it is infeasible to make the algorithm perfectly robust. Fortunately, the Delaunay refinement algorithm is naturally stable with regard to floating-point roundoff error. Problems arise only when triangles are refined to so small a size that it is no longer possible to construct a circumcenter that is distinct from its triangle's vertices.

I have not produced a robust version of the sweepline algorithm for a somewhat technical reason. The sweepline algorithm maintains a priority queue (normally implemented as a heap) containing two types of events: site events, where the sweepline passes over an input point, and circle events, where the sweepline reaches the top of a circle defined by three consecutive vertices on the boundary of the triangulation. Unfortunately, the y-coordinate of such a circle top is expensive to compute exactly, may be irrational, and has a somewhat complicated exact representation. A robust implementation must keep the events correctly ordered, and hence must replace the simple comparisons normally used to maintain a priority queue with a test that correctly compares two circle tops. Even a fast adaptive version of such a test would be so much slower than simple comparisons that event queue maintenance, which is a dominant cost of the sweepline algorithm, would become prohibitively expensive.

Next: Additional Implementation Notes Up: Triangle: Engineering a 2D Mesh Generator Previous: A Negative Result on Quality Triangulations

Jonathan Richard Shewchuk
Mon Aug 12 10:28:49 EDT 1996