Nicola Wolpert

Fast and exact geometric analysis of real algebraic plane curves

An algorithm is presented for the geometric analysis of an algebraic curve f(x, y) = 0 in the real affine plane. It computes a cylindrical algebraic decomposition (CAD) of the plane, augmented with adjacency information. The adjacency information describes the curves topology by a topologically equivalent planar graph. The numerical data in the CAD gives an embedding of the graph. The algorithm is designed to provide the exact result for all inputs but to perform only few symbolic operations for the sake of efficiency. In particular, the roots of f(∝, y) at a critical x-coordinate . The algorithm is implemented as C++ library AlciX in the EXACUS project. Running time comparisons with top by Gonzalez-Vega and Necula (2002), and with cad2d by Brown demonstrate its efficiency.

On the exact computation of the topology of real algebraic curves

We consider the problem of computing a representation of the plane graph induced by one (or more) algebraic curves in the real plane. We make no assumptions about the curves, in particular we allow arbitrary singularities and arbitrary intersection. This problem has been well studied for the case of a single curve. All proposed approaches to this problem so far require finding and counting real roots of polynomials over an algebraic extension of Q, i.e. the coefficients of those polynomials are algebraic numbers. Various algebraic approaches for this real root finding and counting problem have been developed, but they tend to be costly unless speedups via floating point approximations are introduced, which without additional checks in some cases can render the approach incorrect for some inputs.We propose a method that is always correct and that avoids finding and counting real roots of polynomials with non-rational coefficients. We achieve this using two simple geometric approaches: a triple projections method and a curve avoidance method. We have implemented our approach for the case of computing the topology of a single real algebraic curve. Even this prototypical implementation without optimizations appears to be competitive with other implementations.

An exact and efficient approach for computing a cell in an arrangement of quadrics

We present an approach for the exact and efficient computation of a cell in an arrangement of quadric surfaces. All calculations are based on exact rational algebraic methods and provide the correct mathematical results in all, even degenerate, cases. By projection, the spatial problem is reduced to the one of computing planar arrangements of algebraic curves. We succeed in locating all event points in these arrangements, including tangential intersections and singular points. By introducing an additional curve, which we call the Jacobi curve, we are able to find non-singular tangential intersections. We show that the coordinates of the singular points in our special projected planar arrangements are roots of quadratic polynomials. The coefficients of these polynomials are usually rational and contain at most a single square root. A prototypical implementation indicates that our approach leads to good performance in practice.

An exact, complete and efficient implementation for computing planar maps of quadric intersection curves

We present the first exact, complete and efficient implementation that computes for a given set P=p1,...,pn of quadric surfaces the planar map induced by all intersection curves p1∩ pi, 2 ≤ i ≤ n, running on the surface of p1. The vertices in this graph are the singular and x-extreme points of the curves as well as all intersection points of pairs of curves. Two vertices are connected by an edge if the underlying points are connected by a branch of one of the curves. Our work is based on and extends ideas developed in [20] and [9].Our implementation is complete in the sense that it can handle all kind of inputs including all degenerate ones where intersection curves have singularities or pairs of curves intersect with high multiplicity. It is exact in that it always computes the mathematical correct result. It is efficient measured in running times.

EXACUS: efficient and exact algorithms for curves and surfaces

We present the first release of the Exacus C++ libraries. We aim for systematic support of non-linear geometry in software libraries. Our goals are efficiency, correctness, completeness, clarity of the design, modularity, flexibility, and ease of use. We present the generic design and structure of the libraries, which currently compute arrangements of curves and curve segments of low algebraic degree, and boolean operations on polygons bounded by such segments.

Jacobi curves: Computing the exact topology of arrangements of non-singular algebraic curves

We present an approach that extends the Bentley-Ottmann sweep-line algorithm [2] to the exact computation of the topology of arrangements induced by non-singular algebraic curves of arbitrary degrees. Algebraic curves of degree greater than 1 are difficult to handle in case one is interested in exact and efficient solutions. In general, the coordinates of intersection points of two curves are not rational but algebraic numbers and this fact has a great negative impact on the efficiency of algorithms coping with them. The most serious problem when computing arrangements of non-singular algebraic curves turns out be the detection and location of tangential intersection points of two curves. The main contribution of this paper is a solution to this problem, using only rational arithmetic. We do this by extending the concept of Jacobi curves introduced in [11]. Our algorithm is output-sensitive in the sense that the algebraic effort we need for sweeping a tangential intersection point depends on its multiplicity.

Complete, exact, and efficient computations with cubic curves

The Bentley-Ottmann sweep-line method can be used to compute thearrangement of planar curves provided a number of geometricprimitives operating on the curves are available. We discuss themathematics of the primitives for planar algebraic curves of degreethree or less and derive efficient realizations. As a result, weobtain a complete, exact, and efficient algorithm for computingarrangements of cubic curves. Conics and cubic splines are specialcases of cubic curves. The algorithm is complete in that it handles all possibledegeneracies including singularities. It is exact in that itprovides the mathematically correct result. It is efficient in thatit can handle hundreds of curves with a quarter million of segmentsin the final arrangement.

Exact, efficient, and complete arrangement computation for cubic curves

The Bentley-Ottmann sweep-line method can compute the arrangement of planar curves, provided a number of geometric primitives operating on the curves are available. We discuss the reduction of the primitives to the analysis of curves and curve pairs, and describe efficient realizations of these analyses for planar algebraic curves of degree three or less. We obtain a complete, exact, and efficient algorithm for computing arrangements of cubic curves. Special cases of cubic curves are conics as well as implicitized cubic splines and Bezier curves. The algorithm is complete in that it handles all possible degeneracies such as tangential intersections and singularities. It is exact in that it provides the mathematically correct result. It is efficient in that it can handle hundreds of curves with a quarter million of segments in the final arrangement. The algorithm has been implemented in C++ as an Exacus library called CubiX.

Snap rounding of Bézier curves

We present an extension of snap roundingfrom straight-line segments (see Guibas and Marimont, 1998)to Bézier curves of arbitrary degree, and thus the first method for geometric roundingof curvilinear arrangements.Our algorithm takes a set of intersecting Bézier curvesand directly computes a geometric rounding of their true arrangement, without the need of representing the true arrangement exactly.The algorithms output is a deformation of the true arrangementthat has all Bézier control points at integer pointsand comes with the same geometric guarantees as instraight-line snap rounding: during rounding, objects do not movefurther than the radius of a pixel, and features of thearrangement may collapse but do not invert.

Completely randomized RRT-connect: A case study on 3D rigid body motion planning

Nowadays sampling-based motion planners use the power of randomization to compute multidimensional motions at high performance. Nevertheless the performance is based on problem-dependent parameters like the weighting of translation versus rotation and the planning range of the algorithm. Former work uses constant user-adjusted values for these parameters which are defined a priori. Our new approach extends the power of randomization by varying the parameters randomly during runtime. This avoids a preprocessing step to adjust parameters and moreover improves the performance in comparison to existing methods in the majority of the benchmarks. Our method is simple to understand and implement. In order to compare our approach we present a comprehensive experimental analysis about the parameters and the resulting performance. The algorithms and data structures were implemented in our own library RASAND, but we also compare the results of our work with OMPL [12] and the commercial software Kineo™ Kite Lab [15].