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 algorithm's 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

On Multiple Roots in Descartes' Rule and Their Distance to Roots of Higher Derivatives

If an open interval $I$ contains a $k$-fold root $\alpha$ of a real polynomial~$f$, then, after transforming $I$ to $(0,\infty)$, Descartes' Rule of Signs counts exactly $k$ roots of $f$ in~$I$, provided $I$ is such that Descartes' Rule counts no roots of the $k$-th derivative of~$f$. We give a simple proof using the Bernstein basis. The above condition on $I$ holds if its width does not exceed the minimum distance $\sigma$ from $\alpha$ to any complex root of the $k$-th derivative. We relate $\sigma$ to the minimum distance $s$ from $\alpha$ to any other complex root of $f$ using Szeg{\H o}'s composition theorem. For integer polynomials, $\log(1/\sigma)$ obeys the same asymptotic worst-case bound as $\log(1/s)$

Exact and Efficient 2D-Arrangements of Arbitrary Algebraic Curves

We show how to compute the planar arrangement induced by segments of arbitrary algebraic curves with the Bentley-Ottmann sweep-line algorithm. The necessary geometric primitives reduce to cylindrical algebraic decompositions of the plane for one or two curves. We compute them by a new and efficient method that combines adaptive-precision root finding (the Bitstream Descartes method of Eigenwillig et~al.,\ 2005) with a small number of symbolic computations, and that delivers the exact result in all cases. Thus we obtain an algorithm which produces the mathematically true arrangement, undistorted by rounding error, for any set of input segments. Our algorithm is implemented in the EXACUS library AlciX. We report on experiments; they indicate the efficiency of our approach

Sweeping Arrangements of Cubic Segments Exactly and Efficiently

A method is presented to compute the planar arrangement induced by segments of algebraic curves of degree three (or less), using an improved Bentley-Ottmann sweep-line algorithm. Our method is exact (it provides the mathematically correct result), complete (it handles all possible geometric degeneracies), and efficient (the implementation can handle hundreds of segments). The range of possible input segments comprises conic arcs and cubic splines as special cases of particular practical importance

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 curve's 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 $f(\alpha,y)$ at a critical $x$-coordinate $\alpha$ are found with adaptive-precision arithmetic in all cases, using a variant of the Bitstream Descartes method~(Eigenwillig et~al., 2005). The algorithm may choose a generic coordinate system for parts of the analysis but provides its result in the original system. The algorithm has been implemented as C++ library \texttt{AlciX} in the EXACUS project. Running time comparisons with \texttt{top} by Gonzalez-Vega and Necula~(2002), and with \texttt{cad2d} by Brown demonstrate its efficiency

Complete, Exact and Efficient Computations with Cubic Curves

The Bentley-Ottmann sweep-line method can be used to compute the arrangement of planar curves provided a number of geometric primitives operating on the curves are available. We discuss the mathematics of the primitives for planar algebraic curves of degree three or less and derive efficient realizations. As a result, we obtain a complete, exact, and efficient algorithm for computing arrangements of cubic curves. Conics and cubic splines are special cases of cubic curves. The algorithm is complete in that it handles all possible degeneracies including 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