Voronoi diagrams in the max-norm: algorithms, implementation, and applications

Voronoi diagrams and their numerous variants are well-established objects in computational geometry. They have proven to be extremely useful to tackle geometric problems in various domains such as VLSI CAD, Computer Graphics, Pattern Recognition, Information Retrieval, etc. In this dissertation, we study generalized Voronoi diagram of line segments as motivated by applications in VLSI Computer Aided Design. Our work has three directions: algorithms, implementation, and applications of the line-segment Voronoi diagrams. Our results are as follows: (1) Algorithms for the farthest Voronoi diagram of line segments in the Lp metric, 1 ≤ p ≤ ∞. Our main interest is the L2 (Euclidean) and the L∞ metric. We first introduce the farthest line-segment hull and its Gaussian map to characterize the regions of the farthest line-segment Voronoi diagram at infinity. We then adapt well-known techniques for the construction of a convex hull to compute the farthest line-segment hull, and therefore, the farthest segment Voronoi diagram. Our approach unifies techniques to compute farthest Voronoi diagrams for points and line segments. (2) The implementation of the L∞ Voronoi diagram of line segments in the Computational Geometry Algorithms Library (CGAL). Our software (approximately 17K lines of C++ code) is built on top of the existing CGAL package on the L2 (Euclidean) Voronoi diagram of line segments. It is accepted and integrated in the upcoming version of the library CGAL-4.7 and will be released in september 2015. We performed the implementation in the L∞ metric because we target applications in VLSI design, where shapes are predominantly rectilinear, and the L∞ segment Voronoi diagram is computationally simpler. (3) The application of our Voronoi software to tackle proximity-related problems in VLSI pattern analysis. In particular, we use the Voronoi diagram to identify critical locations in patterns of VLSI layout, which can be faulty during the printing process of a VLSI chip. We present experiments involving layout pieces that were provided by IBM Research, Zurich. Our Voronoi-based method was able to find all problematic locations in the provided layout pieces, very fast, and without any manual intervention

Voronoi diagram of orthogonal polyhedra in two and three dimensions

Τα διαγράμματα Voronoi αποτελούν μία θεμελιώδη γεωμετρική δομή δεδομένων και έκφραζουν αποστάσεις σημείων στο χώρο από ένα σύνολο αντικειμένων. Θεωρούμε ορθογώνια πολύεδρα ευθυγραμμισμένα με τους άξονες. Πρόκειται για πολύεδρα των οποίων οι έδρες σχηματίζουν ορθές γωνίες, και οι ακμές είναι παράλληλες προς τους άξονες ενός καρτεσιανού συστήματος συντεταγμένων. Κατασκευάζουμε το διάγραμμα Voronoi στο εσωτερικό ενός ορθογώνιου πολυέδρου με τρύπες που ορίζονται από αντίστοιχα πολύεδρα, χρησιμοποιώντας την max-νόρμα. Πρόκειται για έναν συνδυασμό που βρίσκει πολλές εφαρμογές σε τομείς όπως τα raster graphics και ο σχεδιασμός κυκλωμάτων VLSI. Παρουσιάζουμε έναν αλγόριθμο για την κατασκευή αυτών των διαγραμμάτων Voronoi σε δύο και τρεις διαστάσεις. Ακολουθούμε τη μέθοδο υποδιαίρεσης και βασιζόμαστε σε μία δομή δεδομένων από bounding-volumes: πρόκειται για μία μή τετριμμένη προσέγγιση του προβλήματος. Επιπλέον αναλύουμε την πολυπλοκότητα του αλγορίθμου, η οποία είναι γραμμική κάτω από μία υπόθεση ομοιόμορφα κατανεμημένης εισόδου. Μέρος της παρούσας εργασίας πρόκειται να δημοσιευθεί στα πρακτικά του συνεδρίου SEA^2 2019 (Special Event on Analysis of Experimental Algorithms).Voronoi diagrams are a fundamental geometric data structure for obtaining proximity relations. We consider axis-aligned orthogonal polyhedra in two and three-dimensional space. These are polyhedra whose faces meet at right angles and their edges are aligned with the axes of a coordinate system. We construct the exact Voronoi diagram inside an axis-aligned orthogonal polyhedron with holes defined by such polyhedra, under the max-norm. This is a particularly useful scenario in certain application domains, including raster graphics and VLSI design. Our approach avoids creating full-dimensional elements on the Voronoi diagram and yields a skeletal representation of the input object, equivalent to the straight skeleton. We introduce a complete algorithm in 2D and 3D that follows the subdivision paradigm relying on a bounding-volume hierarchy; this is an original approach to the problem. The algorithm reads in a region bounding the input polyhedron and performs a recursive subdivision into cells (using quadtrees and octrees for 2D and 3D resp.). Then, a reconstruction technique is applied to produce an isomorphic representation of the Voronoi diagram. An hierarchical data structure of bounding volumes is used to accelerate the 2D algorithm for certain inputs and is necessary for the efficiency of the 3D algorithm. The complexity is adaptive and comparable to that of previous methods. Under a mild assumption it is O(n / D+1 / D^2) in 2D or O(n a ^2 / D^2+1 / D^3) in 3D, where n is the number of sites, namely edges or facets respectively, D is the maximum cell size for the subdivision to stop (and is <1 under the appropriate scaling), and a bounds vertex cardinality per facet. We also provide a numerically stable, open-source implementation in Julia, illustrating the practical nature of our algorithm. Part of the current thesis is given in the paper "Voronoi diagram of orthogonal polyhedra in two and three dimensions", co-authored with Prof. Ioannis Z. Emiris, that is about to appear in Proceedings of SEA^2 2019 (Special Event on Analysis of Experimental Algorithms)

Collection of abstracts of the 24th European Workshop on Computational Geometry

International audienceThe 24th European Workshop on Computational Geomety (EuroCG'08) was held at INRIA Nancy - Grand Est & LORIA on March 18-20, 2008. The present collection of abstracts contains the 63 scientific contributions as well as three invited talks presented at the workshop

Robust and efficient software for problems in 2.5-dimensional non-linear geometry : algorithms and implementations

We discuss how to compute and implement three geometric problems dealing with nonlinear three-dimensional surfaces. As a main tool we rely on planar subdivisions induced by algebraic curves, developed in CGAL (Computational Geometry Algorithm Library). First, we achieve lower envelopes of quadrics using CGAL'S Envelope_3 package. Second, we extend CGAL's Arrangement_2 package to support two-dimensional arrangements on a parametric reference surface. Two main examples are discussed: Arrangements induced by algebraic surfaces on an elliptic quadric and on a ring Dupin cyclide. Third, we decompose a set of quadrics or a set of algebraic surfaces into cells using projection. Our goal is to achieve topological information for the surfaces, while preserving their geometric properties. We maintain a special two-dimensional arrangement; the lifting to the third dimension benefits from the recently presented bitstream Descartes method. The obtained cell decomposition supports a set of other geometric applications on surfaces. Our implementations follow the geometric programming paradigm. That is, we split combinatorial tasks from geometric operations by generic programming techniques. It is also ensured that each geometric predicate returns the mathematically correct result, even if it internally exploits approximative methods to speed up the computation. The thesis is written in English.Wir besprechen die Berechnung und Implementierung dreier Probleme aus der algorithmischen Geometrie, deren Eingabe aus gekrümmten Oberflächen besteht. Als Werkzeug benutzen wir in CGAL (Computational Geometry Algorithm Library) entwickelte Zerlegungen der Ebene durch algebraische Kurven. Zunächst berechnen wir die untere Einhüllende einer Menge von Quadriken. Danach erweitern wir CGALs Arrangement_2 Paket, so dass zweidimensionale Zerlegungen auf para-meterisierbaren Oberflächen berechnet werden können, und führen zwei konkrete Beispiele aus: Zerlegungen induziert durch algebraische Oberflächen auf einer Quadrik und auf einem ringförmigen Zykliden nach Dupin. Zum Abschluss unterteilen wir eine Menge von Quadriken bzw. algebraischen Oberflächen in disjunkte Untermannigfaltigkeiten mit Hilfe einer Projektion. Die Hebung erfolgt mit einem kürzlich vorgestellten approximativen Verfahren zur Nullstellenisolation (bitstream Descartes). Ingesamt erhalten wir geometrische Eigenschaften der Eingabe und erfahren mehr über deren topologische Zusammensetzung. Die kombinatorische Ausgabe hilft bei der Berechnung anderer geometrischer Probleme auf den Oberflächen. Unsere Implementierungen trennen kombinatorische Aufgaben von geometrischen durch Anwenden von generischen Programmiertechniken. Wir stellen außerdem sicher, dass Prädikate stets das mathematisch korrekte Ergebnis ausgeben, auch wenn sie intern mit approximativen Methoden rechnen. Die Arbeit ist in englischer Sprache verfasst

Delaunay triangulation in R3 on the GPU

Ph.DDOCTOR OF PHILOSOPH

Skeletal representations of orthogonal shapes

Skeletal representations are important shape descriptors which encode topological and geometrical properties of shapes and reduce their dimension. Skeletons are used in several fields of science and attract the attention of many researchers. In the biocad field, the analysis of structural properties such as porosity of biomaterials requires the previous computation of a skeleton. As the size of three-dimensional images become larger, efficient and robust algorithms that extract simple skeletal structures are required. The most popular and prominent skeletal representation is the medial axis, defined as the shape points which have at least two closest points on the shape boundary. Unfortunately, the medial axis is highly sensitive to noise and perturbations of the shape boundary. That is, a small change of the shape boundary may involve a considerable change of its medial axis. Moreover, the exact computation of the medial axis is only possible for a few classes of shapes. For example, the medial axis of polyhedra is composed of non planar surfaces, and its accurate and robust computation is difficult. These problems led to the emergence of approximate medial axis representations. There exists two main approximation methods: the shape is approximated with another shape class or the Euclidean metric is approximated with another metric. The main contribution of this thesis is the combination of a specific shape and metric simplification. The input shape is approximated with an orthogonal shape, which are polygons or polyhedra enclosed by axis-aligned edges or faces, respectively. In the same vein, the Euclidean metric is replaced by the L infinity or Chebyshev metric. Despite the simpler structure of orthogonal shapes, there are few works on skeletal representations applied to orthogonal shapes. Much of the efforts have been devoted to binary images and volumes, which are a subset of orthogonal shapes. Two new skeletal representations based on this paradigm are introduced: the cube skeleton and the scale cube skeleton. The cube skeleton is shown to be composed of straight line segments or planar faces and to be homotopical equivalent to the input shape. The scale cube skeleton is based upon the cube skeleton, and introduces a family of skeletons that are more stable to shape noise and perturbations. In addition, the necessary algorithms to compute the cube skeleton of polygons and polyhedra and the scale cube skeleton of polygons are presented. Several experimental results confirm the efficiency, robustness and practical use of all the presented methods

Large bichromatic point sets admit empty monochromatic 4-gons

We consider a variation of a problem stated by Erd˝os and Szekeres in 1935 about the existence of a number fES(k) such that any set S of at least fES(k) points in general position in the plane has a subset of k points that are the vertices of a convex k-gon. In our setting the points of S are colored, and we say that a (not necessarily convex) spanned polygon is monochromatic if all its vertices have the same color. Moreover, a polygon is called empty if it does not contain any points of S in its interior. We show that any bichromatic set of n ≥ 5044 points in R2 in general position determines at least one empty, monochromatic quadrilateral (and thus linearly many).Postprint (published version

LIPIcs, Volume 258, SoCG 2023, Complete Volume

LIPIcs, Volume 258, SoCG 2023, Complete Volum

New Applications of the Nearest-Neighbor Chain Algorithm

The nearest-neighbor chain algorithm was proposed in the eighties as a way to speed up certain hierarchical clustering algorithms. In the first part of the dissertation, we show that its application is not limited to clustering. We apply it to a variety of geometric and combinatorial problems. In each case, we show that the nearest-neighbor chain algorithm finds the same solution as a preexistent greedy algorithm, but often with an improved runtime. We obtain speedups over greedy algorithms for Euclidean TSP, Steiner TSP in planar graphs, straight skeletons, a geometric coverage problem, and three stable matching models. In the second part, we study the stable-matching Voronoi diagram, a type of plane partition which combines properties of stable matchings and Voronoi diagrams. We propose political redistricting as an application. We also show that it is impossible to compute this diagram in an algebraic model of computation, and give three algorithmic approaches to overcome this obstacle. One of them is based on the nearest-neighbor chain algorithm, linking the two parts together