Implementing Delaunay Triangulations of the Bolza Surface

The CGAL library offers software packages to compute Delaunay triangulations of the (flat) torus of genus one in two and three dimensions. To the best of our knowledge, there is no available software for the simplest possible extension, i.e., the Bolza surface, a hyperbolic manifold homeomorphic to a torus of genus two. In this paper, we present an implementation based on the theoretical results and the incremental algorithm proposed last year at SoCG by Bogdanov, Teillaud, and Vegter. We describe the representation of the triangulation, we detail the different steps of the algorithm, we study predicates, and report experimental results

Implémentation des triangulations de Delaunay de la surface de Bolza

The CGAL library offers software packages to compute Delaunay triangulations of the (flat) torus of genus one in two and three dimensions. To the best of our knowledge, there is no available software for the simplest possible extension, i.e., the Bolza surface, a hyperbolic manifold homeomorphic to a torus of genus two.In this paper, we present an implementation based on the theoretical results and the incremental algorithm proposed recently. We describe the representation of the triangulation, we detail the different steps of the algorithm, we study predicates, and report experimental results.La bibliothèque logicielle CGAL offre des modules pour calculer des triangulations de Delaunay du tore plat de genre un en dimension deux et trois. À notre connaissance, il n’existe pas de logiciel pour l’extension la plus simple possible, c’est-à-dire la surface de Bolza, qui est une variété hyperbolique homéomorphe à un double tore.Dans cet article, nous présentons une implémentation basée sur les résultats théoriques et l’algorithme incrémental proposé récemment. Nous décrivons la représentation d’une triangulation, nous détaillons les différentes étapes de l’algorithme, nous étudions les prédicats et présentons des résultats expérimentaux

Delaunay triangulations of generalized Bolza surfaces

The Bolza surface can be seen as the quotient of the hyperbolic plane, represented by the Poincar\'e disk model, under the action of the group generated by the hyperbolic isometries identifying opposite sides of a regular octagon centered at the origin. We consider generalized Bolza surfaces $\mathbb{M}_g$, where the octagon is replaced by a regular $4g$-gon, leading to a genus $g$ surface. We propose an extension of Bowyer's algorithm to these surfaces. In particular, we compute the value of the systole of $\mathbb{M}_g$. We also propose algorithms computing small sets of points on $\mathbb{M}_g$ that are used to initialize Bowyer's algorithm.Comment: 50 pages, 28 figure

Delaunay triangulations of hyperbolic surfaces

Triangulations are among the most important and well-studied objects in computational geometry. A triangulation is a subdivision of a surface into triangles. This allows the use of computer algorithms to analyze the geometry of the surface or perform simulations. A Delaunay triangulation is a particular kind of triangulation that is often used because of its favorable properties. In this thesis we studied Delaunay triangulations of hyperbolic surfaces. Hyperbolic surfaces are surfaces with a constant negative curvature and can be used to model shapes or structures that, intuitively speaking, cannot be "flattened" in the Euclidean plane. In the thesis we describe the properties of a specific class of hyperbolic surfaces that allow a well-known algorithm for computing Delaunay triangulations to be generalized to these surfaces. In particular, we compute the systole of these surfaces, which is an important parameter in the algorithm. Moreover, we provide upper and lower bounds for the minimal number of vertices of Delaunay triangulations of hyperbolic surfaces and show that these bounds are asymptotically optimal

Delaunay triangulations of symmetric hyperbolic surfaces

International audienceOf the several existing algorithms for computing Delaunay triangulations of point sets in Euclidean space, the incremental algorithm has recently been extended to the Bolza surface, a hyperbolic surface of genus 2. We will generalize this algorithm to so called symmetric hyperbolic surfaces of arbitrary genus. Delaunay triangulations of point sets on hyperbolic surfaces can be constructed by using the fact that such point sets can be regarded as periodic point sets in the hyperbolic plane. However, one of the main issues is then that the result might contain 1-or 2-cycles, which means that the triangulation is not simplicial. As the incremental algorithm that we use can only work with simplicial complexes, this situation must be avoided. In this work, we will first compute the systole of the symmetric hyperbolic surfaces, i.e., the length of the shortest non-contractible loop. The value of the systole is used in a condition to ensure that the triangulations will be simplicial. Secondly, we will show that it is sufficient to consider only a finite subset of the infinite periodic point set in the hyperbolic plane. Finally, we will algorithmically construct a point set with which we can initialize the algorithm

Systole of regular hyperbolic surfaces with an application to Delaunay triangulations

International audienceThe talk presents results regarding the properties of some symmetric hyperbolic surfaces of genus at least 2, with an application to the computation of Delaunay triangulations of such surfaces

Triangulations de Delaunay d'une famille de surfaces hyperboliques symétriques en pratique

The Bolza surface is the most symmetric compact orientable hyperbolic surface of genus 2. For any genus higher than 2, there exists one compact orientable surface constructed in a similar way as the Bolza surface having the same kind of symmetry. We refer to this family of surfaces as symmetric hyperbolic surfaces. This thesis deals with the computation of Delaunay triangulations of symmetric hyperbolic surfaces.Delaunay triangulations of compact surfaces can be seen as periodic Delaunay triangulations of their universal cover (in our case, the hyperbolic plane). A Delaunay triangulation is for us a simplicial complex. However, not all sets of points define a simplicial decomposition of a symmetric hyperbolic surface. In the literature, an algorithm has been proposed to deal with this issue by using so-called dummy points: initially a triangulation of the surface is constructed with a set of dummy points that defines a Delaunay triangulation of the surface, then input points are inserted with the well-known incremental algorithm by Bowyer, and finally the dummy points are removed, if the triangulation remains a simplicial complex after their removal. For the Bolza surface, the set of dummy points to initialize the triangulation is given. The existing algorithm computes a triangulation of the Bolza surface as a periodic triangulation of the hyperbolic plane and requires to identify a suitable subset of the hyperbolic plane in which to work.We study the properties of Delaunay triangulations of the Bolza surface defined by sets of points containing the proposed set of dummy points, and we describe in detail an implementation of the incremental algorithm for it. We begin by identifying a subset of the hyperbolic plane that contains at least one representative for each face of a Delaunay triangulation of the surface, which enables us to define a unique canonical representative in the hyperbolic plane for each face on the surface. We give a data structure to represent a Delaunay triangulation of the Bolza surface via the canonical representatives of its faces in the hyperbolic plane. We detail the construction of such a triangulation and additional operations that enable the location of points and the removal of vertices. We also report results on the algebraic degree of predicates needed for all operations. We provide a fully dynamic implementation for the Bolza surface, supporting insertion of new points, removal of existing vertices, point location, and construction of dual objects. Our implementation is based on CGAL, the Computational Geometry Algorithms Library, and is currently under revision for integration in the library. To incorporate our code into CGAL, all the objects that we introduce must be compatible with the existing framework and comply with the standards adopted by the library. We give a detailed description of the classes used to represent and handle periodic hyperbolic triangulations and related objects. Benchmarks and tests are performed to evaluate our implementation, and a simple application is given in the form of a CGAL demo.We discuss an extension of our implementation to symmetric hyperbolic surfaces of genus higher than 2. We propose three methods to generate sets of dummy points for each surface and present the advantages and shortcomings of each method. We identify a suitable subset of the hyperbolic plane that contains at least one representative for each face of a Delaunay triangulation of the surface, and we define a canonical representative in the hyperbolic plane for each face on the surface. We describe a data structure to represent such a triangulation via the canonical representatives of its faces, and give algorithms for the initialization of the triangulation with dummy points. Finally, we discuss a preliminary implementation in which we examine the difficulties of having efficient exact predicates for the construction of Delaunay triangulations of symmetric hyperbolic surfaces.La surface de Bolza est la surface hyperbolique orientable compacte la plus symétrique de genre 2. Pour tout genre supérieur à 2, il existe une surface orientable compacte construite de manière similaire à la surface de Bolza et ayant le même type de symétries. Nous appelons ces surfaces des surfaces hyperboliques symétriques. Cette thèse porte sur le calcul des triangulations de Delaunay (TD) de surfaces hyperboliques symétriques.Les TD de surfaces compactes peuvent être considérées comme des TD périodiques de leur revêtement universel (dans notre cas, le plan hyperbolique). Une TD est pour nous un complexe simplicial. Cependant, les ensembles de points ne définissent pas tous une décomposition simpliciale d'une surface hyperbolique symétrique. Dans la littérature, un algorithme a été proposé pour traiter ce problème avec l'utilisation de points factices : initialement une TD de la surface est construite avec un ensemble de points connu, puis des points d'entrée sont insérés avec le célèbre algorithme incrémental de Bowyer, et enfin les points factices sont supprimés, si la triangulation reste toujours un complexe simplicial. Pour la surface de Bolza, les points factices sont spécifiés. L'algorithme existant calcule une DT de la surface de Bolza comme une DT périodique du plan hyperbolique, ce qui nécessite de travailler dans un sous-ensemble approprié du plan hyperbolique.Nous étudions les propriétés des TD de la surface de Bolza définies par des ensembles de points contenants l'ensemble proposé de points factices, et nous décrivons en détail une implémentation de l'algorithme incrémentiel pour cette surface. Nous commençons par définir un représentant canonique unique qui est contenu dans un sous-ensemble borné du plan hyperbolique pour chaque face d'une TD de la surface. Nous donnons une structure de données pour représenter une TD de la surface de Bolza via les représentants canoniques de ses faces. Nous détaillons les étapes de la construction d'une telle triangulation et les opérations supplémentaires qui permettent de localiser les points et de retirer des sommets. Nous présentons également les résultats sur le degré algébrique des prédicats nécessaires pour toutes les opérations.Nous fournissons une implémentation entièrement dynamique pour la surface de Bolza, en offrant l'insertion de nouveaux points, la suppression des sommets existants, la localisation des points, et la construction d'objets duaux. Notre implémentation est basée sur la bibliothèque CGAL (Computational Geometry Algorithms Library), et est actuellement en cours de révision pour être intégrée dans la bibliothèque. L'intégration de notre code dans CGAL nécessite que tous les objets que nous introduisons soient compatibles avec le cadre existant et conformes aux standards adoptés par la bibliothèque. Nous donnons une description détaillée des classes utilisées pour représenter et traiter les triangulations hyperboliques périodiques et les objets associés. Des analyses comparatives et des tests sont effectués pour évaluer notre implémentation, et une application simple est donnée sous la forme d'une démonstration CGAL.Nous discutons une extension de notre implémentation à des surfaces hyperboliques symétriques de genre supérieur à 2. Nous proposons trois méthodes pour engendrer des ensembles de points factices pour chaque surface et présentons les avantages et les inconvénients de chaque méthode. Nous définissons un représentant canonique contenu dans un sous-ensemble borné du plan hyperbolique pour chaque face d'une TD de la surface. Nous décrivons une structure de données pour représenter une telle triangulation via les représentants canoniques de ses faces, et donnons des algorithmes pour l'initialisation de la triangulation. Enfin, nous discutons une implémentation préliminaire dans laquelle nous examinons les difficultés d'avoir des prédicats exacts efficaces pour la construction de TD de surfaces hyperboliques symétriques

Delaunay triangulations of regular hyperbolic surfaces

International audienceThe talk presents work on computing Delaunay triangulations of some symmetric hyperbolic surfaces of genus at least 2

Delaunay triangulations of generalized Bolza surfaces

International audienceThe Bolza surface can be seen as the quotient of the hyperbolic plane, represented by the Poincaré disk model, under the action of the group generated by the hyperbolic isometries identifying opposite sides of a regular octagon centered at the origin. We consider generalized Bolza surfaces Mg, where the octagon is replaced by a regular 4g-gon, leading to a genus g surface. We propose an extension of Bowyer's algorithm to these surfaces. In particular, we compute the value of the systole of Mg. We also propose algorithms computing small sets of points on Mg that are used to initialize Bowyer's algorithm