Courbe de Bézier | EPFL Graph Search

Les courbes de Bézier sont des courbes polynomiales paramétriques développées pour concevoir des pièces de carrosserie d'automobiles. Elles ont été conçues par Paul de Casteljau en 1959 pour Citroën et, indépendamment, par Pierre Bézier en 1962 pour Renault (les travaux de Paul de Casteljau étant confidentiels, c'est le nom de Bézier qui est passé à la postérité). Elles ont de nombreuses applications dans la et le rendu de polices de caractères. Elles ont donné naissance à de nombreux autres objets mathématiques. Il existait avant Bézier des courbes d'ajustement nommées splines, mais dont le défaut était de changer d'aspect lors d'une rotation de repère, ce qui les rendait inutilisables en CAO. Bézier partit d'une approche géométrique fondée sur la linéarité de l'espace euclidien et la théorie, déjà existante, du barycentre : si la définition est purement géométrique, aucun repère n'intervient puisque la construction en est indépendante, ce qui n'était pas le cas pour les splines (l

G1 continuous approximate curves on NURBS surfaces

Bailin Deng, Wei Zeng

Curves on surfaces play an important role in computer aided geometric design. In this paper, we present a parabola approximation method based on the cubic reparameterization of rational Bezier surfaces, which generates G1 continuous approximate curves lying completely on the surfaces by using iso-parameter curves of the reparameterized surfaces. The Hausdorff distance between the approximate curve and the exact curve is controlled under the user-specified tolerance. Examples are given to show the performance of our algorithm. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.

2012

Optimal control of geodesics in Riemannian manifolds

Roland Rozsnyo

The aim of this dissertation is to solve numerically the following problem, denoted by P : given a Riemannian manifold and two points a and b belonging to that manifold, find a tangent vector T at a, such that expa(T) = b, assuming that T exists. This problem is set under an optimal control formulation, which requires the definition of an objective function and a space of control, the choice of a method for the calculation of the descent direction of that function in the space of control and the use of an optimization algorithm to find its minimum, which corresponds to the solution of the original problem by construction. Several techniques are necessary to be put together, coming from the fields of geometry, numerical analysis and optimization. The first part will concern a recalling of the mathematical context in which this formulation takes place. The general principles of optimal control will also be given. In the second part, we will present an intrinsic formulation of the optimal control problem associated to P, based on Jacobi fields, which will play the role of the so called adjoint state. This derivation leads to necessary optimality conditions. We will illustrate explicitly that formulation by treating the specific case of Riemannian manifolds with constant sectional curvature. Then, we will derive the optimal control problem in coordinates, not only to check the intrinsic formulation but also to reveal how it is hidden behind the expressions in coordinates. Their use reveals some quantities whose interpretation may be given this way. Moreover, we will show that more possibilities exist to chose the cost function and the control space in coordinates. In a second step, an alternative approach will consider the Hamiltonian formulation of geodesics. This is an incursion into symplectic geometry. We will then reformulate the Riemannian optimal control problem in its Hamiltonian version. In the third part, the numerical methods used for solving P will be presented. The discretization imposes the definition of new discrete optimal control problems. The technique shows that the discrete adjoint state equation strongly depends on the numerical scheme used to solve the direct problem. We will give a collection of numerical computations in the specific case of parametric piece of surfaces, where the surface can be defined by one or several Bézier patches, each one corresponding to a chart, which is representative of a Riemannian manifold. We will compare the different numerical approaches. The last but one part will be devoted to the interesting application of wooden roof building, where the structure is made of wooden boards, with geodesic trajectories on the designed piece of surface. The Geos (Geodesic solver) software has been developed for that purpose. After having introduced some specific numerical methods used in the code, we present the Geos application interface (AI) developed as a tool for the conception of such a roof. We then show an existing wooden structure built according to that mean. Finally, we will summarize the results of our research and discuss future possible prospects.

EPFL2007

Optimal control of geodesics in Riemannian manifolds

Roland Rozsnyo

EPFL2007