Déformation et découpe interactive de solides à géométrie complexe

• Main Author: Bousquet, Guillaume
• Other Authors: Grenoble
• Language: fr
• Published: 2012
• Subjects: Simulation; Repères déformables; Skinning; Méthodes particulaires; Éléments finis; Simulation chirurgicale.; Deformable reference frames; Meshless method; Finite elements; Surgery simulation.
• Online Access: http://www.theses.fr/2012GRENM061/document

Cette thèse consiste à explorer une nouvelle approche pour la simulation d'objets flexibles par la mécanique des milieux continus, dans le cadre d'applications graphiques interactives telles que le jeu vidéo ou l'entraînement aux gestes chirurgicaux. Elle s'inscrit en continuité d'un stage de M2-R sur ce même sujet. Il est important de pouvoir régler simplement un compromis entre précision et temps de calcul suivant la nature de l'application. Les approches actuelles de simulation utilisent principalement la méthode des éléments finis. Celle-ci repose sur un maillage volumique des objets qu'il est souvent difficile d'adapter dynamiquement aux besoins de l'application. La nouveauté introduite par cette thèse est d'utiliser des repères déformables comme primitives cinématiques, avec des champs de déplacements inspirés des méthodes de 'skinning' utilisées en informatique graphique. Le but est d'éviter ainsi les difficultés liées au maillage volumique, ainsi que de faciliter le raffinement et la simplification adaptatives par simple ajout ou suppression de repère déformable là où c'est souhaitable. Ce travail est financé par le projet européen 'Passport for Virtual Surgery', dont le but est de créer automatiquement des modèles physiques pour l'entraînement aux gestes de chirurgie hépatique, à partir de données médicales et anatomiques personnalisées. Dans ce contexte, Guillaume, en collaboration avec d'autres membres du projet, mettra en place les outils nécessaires pour construire la scène physique à partir d'images médicales segmentées et de connaissances anatomiques génériques. Le foie sera dans un premier temps représenté par des modèles physiques précédemment développés à EVASION et étendus aux opérations de découpe. Par la suite, il y appliquera son nouveau modèle mécanique basé sur des repères déformables. The aim of this thesis is to develop a new approach for the simulation of flexible objects based on the continous middle method, related with interactive graphics applications such as video games or training in surgery. It is a continuity of the M2 research internship on the same topic. It is important to simply settle a compromise between accuracy and time computing according to the application. Current simulation approaches mainly use the finite element method, which is based on a volumetric mesh of the simulated objects. It is often difficult to dynamically adapt the needs to the application. The novelty of this thesis is to use deformable reference frames as kinematic primitives, with displacement fields based on 'skinning' methods used in computer graphics. The aim is to avoid the difficulties associated with volumetric mesh, and make the refinement and the adaptive simplification easier by adding or deleting deformable reference frames if necessary. This work is funded by the European project 'Passport for Virtual Surgery', which aims to automatically create models for physical training in gestures of liver surgery, from medical and anatomical custom data. In this context, Guillaume, in collaboration with other members of the project, will develop the tools necessary to build the physical scene from segmented medical images and generic anatomical knowledge. The liver will initially be represented by physical models previously developed in the EVASION team and then extended to cutting operations. Thereafter, Guillaume will apply his new mechanical model based on deformable reference frames. === Physically based deformable models have become ubiquitous in computer graphics. It allow to synthetize real behaviors, based on the physical laws from continuum mechanics. In this thesis, we focus on interactive simulations such as to video games or surgical simulators. The majority of the existing works focused up to here on the animation of objects made of homogeneous materials. Nevertheless, plenty of real objects, for instance like the biological structures, consist of multiple imbricated materials. Their decomposition in homogeneous zones requires a high-resolution spatial discretization to solve the variations of the material properties, which requires prohibitive computation time. In this context, we present new real time simulation techniques for deformable objects which can be cut. First of all, we present a real time method for cutting deformable objects in which, contrary to the previous methods, the object deforms on the cutting tool contact and cuts occur only when the pressure reaches a certain level. The independence of the physical, collision and visual models makes the topological changes easier. The GPU computing and local modifications enable fast execution. Then, a dynamic meshless method is described, which uses reference frames as control nodes instead of using points, with a displacement field formulation similar to skinning. It allows to easily tune the weights and benefits from the rigor of physical methods as the finite elements. The introduction of integration points, reducing the samples number by a least squares approximation, speeds up the spatial integrations. Other pre-computations are proposed in order to speed up the simulation time. Finally, new anisotropic shape functions are defined to encode the variations of material properties thanks to the introduction of the compliance distance. These complex shape functions uncouple the material resolution of the displacement functions ones. It allow an extremely sparse nodes sampling. The use of the compliance distance allows an automatic nodes distribution with regard to the material properties.