Association Mouvement/Géométrie pour représentations volumiques

Session: AnimationNational audienceLes modèles particulaires permettent de produire des animations riches et variées. Ils sont particulièrement adaptés à certains effets d'animation. Mais intrinsèquement, ils ne sont pas basés sur des représentations surfaciques ou volumiques d'objets. Ainsi, visualiser le mouvement qu'ils décrivent peut poser problème car ils ne contiennent souvent pas assez d'information pour reconstruire la moindre topologie spatiale sous-jacente. Plus précisément, un mouvement produit par de tels modèles peut être rendu via différentes formes géométriques et mener à autant d'interprétations visuelles, sans contrôle de l'utilisateur. À notre connaissance, il n'existe pas de méthode générique associant des mouvements basés points, comme ceux produits par un modèle particulaire, ou n'importe quel ensemble de points en mouvement, à une structure topologique. Dans cet article, nous proposons un "framework" permettant d'associer, selon les souhaits de l'utilisateur, n'importe quelle forme volumique à n'importe quel mouvement basé points, et de contrôler les changements topologiques. Il est ainsi possible de créer différents résultats visuels avec une unique description de mouvement. Ce "framework" est séparé en trois processus distincts : l'association entre particules et sommets, la définition de l'application du mouvement aux sommets du maillage, et les modifications topologiques et les événements qui les déclenchent. Nous montrons comment la manipulation de ces paramètres permet d'expérimenter différentes associations sur un même mouvement

Exploring the Use of Adaptively Restrained Particles for Graphics Simulations

International audienceIn this paper, we explore the use of Adaptively Restrained (AR) particles for graphics simulations. Contrary to previous methods, Adaptively Restrained Particle Simulations (ARPS) do not adapt time or space sampling, but rather switch the positional degrees of freedom of particles on and off, while letting their momenta evolve. Therefore, inter-particles forces do not have to be updated at each time step, in contrast with traditional methods that spend a lot of time there. We present the initial formulation of ARPS that was introduced for molecular dynamics simulations, and explore its potential for Computer Graphics applications: We first adapt ARPS to particle-based fluid simulations and propose an efficient incremental algorithm to update forces and scalar fields. We then introduce a new implicit integration scheme enabling to use ARPS for cloth simulation as well. Our experiments show that this new, simple strategy for adaptive simulations can provide significant speedups more easily than traditional adaptive models

Seamless Adaptivity of Elastic Models

International audienceA new adaptive model for viscoelastic solids is presented. Unlike previous approaches, it allows seamless transitions, and simplifications in deformed states. The deformation field is generated by a set of physically animated frames. Starting from a fine set of frames and mechanical energy integration points, the model can be coarsened by attaching frames to others, and merging integration points. Since frames can be attached in arbitrary relative positions, simplifications can occur seamlessly in deformed states, without returning to the original shape, which can be recovered later after refinement. We propose a new class of velocity-based simplification criterion based on relative velocities. Integration points can be merged to reduce the computation time even more, and we show how to maintain constant elastic forces through the levels of detail. This meshless adaptivity allows significant improvements of computation time

Macro-mechanical modelling and simulation of textile fabric and clothing with S-FEM

Tese de Doutoramento Programa Doutoral em Engenharia TêxtilEsta tese propõe um método de elementos finitos, designado por S-FEM (Smoothed Finite Element Method), para modelação e análise mecânica de estruturas têxteis planas. Neste enquadramento teórico, supõe-se que a estrutura têxtil não-tecida é um material isotrópico elástico, enquanto a estrutura têxtil tecida é um material elástico com anisotropia ortotrópica, para os quais as leis constitutivas utilizam propriedades mecânicas de baixa pressão (low stress) com base na Medição Objetiva de Tecidos (FOM - Fabric Objective Measurement). As formulações de elementos finitos de baixa ordem baseadas em deslocamento quando aplicadas a elementos finitos de placas (plate/shell) quadriláteras de 4 nós, incluindo campos de tensão de cisalhamento transversal, baseiam-se nas contribuições de Raymond Mindlin e por Eric Reissner, no que agora se designa teoria de deformação por cisalhamento de primeira ordem (first-order shear deformation, do inglês, ou FSDT de forma abreviada), ou simplesmente teoria de Mindlin-Reissner, e nas abordagens MITC (Mixed Interpolation of Tensorial Components), são nesta tese combinadas com a técnica de suavização do/da gradiente/tensão nos termos dos modelos S-FEM por forma a mitigar problemas como são o caso da distorção de elementos finitos, da granularidade grosseira da malha, bem como dos bem conhecidos fenómenos de bloqueio. As malhas de quadriláteros são utilizadas nesta tese devido à sua capacidade de representar geometrias complexas de tecidos em resultado de deformações mecânicas como são os casos da recuperação face à pressão planar, flexão, deformação, vibração, drapejamento, etc. Refira-se que foi desenvolvido e implementado em Matlab um software para os novos modelos de elementos finitos, em grande medida devido à inexistência de modelos S-FEM em softwares de análise de elementos finitos (finite element analysis ou FEA), lacuna esta que ocorre quer em softwares comerciais, quer não comerciais, e até em códigos abertos. Exemplos numéricos para as aplicações básicas de engenharia no que respeita à modelação mecânica de folhas de tecido fino e de folhas de tecido de espessura média em estudos de casos típicos, como é o caso da recuperação face a pressão planar, flexão, deformação e comportamento livre de vibrações, indicam que os elementos finitos (plate/shell) desenvolvidos com a técnica de suavização de tensão e MITC acabam por aliviar os efeitos de distorção dos elementos, a granularidade grosseira da malha e efeito de bloqueio na modelação e análise mecânica de tecidos muito finos e até mesmo de tecidos de espessura média. Os modelos de elementos finitos de placas (plate/shell) desenvolvidos durante o trajeto desta tese, bem como as suas propriedades mecânicas de baixa tensão em termos de FOM, são, portanto, bem adaptados à modelação e análise numérica de deformação macro-mecânica de folhas de tecido muito fino e de folhas de tecido de espessura média, incluindo ao mesmo tempo análise de deformação mecânica simples e complexa.An S-FEM (Smoothed Finite Element Method) for mechanical analysis and modelling of the textile fabrics is proposed. In this theoretical framework, one assumes that the non-woven fabric is an elastic isotropic material, while the woven fabric is an elastic with orthotropic anisotropy for which the constitutive laws formulated are using low-stress mechanical properties based on FOM (Fabric Objective Measurement). The displacement-based low-order finite element formulations for four-node quadrilateral plate/shell finite element, including assumed transverse shear strain fields, are based on the contributions of Raymond Mindlin and by Eric Reissner as FSDT (first-order shear deformation theory and so-called the Mindlin-Reissner theory) together with MITC (Mixed Interpolation of Tensorial Components) approaches, which are combined with the gradient/strain smoothing technique in terms of S-FEM models contributed by G. R. Liu et al. in order to mitigate problems as element distortion, mesh coarseness as well as the well-known locking phenomena. Quadrilateral meshes are used due to ability to represent complicated geometries of complex mechanical deformation of the fabric such as plane stress recovery, bending, buckling, vibration, draping behavior, etc. The finite element computer codes were developed in MATLAB for the new formulated plate/shell finite element models due to the lack of FEM (Finite Element Method) packages for S-FEM models in both commercial and non-commercial FEA (Finite Element Analysis) computer applications, and even from open-source platforms. Numerical examples for the basic engineering applications of mechanical modelling of thin to moderately thick fabric sheet in the typical case studies such as in-plane stress recovery, bending, buckling and free-vibration behavior, indicate that the developed plate/shell finite elements with assumed strain smoothing technique and MITC, do alleviate element distortion, mesh coarseness, and locking effect even for mechanical analysis and modelling very thin to moderately thick fabric. The developed plate/shell finite element models and low-stress mechanic properties in terms of FOM are, therefore, well adapted for numerical analysis and modelling of macro-mechanical deformation of the thin to moderately thick fabric sheet including both simple and complex mechanical deformation analysis.EMECW L12 MOBILITY GRANT AWARD CONTRACT BTG_559 Grant agreement n 2009/1661-001 001EC

Simulation inkompressibler deformierbarer Körper

Die computergestützte Simulation von Bewegungsabläufen wird immer wichtiger in vielen Anwendungsgebieten. Einsatzgebiete von dynamischen Simulationen sind beispielsweise die Erstellung von Computeranimationen für Filme, Anwendungen in der virtuellen Realität oder für Computerspiele. In diesen Gebieten genügen oft plausible Ergebnisse, die dem Anwender das Gefühl einer realistischen Bewegung vermitteln. Hier kann die Simulation volumenerhaltender Körper zur Verbesserung der visuellen Plausibilität

Dynamiksimulation in der Computergraphik

Die interaktive physikalisch-basierte Simulation von Starrkörpern und deformierbaren Festkörpern ist ein wichtiges und aktuelles Forschungsgebiet in der Computergraphik und ein essentieller Bestandteil in vielen Anwendungen, wie z.B. Virtual Prototyping, Computerspiele oder Trainingssimulatoren. In dieser Arbeit werden interaktive Simulationsmethoden für Mehrkörpersysteme, Textilien und inkompressible deformierbare Volumenkörper vorgestellt