Dr. Simon Pabst WSI-GRIS, University of Tübingen email: spabst -at- gris.uni-tuebingen.de phone: [+49] (7071) 29-70425 office: Sand 14, C409

Publications and Projects

	Advances in Physically Based Deformable Object Simulation PhD thesis University of Tuebingen, 2011 Abstract: Physically based simulation of thin deformable objects is a well established, fascinating field of research in Computer Graphics. It is influenced by many different disciplines, ranging from physics, computational mathematics and computer science to material sciences and is a rewarding topic for study. In this thesis, we present a number of new techniques that improve the state of the art in the field. Chapter 2 describes a new nonlinear and efficient bending model for cloth simulations that can make use of measured material data and is also able to include the influence of seams and interlinings on fabric drape. Simulations using the new approach are compared to real-world counterparts for quality comparison. Physically based simulation is computationally expensive. Real-time performance is difficult to achieve due to high resolution geometry and mathematically involved computations, and keeping the highly deformable geometry intersection-free further aggravates the problem. In Chapter 3 we present techniques for parallel simulation on modern off-the-shelf multi-processor systems. We propose a parallel preconditioned conjugate gradients algorithm as the core of our simulation engine. Furthermore, we introduce parallel collision detection and handling with a novel dynamic task decomposition scheme based on temporal coherence data. Building on the insights gained in Chapter 3, we propose a second approach to parallel collision detection. Chapter 4 shows how to exploit modern graphics processors (GPUs) in addition to multiple CPUs. Specifically, we describe a spatial subdivision method that reveals a maximum of parallelism while minimizing the required communication between the computational units. In Chapter 5 we present a generalized dry Coulomb friction model in the anisotropic domain. The method is applicable to both elastic surfaces, like cloth and shells, and volumetric solids. It can realistically model anisotropic, heterogeneous, and asymmetric friction. Only negligibly higher computational costs compared to isotropic approaches are incurred and the additional effort is generously rewarded by a variety of interesting new effects. Finally, in Chapter 6 we propose a technique that simulates wet textiles using translational diffusion of liquids inside porous textiles. We combine this approach with our cloth simulation and additionally couple it to a particle-based fluid simulation. The fluid can interact with the textile and also wet it, thereby changing its material properties.
	Wet Cloth Simulation M. Huber, S. Pabst and W. Strasser Computer Graphics International Workshop, 2011 Abstract: We present a new technique to simulate wetting and wicking in fabrics based on translational diffusion theory. Our approach is physically-based and tracks the liquid state using a 2D cellular automaton. The liquid changes the local properties of the underlying fabric, making it heavier and softer. We use a simple adhesion model to simulate the stickiness of wet clothes. Previous approaches were based on full-fledged particle fluid simulations and we achieve orders of magnitudes faster computation times in comparison. The technique is easy to incorporate into state-of-the-art cloth simulation engines.
	Fast and Scalable CPU/GPU Collision Detection for Rigid and Deformable Surfaces (qt) S. Pabst, A. Koch and W. Strasser Symposium on Geometry Processing (SGP), 2010 Abstract: We present a new hybrid CPU/GPU collision detection technique for rigid and deformable objects based on spatial subdivision. Our approach efficiently exploits the massive computational capabilities of modern CPUs and GPUs commonly found in off-the-shelf computer systems. The algorithm is specifically tailored to be highly scalable on both the CPU and the GPU sides. We can compute discrete and continuous external and self-collisions of non-penetrating rigid and deformable objects consisting of many tens of thousands of triangles in few milliseconds on a modern PC. Our approach is orders of magnitude faster than earlier CPU-based approaches and up to twice as fast as the most recent GPU-based techniques.
	Anisotropic Friction for Deformable Surfaces and Solids (qt or avi) S. Pabst, B. Thomaszewski and W. Strasser Symposium on Computer Animation (SCA), 2009 Abstract: This paper presents a method for simulating anisotropic friction for deforming surfaces and solids. Frictional contact is a complex phenomenon that fuels research in mechanical engineering, computational contact mechanics, composite material design and rigid body dynamics, to name just a few. Many real-world materials have anisotropic surface properties. As an example, most textile materials exhibit direction-dependent frictional behavior, but despite its tremendous impact on visual appearance, only simple isotropic models have been considered for cloth and solid simulation so far. In this work, we propose a simple, application-oriented but physically sound model that extends existing methods to account for anisotropic friction. The sliding properties of surfaces are encoded in friction tensors, which allows us to model frictional resistance freely along arbitrary directions. We also consider heterogeneous and asymmetric surface roughness and demonstrate the increased simulation quality on a number of two- and three-dimensional examples. Our method is computationally efficient and can easily be integrated into existing systems.
	Continuum-based Strain Limiting (Video) B. Thomaszewski, S. Pabst and W. Strasser Computer Graphics Forum (Proceedings of Eurographics), 2009 Abstract: We present Continuum-based Strain Limiting (CSL) – a new method for limiting deformations in physically-based cloth simulations. Recent developments have led to methods which excel at simulating nearly inextensible materials, but the efficient simulation of general biphasic textiles and their anisotropic behavior remains challenging. Other approaches use softer materials and enforce limits on edge elongations, leading to discretization-dependent behavior. Moreover, they offer no explicit control over shearing and stretching unless specifically aligned meshes are used, which makes them less attractive for practical animation of anisotropic textiles. Based on a continuum deformation measure, our method allows accurate deformation control using individual thresholds for all types of strain. We impose deformation limits element-wise and cast the problem as a 6x6-system of linear equations. We show how to further improve efficiency using an approximate formulation. CSL can be combined with any type of cloth simulator and, as a velocity filter, integrates seamlessly into standard collision handling frameworks.
	Interactive Physically-Based Shape Editing J. Mezger, B. Thomaszewski, S. Pabst and W. Strasser Computer Aided Geometric Design, 2008 Abstract: We present an alternative approach to standard geometric shape editing using physically-based simulation. With our technique, the user can deform complex objects in real-time. The basis of our method is formed by a fast and accurate finite element implementation of an elasto-plastic material model, specifically designed for interactive shape manipulation. Using quadratic shape functions, we reduce approximation errors inherent to methods based on linear finite elements. The physical simulation uses a volume mesh comprised of quadratic tetrahedra, which are constructed from a coarser approximation of the detailed surface. In order to guarantee stability and real-time frame rates during the simulation, we cast the elasto-plastic problem into a linear formulation. For this purpose, we present a corotational formulation for quadratic finite elements. We demonstrate the versatility of our approach in interactive manipulation sessions and show that our animation system can be coupled with further physics-based animations like, e.g. fluids and cloth, in a bi-directional way.
	Magnets in Motion (project page) B. Thomaszewski, A. Gumann, S. Pabst and W. Strasser SIGGRAPH Asia (ACM Transactions on Graphics), 2008 Abstract: We introduce magnetic interaction for rigid body simulation. Our approach is based on an equivalent dipole method and as such it is discrete from the ground up. Our approach is symmetric as we base both field and force computations on dipole interactions. Enriching rigid body simulation with magnetism allows for many new and interesting possibilities in computer animation and special effects. Our method also allows the accurate computation of magnetic fields for arbitrarily shaped objects, which is especially interesting for pedagogy as it allows the user to visually discover properties of magnetism which would otherwise be difficult to grasp. We demonstrate our method on a variety of problems and our results reflect intuitive as well as surprising effects. Our method is fast and can be coupled with any rigid body solver to simulate dozens of magnetic objects at interactive rates.
	Clothfighters (final version: AVC, WMV Video) G. Heiss, K. Franke, C. Kaese, K. Juarez, S. Pabst Animation Theatre (juried program) of the Computer Animation Festival, SIGGRAPH Asia, 2008 Abstract: Clothfighters was produced by students of audiovisual media at the Hochschule der Medien in Stuttgart, Germany, in cooperation with the WSI-GRIS of University of Tübingen. The idea of this short animation was to work with and test an Autodesk Maya plug-in for cloth simulation developed at the GRIS. The shape of the characters should only be defined by the clothes they were wearing; the actual geometries of their bodies were to be invisible. To challenge the simulator, a fight scene with fast movements was designed and captured. The base for the simulation was motion capture data, captured at 120fps in a studio at HdM using 12 Vicon cameras. This data was cleaned in Vicon IQ and then prepared for Maya in Motionbuilder. It was skinned in Maya to a low resolution polygon model. The simulation of the clothes, which are based on NURBS pattern, was done in Maya using the proprietary tcCloth plug-in, developed at the University of Tübingen. The highly efficient cloth simulation engine uses the Finite Element method to model realistic orthotropic materials and is coupled with a very robust collision detection and handling stage. The red character consists of 5 different cloth layers and the white one of 4 layers. The final shots were rendered using Mental Ray. Matte painting and the compositing was done in After Effects.
	Seams and Bending in Cloth Simulation S. Pabst, S. Krzywinski, A. Schenk, B. Thomaszewski Eurographics Workshop on Virtual Reality Interaction and Physical Simulation (VRIPHYS), 2008 Abstract: Accurate modeling of bending behavior is one of the most important tasks in the field of cloth simulation. Bending stiffness is probably the most significant material parameter describing a given textile. Much work has been done in recent years to allow a fast and authentic reproduction of the effect of bending in cloth simulation systems. However, these approaches usually treat the textiles as consisting of a single, homogeneous material. The effects of seams, interlining and multilayer materials have not been considered so far. Recent work showed that the bending stiffness of a textile is greatly influenced by the presence of seams and that a good cloth simulation system needs to consider these effects. In this work we show how accurate modeling of bending and seams can be achieved in a state-of-the-art cloth simulation system. Our system can make use of measured bending stiffness data, but also allows intuitive user control, if desired. We verify our approach using virtual draping tests and garments in the simulation and comparing the results to their real-world counterparts. Furthermore, we provide heuristics derived from measurements that can be used to approximate the influence of several common types of seams.
	Asynchronous Cloth Simulation (Video Bibtex) B. Thomaszewski, S. Pabst and W. Strasser Computer Graphics International (CGI), 2008 Abstract: This paper presents a new method for cloth simulation, which uses asynchronous variants of both time integration and collision handling. Implicit integration methods like backward Euler and BDF-2 are very popular in computer graphics, since they allow for fast and stable animations. However, when combined with large time steps, their inherent numerical dissipation results in over-damped simulations, which lack high frequency details such as small folds and wrinkles. In this paper, we present a computationally efficient method which does not suffer from these restrictions. The time integration component uses an asynchronous variational integrator (AVI), which allows dedicated time steps for every element. Thanks to its energy preserving nature, low-damped cloth materials can be simulated without compromising dynamic motion or suppressing important details. Our collision handling scheme combines both synchronous and asynchronous strategies and, in this way, allows focusing computation power on the important regions where collisions actually occur. We provide timings for several integration methods and show that our AVI-based scheme consistently outperforms synchronous explicit variants. Compared to implicit schemes, superior quality is obtained while remaining comparable in terms of computation times. Finally, we demonstrate the robustness of our method on a series of challenging animations.
	Interactive Physically-Based Shape Editing (Video Bibtex) J. Mezger, B. Thomaszewski, S. Pabst and W. Strasser ACM Solid and Physical Modeling Conference (SPM), 2008 My golden dragons rendering to the left won the Computer Graphics forum cover contest 2008. Abstract: We present an alternative approach to standard geometric shape editing using physically-based simulation. With our technique, the user can deform complex objects in real-time. The basis of our method is formed by a fast and accurate finite element implementation of an elasto-plastic material model, specifically designed for interactive shape manipulation. Using quadratic shape functions, we reduce approximation errors inherent to methods based on linear finite elements. The physical simulation uses a volume mesh comprised of quadratic tetrahedra, which are constructed from a coarser approximation of the detailed surface. In order to guarantee stability and real-time frame rates during the simulation, we cast the elasto-plastic problem into a linear formulation. For this purpose, we present a corotational formulation for quadratic finite elements. We demonstrate the versatility of our approach in interactive manipulation sessions and show that our animation system can be coupled with further physics-based animations like, e.g. fluids and cloth, in a bi-directional way.
	Parallel Techniques for Physically-Based Simulation on Multi-Core Processor Architectures (Video Bibtex) B. Thomaszewski, S. Pabst and W. Blochinger Computers & Graphics, 31(1):25-40, 2008 Abstract: As multi-core processor systems become more and more widespread, the demand for efficient parallel algorithms also propagates into the field of computer graphics. This is especially true for physically-based simulation, which is notorious for expensive numerical methods. In this work, we explore possibilities for accelerating physically-based simulation algorithms on multi-core architectures. Two components of physically-based simulation represent a great potential for bottlenecks in parallelisation: implicit time integration and collision handling. From the parallelisation point of view these two components are substantially different. Implicit time integration can be treated efficiently using static problem decomposition. The linear system arising in this context is solved using a data-parallel preconditioned conjugate gradient algorithm. The collision handling stage, however, requires a different approach, due to its dynamic structure. This stage is handled using multi-threaded programming with fully dynamic task decomposition. In particular, we propose a new task splitting approach based on a reasonable estimation of work, which analyses previous simulation steps. Altogether, the combination of different parallelisation techniques leads to a concise and yet versatile framework for highly efficient physical simulation.
	Multilayer-Clothing and Seams in Cloth Simulation DFG Project, 2006-2010, Joint Work with Institut für Textil- und Bekleidungstechnik (ITB).
	A Finite Element Method for Interactive Physically Based Shape Modelling with Quadratic Tetrahedra (Bibtex) J. Mezger, B. Thomaszewski, S. Pabst and W. Strasser Technical Report, Universität Tübingen, 2007 Abstract: We present an alternative approach to standard geometric shape editing using physically-based simulation. With our technique, the user can deform complex objects in real-time. The enabling technology of this approach is a fast and accurate finite element implementation of an elasto-plastic material model, specifically designed for interactive shape manipulation. Using quadratic shape functions, we avoid the inherent drawback of volume locking exhibited by methods based on linear finite elements. The physical simulation uses a tetrahedral mesh, which is constructed from a coarser approximation of the detailed surface. Having computed a deformed state of the tetrahedral mesh, the deformation is transferred back to the high detail surface. This can be accomplished in an accurate and efficient way using the quadratic shape functions. In order to guarantee stability and real-time frame rates during the simulation, we cast the elasto-plastic problem into a linear formulation. For this purpose, we present a corotational formulation for quadratic finite elements. We demonstrate the versatility of our approach in interactive manipulation sessions and show that our animation system can be coupled with further physics-based animations like, e.g. fluids and cloth, in a bi-directional way.
	Exploiting Parallelism in Physically-Based Simulations on Multi-Core Processor Architectures (Video Bibtex) B. Thomaszewski, S. Pabst and W. Blochinger EG Symposium on Parallel Graphics and Visualization (EGPGV), 2007 Abstract: Physically based simulation of cloth in virtual environments is a computationally demanding problem. It involves modeling the internal material properties of the textile (physical modeling) and also treating interactions with the surrounding scene (collision handling). In this paper, we present an approach to parallel cloth simulation designed for distributed memory parallel architectures, particularly clusters built of commodity components. We discuss parallel techniques for the physical modeling phase as well as for the collision handling phase which can significantly reduce the respective computation times. To deal with the very fine granularity of the physical modeling phase we apply a static data decomposition approach based on graph partitioning. In order to cope with the high irregularity of the collision handling phase we employ task-parallel techniques based on fully dynamic problem decomposition. We show how both techniques can be integrated into a robust parallel cloth simulation method which can deal with considerably complex scenes.
	Walk on Glass S. Pabst, E. Bachmann, S. Kimmerle Eurographics Animation Theatre, 2005 Abstract: This video is a demonstration of the Finite-Element GrisTex cloth simulator developed at the University of Tübingen. The model was completely animated and modeled using Alias Maya while the dress was simulated using a Maya-plug-in of the GrisTex simulator.
	Robust Collision Handling for Cloth Simulations S. Pabst Diploma Thesis, Universität Tübingen, 2005
	tcCloth - An interactive cloth modeling and animation system (Bibtex) M. Gruber, C. Michel, S. Pabst, M. Wacker, M. Keckeisen and Stefan Kimmerle Proc. Graphiktag, 2004 Abstract: We present tcCloth, a cloth creation and simulation system for the standard 3D modeling software Alias Maya. It is based on the TüTex cloth simulation engine developed at WSI/GRIS, Uni Tübingen. The system we describe supports the whole pipeline from cloth creation to simulation, ranging from cutting and sewing of garments to the simulation of their drape using various real-world cloth materials with realistic dynamic effects like wind or air resistance.