Simon Pabst

Continuum‐based Strain Limiting

We present Continuum‐based Strain Limiting (CSL) – a new method for limiting deformations in physically‐based cloth simulations. Despite recent developments for nearly inextensible materials, the efficient simulation of general biphasic textiles and their anisotropic behavior remains challenging. Many approaches use soft 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. Based on a continuum deformation measure, our method allows accurate control over all strain components using individual thresholds. We impose deformation limits element‐wise and cast the problem as a 6×6 system of linear equations. CSL can be combined with any cloth simulator and, as a velocity filter, integrates seamlessly into standard collision handling.

Fast and Scalable CPU/GPU Collision Detection for Rigid and Deformable Surfaces

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 a 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.

Parallel techniques for physically based simulation on multi-core processor architectures

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.

Anisotropic friction for deformable surfaces and solids

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 textiles 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.

Interactive physically-based shape editing

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

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.

Seams and Bending in Cloth Simulation

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.

Wet cloth simulation

Both cloth and fluid simulation are important areas of research in Computer Graphics. However, only a surprisingly small amount of authors have investigated the two-way coupling of these simulations. Lenaerts et al. [2008] present a porous flow technique based on Darcys law using a 3D particle simulation and apply their approach to textiles as well, however, their approach is computationally expensive and they use a rather simple cloth simulation system that cannot fully represent the complex effects of wet textiles. [Morimoto et al. 2007] present a technique that allows the visualization of dyeing based on diffusion terms. However, they do not consider their approach in a simulation context.

Exploiting parallelism in physically-based simulations on multi-core processor architectures

As multi-core processor systems become more and more widespread, the demand for designing efficient parallel algorithms propagates also into the field of computer graphics. This is especially true for the physically-based simulation, which is notorious for expensive numerical methods. In this paper we explore possibilities for accelerating these algorithms on modern multi-core architectures. As an application we focus on physically-based cloth simulation. In this context, two distinct problems can be identified: the physical model and the collision handling stage — both bearing potential bottlenecks for the simulation. From the parallelization point of view these two components are substantially different. The physical model can be treated efficiently using static problem decomposition. The collision handling problem, however, requires a different approach, due to its dynamically changing structure. We address this problem using multi-threaded programming with fully dynamic task decomposition. Furthermore, we propose a new task splitting approach based on a robust work estimate. The associated data is derived from temporal coherence. Altogether, the combination of different parallelization techniques leads to a concise and yet versatile framework for highly efficient physical simulation.

CGForum 2008 Cover Image

After the silver-coloured feet had been fixed on the ground, the deformation was modelled during a short interactive session. A dedicated plasticity model facilitates realistic, intuitive modelling while volumetric effects are appropriately incorporated. The surface mesh consists of 250.000 vertices (original mesh courtesy of Stanford Computer Graphics Laboratory), which are transformed smoothly and interactively by the quadratic shape functions of the underlying tetrahedral mesh.