Physics-Based Animation

Inyong Jeon, Kwang-Jin Choi, Tae-Yong Kim, Bong-Ouk Choi, and Hyeong-Seok Ko

We present a new technique which can handle both point and sliding constraints in the multigrid (MG) framework. Although the MG method can theoretically perform as fast as O(N), the development of a clothing simulator based on the MG method calls for solving an important technical challenge: handling the constraints. Resolving constrains has been difficult in MG because there has been no clear way to transfer the constraints existing in the finest level mesh to the coarser level meshes. This paper presents a new formulation based on soft constraints, which can coarsen the constraints defined in the finest level to the coarser levels. Experiments are performed which show that the proposed method can solve the linear system up to 4–9 times faster in comparison with the modified preconditioned conjugate gradient method (MPCG) without quality degradation. The proposed method is easy to implement and can be straightforwardly applied to existing clothing simulators which are based on implicit time integration.

Constrainable Multigrid for Cloth

N. Schmitt, Martin Knuth, Jan Bender, A. Kuijper

Today most cloth simulation systems use triangular mesh models. However, regular grids allow many optimizations as connectivity is implicit, warp and weft directions of the cloth are aligned to grid edges and distances between particles are equal. In this paper we introduce a cloth simulation that combines both model types. All operations that are performed on the CPU use a low-resolution triangle mesh while GPU-based methods are performed efficiently on a high-resolution grid representation. Both models are coupled by a sampling operation which renders triangle vertex data into a texture and by a corresponding projection of texel data onto a mesh. The presented scheme is very flexible and allows individual components to be performed on different architectures, data representations and detail levels. The results are combined using shader programs which causes a negligible overhead. We have implemented CPU-based collision handling and a GPU-based hierarchical constraint solver to simulate systems with more than 230k particles in real-time.

Multilevel Cloth Simulation using GPU Surface Sampling

Crispin Deul, Jan Bender

In this paper we present a novel multi-layer model for physically-based character skinning. In contrast to geometric approaches which are commonly used in the field of character skinning, physically-based methods can simulate secondary motion effects. Furthermore, these methods can handle collisions and preserve the volume of the model without the need of an additional post-process. Physically-based approaches are computationally more expensive than geometric methods but they provide more realistic results. Recent works in this area use finite element simulations to model the elastic behavior of skin. These methods require the generation of a volumetric mesh for the skin shape in a pre-processing step. It is not easy for an artist to model the different elastic behaviors of muscles, fat and skin using a volumetric mesh since there is no clear assignment between volume elements and tissue types. For our novel multi-layer model the mesh generation is very simple and can be performed automatically. Furthermore, the model contains a layer for each kind of tissue. Therefore, the artist can easily control the elastic behavior by adjusting the stiffness parameters for muscles, fat and skin. We use shape matching with oriented particles and a fast summation technique to simulate the elastic behavior of our skin model and a position-based constraint enforcement to handle collisions, volume conservation and the coupling of the skeleton with the deformable model. Position-based methods have the advantage that they are fast, unconditionally stable, controllable and provide visually plausible results.

Physically Based Character Skinning

Nadir Akinci, Gizem Akinci, Matthias Teschner

Realistic handling of fluid-air and fluid-solid interfaces in SPH is a challenging problem. The main reason is that some important physical phenomena such as surface tension and adhesion emerge as a result of inter-molecular forces in a microscopic scale. This is different from scalar fields such as fluid pressure, which can be plausibly evaluated on a macroscopic scale using particles. Although there exist techniques to address this problem for some specific simulation scenarios, there does not yet exist a general approach to reproduce the variety of effects that emerge in reality from fluid air and fluid-solid interactions. In order to address this problem, we present a new surface tension force and a new adhesion force. Different from the existing work, our surface tension force can handle large surface tensions in a realistic way. This property lets our approach handle challenging real scenarios, such as water crown formation, various types of fluid-solid interactions, and even droplet simulations. Furthermore, it prevents particle clustering at the free surface where inter-particle pressure forces are incorrect. Our adhesion force allows plausible two-way attraction of fluids and solids and can be used to model different wetting conditions. By using our forces, modeling surface tension and adhesion effects do not require involved techniques such as generating a ghost air phase or surface tracking. The forces are applied to the neighboring fluidfluid and fluid-boundary particle pairs in a symmetric way, which satisfies momentum conservation. We demonstrate that combining both forces allows simulating a variety of interesting effects in a plausible way.

Versatile Surface Tension and Adhesion for SPH Fluids

Zherong Pan, Jin Huang, Yiying Tong, Changxi Zheng, and Hujun Bao

Animation techniques for controlling liquid simulation are challenging: they commonly require carefully setting initial and boundary conditions or performing a costly numerical optimization scheme against user-provided keyframes or animation sequences. Either way, the whole process is laborious and computationally expensive.

We introduce a novel method to provide intuitive and interactive control of liquid simulation. Our method enables a user to locally edit selected keyframes and automatically propagates the editing in a nearby temporal region using geometric deformation. We formulate our local editing techniques as a small-scale nonlinear optimization problem which can be solved interactively. With this uniformed formulation, we propose three editing metaphors, including (i) sketching local fluid features using a few user strokes, (ii) dragging a local fluid region, and (iii) controlling a local shape with a small mesh patch. Finally, we use the edited liquid animation to guide an of offline high-resolution simulation to recover more surface details. We demonstrate the intuitiveness and efficacy of our method in various practical scenarios.

Interactive Localized Liquid Motion Editing

David A. Stuart, Joshua A. Levine, Ben Jones, Adam Bargteil

Embedding high-resolution surface geometry in coarse control meshes is a standard approach to achieving high-quality computer animation at low computational expense. In this paper we present an effective, automatic method for generating such control meshes. The resulting high-quality, tetrahedral meshes enclose and approximate an input surface mesh, avoiding extrapolation artifacts and ensuring that the resulting coarse volumetric meshes are adequate collision proxies. Our approach comprises three steps: we begin with a tetrahedral mesh built from the body-centered cubic lattice that tessellates the bounding box of the input surface; we then perform a sculpting phase that carefully removes elements from the lattice; and finally a variational vertex adjustment phase iteratively adjusts vertex positions to more closely approximate the surface geometry. Our approach provides explicit trade-offs between mesh quality, resolution, and surface approximation. Our experiments demonstrate the technique can be used to build high-quality meshes appropriate for simulations within games.

Automatic Construction of Coarse, High-Quality Tetrahedralizations that  Enclose and Approximate Surfaces for Animation

John Koenig, Ioannis Karamouzas, Stephen J. Guy

We present an object-centric formulation for parallel rigid body simulation that supports variable length integration time steps through rollbacks. We combine our object-centric simulation
framework with a novel spatiotemporal data structure to reduce global synchronization and achieve interactive, real-time simulations which scale across many CPU cores. Additionally, we provide proofs that both our proposed data structure and our object-centric formulation are deadlock-free. We implement our approach with the functional programming language Erlang, and test the performance and scalability of our method over several scenarios consisting of hundreds of interacting objects.

Object-Centric Parallel Rigid Body Simulation with Timewarp

Min Tang, Ruofeng Tong, Rahul Narain, Chang Meng, Dinesh Manocha

We present a GPU-based streaming algorithm to perform high-resolution and accurate cloth simulation. We map all the components of cloth simulation pipeline, including time integration, collision detection, collision response, and velocity updating to GPU-based kernels and data structures. Our algorithm perform intra-object and inter-object collisions, handles contacts and friction, and is able to accurately simulate folds and wrinkles. We describe the streaming pipeline and address many issues in terms of obtaining high throughput on many-core GPUs. In practice, our algorithm can perform high-fidelity simulation on a cloth mesh with 2M triangles using 3GB of GPU memory. We highlight the parallel performance of our algorithm on three different generations of GPUs. On a high-end NVIDIA Tesla K20c, we observe up to two orders of magnitude performance improvement as compared to a single-threaded CPU-based algorithm, and about one order of magnitude improvement over a 16-core CPU-based parallel implementation.

A GPU-Based Streaming Algorithm for High Resolution Cloth Simulation

Yahan Zhou, Zhaoliang Lun, Evangelos Kalogerakis, Rui Wang

We present a novel particle-based method for stable simulation of elasto-plastic materials. The main contribution of our method is an implicit numerical integrator, using a physically-based model, for computing particles that undergo both elastic and plastic deformations. The main advantage of our implicit integrator is that it allows the use of large time steps while still preserving stable and physically plausible simulation results. As a key component of our algorithm, at each time step we compute the particle positions and velocities based on a sparse linear system, which we solve efficiently on the graphics hardware. Compared to existing techniques, our method allows for a much wider range of stiffness and plasticity settings. In addition, our method can significantly reduce the computation cost for certain range of material types. We demonstrate fast and stable simulations for a variety of elasto-plastic materials, ranging from highly stiff elastic materials to highly plastic ones.

Implicit Integration for Particle-based Simulation of Elasto-plastic Solids

Eder Miguel, Rasmus Tamstorf, Derek Bradley, Sara C. Schvartzman, Bernhard Thomaszewski, Bernd Bickel, Wojciech Matusik, Steve Marschner, Miguel A. Otaduy

Force-deformation measurements of cloth exhibit significant hysteresis, and many researchers have identified internal friction as the source of this effect. However, it has not been incorporated into computer animation models of cloth. In this paper, we propose a model of internal friction based on an augmented reparameterization of Dahl’s model, and we show that this model provides a good match to several important features of cloth hysteresis even with a minimal set of parameters. We also propose novel parameter estimation procedures that are based on simple and inexpensive setups and need only sparse data, as opposed to the complex hardware and dense data acquisition of previous methods. Finally, we provide
an algorithm for the efficient simulation of internal friction, and we demonstrate it on simulation examples that show disparate behavior with and without internal friction.

Modeling and Estimation of Internal Friction in Cloth