Physics-Based Animation

We propose a novel, multi-resolution method to efficiently perform large-scale cloth simulation. Our cloth simulation method is based on a triangle-based energy model constructed from a cloth mesh. We identify that solutions of the linear system of cloth simulation are smooth in certain regions of the cloth mesh and solve the linear system on those regions in a reduced solution space. Then we reconstruct the original solutions by performing a simple interpolation from solutions computed in the reduced space. In order to identify regions where solutions are smooth, we propose simplification metrics that consider stretching, shear, and bending forces, as well as geometric collisions. Our multi-resolution method can be applied to many existing cloth simulation methods, since our method works on a general linear system. In order to demonstrate benefits of our method, we apply our method into four large-scale cloth benchmarks that consist of tens or hundreds of thousands of triangles. Because of the reduced computations, we achieve a performance improvement by a factor of up to one order of magnitude, with a little loss of simulation quality.

Multi-Resolution Cloth Simulation

In this paper we present a novel surface reconstruction method for particle-based fluid simulators such as Smoothed Particle Hydrodynamics. In particle-based simulations, fluid surfaces are usually defined as a level set of an implicit function. We formulate the implicit function as a sum of anisotropic smoothing kernels, and the direction of anisotropy at a particle is determined by performing Principal Component Analysis (PCA) over the neighboring particles. In addition, we perform a smoothing step that re-positions the centers of these smoothing kernels. Since these anisotropic smoothing kernels capture the local particle distributions more accurately, our method has advantages over existing methods in representing smooth surfaces, thin streams and sharp features of fluids. Our method is fast, easy to implement, and our results demonstrate a significant improvement in the quality of reconstructed surfaces as compared to existing methods.

Reconstructing Surface of Particle-Based Fluids Using Anisotropic Kernels

We propose a practical approach to integrating shock wave dynamics into traditional smoke simulations. Previous methods either simplify away the compressible component of the flow and are unable to capture shock fronts or use a prohibitively expensive explicit method that limits the time step of the simulation long after the relevant shock waves and rarefactions have left the domain. Instead, we employ a semi-implicit formulation of Euler’s equations, which allows us to take time steps on the order of the fluid velocity (ignoring the more stringent acoustic wave-speed restrictions) and avoids the expensive characteristic decomposition typically required of compressible flow solvers. We also propose an extension to Euler’s equations to model combustion of fuel in explosions. The flow is two-way coupled with rigid and deformable solid bodies, treating the solid-fluid interface effects implicitly in a projection step by enforcing a velocity boundary condition on the fluid and integrating pressure forces along the solid surface. As we handle the acoustic fluid effects implicitly, we can artificially drive the sound speed c of the fluid to infinity without going unstable or driving the time step to zero. This permits the fluid to transition from compressible flow to the far more tractable incompressible flow regime once the interesting compressible flow phenomena (such as shocks) have left the domain of interest, and allows the use of state-of-the-art smoke simulation techniques.

Practical Animation of Compressible Flow for Shockwaves and Related Phenomena

Achieving interactive performance in cloth animation has significant implications in computer games and other interactive graphics applications. Although much progress has been made, it is still much desired to have real-time high-quality results that well preserve dynamic folds and wrinkles. In this paper, we introduce a hybrid method for real-time cloth animation. It relies on data-driven models to capture the relationship between cloth deformations at two resolutions. Such data-driven models are responsible for transforming low-quality simulated deformations at the low resolution into high-resolution cloth deformations with dynamically introduced fine details. Our data-driven transformation is trained using rotation invariant quantities extracted from the cloth models, and is independent of the simulation technique chosen for the lower resolution model. We have also developed a fast collision detection and handling scheme based on dynamically transformed bounding volumes. All the components in our algorithm can be efficiently implemented on programmable graphics hardware to achieve an overall real-time performance on high-resolution cloth models.

A Deformation Transformer for Real-Time Cloth Animation

This paper describes a method for animating the appearance of clothing, such as pants or a shirt, that fits closely to a figure’s body. Compared to flowing cloth, such as loose dresses or capes, these types of garments involve nearly continuous collision contact and small wrinkles, that can be troublesome for traditional cloth simulation methods. Based on the observation that the wrinkles in close-fitting clothing behave in a predominantly kinematic fashion, we have developed an example-based wrinkle synthesis technique. Our method drives wrinkle generation from the pose of the figure’s kinematic skeleton. This approach allows high quality clothing wrinkles to be combined with a coarse cloth simulation that computes the global and dynamic aspects of the clothing motion. While the combined results do not exactly match a high-resolution reference simulation, they do capture many of the characteristic fine-scale features and wrinkles. Further, the combined system runs at interactive rates, making it suitable for applications where high-resolution offline simulations would not be a viable option. The wrinkle synthesis method uses a precomputed database built by simulating the high-resolution clothing as the articulated figure is moved over a range of poses. In principle, the space of poses is exponential in the total number of degrees of freedom; however clothing wrinkles are primarily affected by the nearest joints, allowing each joint to be processed independently. During synthesis, mesh interpolation is used to consider the influence of multiple joints, and combined with a coarse simulation to produce the final results at interactive rates.

Example-Based Wrinkle Synthesis for Clothing Animation

We present a technique for learning clothing models that enables the simultaneous animation of thousands of detailed garments in real-time. This surprisingly simple conditional model learns and preserves the key dynamic properties of a cloth motion along with folding details. Our approach requires no a priori physical model, but rather treats training data as a ‘black box’. We show that the models learned with our method are stable over large time-steps and can approximately resolve cloth-body collisions. We also show that within a class of methods, no simpler model covers the full range of cloth dynamics captured by ours. Our method bridges the current gap between skinning and physical simulation, combining benefits of speed from the former with dynamic effects from the latter. We demonstrate our approach on a variety of apparel worn by male and female human characters performing a varied set of motions typically used in video games (e.g., walking, running, jumping, etc.).

Stable Spaces for Real-Time Clothing

Real-Time Simulation of Large Bodies of Water with Small Scale Details

Kesen’s page of 2010 SIGGRAPH papers is up.

Here’s the list of SIGGRAPH 2010 physics-based animation papers to appear so far:

• Subspace Self-Collision Culling
• Star-Contours for Efficient Hierarchical Self-Collision Detection
• Efficient Yarn-based Cloth with Adaptive Contact Linearization
• Physics-Inspired Topology Changes for Thin Fluid Features
• A Multiscale Approach to Mesh-based Surface Tension Flows
• A Practical Simulation of Dispersed Bubble Flow
• Dynamic Local Remeshing for Elastoplastic Simulation
• Matching Fluid Simulation Elements to Surface Geometry and Topology
• Discrete Viscous Threads
• Unified Simulation of Elastic Rods, Shells, and Solids
• Example-Based Wrinkle Synthesis for Clothing Animation
• Filament Based Smoke With Vortex Shedding and Variational Reconnection
• Volume Contact Constraints at Arbitrary Resolution
• A Simple Geometric Model for Elastic Deformations
• A Novel Algorithm for Incompressible Flow Using Only a Coarse Grid Projection
• Stable Spaces for Real-Time Cloth
• Example-Based Wrinkle Synthesis for Clothing Animation
• A Deformation Transformer for Real-Time Cloth Animation

And a few TOG papers that I understand will be presented there too…

• An Efficient Multigrid Method for the Simulation of High-Resolution Elastic Solids
• Comprehensive Biomechanical Modeling and Simulation of the Upper Body
• A Simple Approach to Nonlinear Tensile Stiffness for Accurate Cloth Simulation
• Underwater Cloth Simulation with Fractional Derivatives

The full list of accepted papers for SCA 2010 has been posted, and Ke-Sen’s collection is here.  Here’s the physics ones:

• Linear-Time Dynamics for Multibody Systems with General Joint Models
• Constraint-Based Simulation of Adhesive Contact
• Point Cloud Glue: Constraining simulations using the Procrustes transform
• Interactive SPH Simulation and Rendering on the GPU
• Real-Time Simulation of Large Bodies of Water with Small Scale Details
• Enhancing Fluid Animation with Adaptive, Controllable, and Intermittent Turbulence
• FASTCD: Fracturing-Aware Stable Collision Detection
• A parallel multigrid Poisson solver for fluids simulation on large grids
• Practical Animation of Compressible Flow for Shock Waves and Related Phenomena
• Reconstructing Surface of Particle-Based Fluids Using Anisotropic Kernels

Yarn-based cloth simulation can improve visual quality but at high computational costs due to the reliance on numerous persistent yarn-yarn contacts to generate material behavior. Finding so many contacts in densely interlinked geometry is a pathological case for traditional collision detection, and the sheer number of contact interactions makes contact processing the simulation bottleneck. In this paper, we propose a method for approximating penalty-based contact forces in yarn-yarn collisions by computing the exact contact response at one time step, then using a rotated linear force model to approximate forces in nearby deformed configurations. Because contacts internal to the cloth exhibit good temporal coherence, sufficient accuracy can be obtained with infrequent updates to the approximation, which are done adaptively in space and time. Furthermore, by tracking contact models we reduce the time to detect new contacts. The end result is a 7- to 9-fold speedup in contact processing and a 4- to 5-fold overall speedup, enabling simulation of character-scale garments.

Efficient Yarn-Based Cloth Simulation With Adaptive Contact Linearization