Physics-Based Animation

Yifei Li, Tao Du, Kui Wu, Jie Xu, Wojciech Matusik

Cloth simulation has wide applications in computer animation, garment design, and robot-assisted dressing. This work presents a differentiable cloth simulator whose additional gradient information facilitates cloth-related applications. Our differentiable simulator extends a state-of-the-art cloth simulator based on Projective Dynamics (PD) and with dry frictional contact. We draw inspiration from previous work to propose a fast and novel method for deriving gradients in PD-based cloth simulation with dry frictional contact. Furthermore, we conduct a comprehensive analysis and evaluation of the usefulness of gradients in contact-rich cloth simulation. Finally, we demonstrate the efficacy of our simulator in a number of downstream applications, including system identification, trajectory optimization for assisted dressing, closed-loop control, inverse design, and real-to-sim transfer. We observe a substantial speedup obtained from using our gradient information in solving most of these applications.

DiffCloth: Differentiable Cloth Simulation with Dry Frictional Contact

Han Shao, Libo Huang, Dominik L. Michels

Multigrid methods are quite efficient for solving the pressure Poisson equation in simulations of incompressible flow. However, for viscous liquids, geometric multigrid turned out to be less efficient for solving the variational viscosity equation. In this contribution, we present an Unsmoothed Aggregation Algebraic MultiGrid (UAAMG) method with a multi-color Gauss-Seidel smoother, which consistently solves the variational viscosity equation in a few iterations for various material parameters. Moreover, we augment the OpenVDB data structure with Intel SIMD intrinsic functions to perform sparse matrix-vector multiplications efficiently on all multigrid levels. Our framework is 2.0 to 14.6 times faster compared to the state-of-the-art adaptive octree solver in commercial software for the large-scale simulation of both non-viscous and viscous flow.

A Fast Unsmoothed Aggregation Algebraic Multigrid Framework for the Large-Scale Simulation of Incompressible Flow

Shiying Xiong, Zhecheng Wang, Mengdi Wang, Bo Zhu

We propose a novel Clebsch method to simulate the free-surface vortical flow. At the center of our approach lies a level-set method enhanced by a wave-function correction scheme and a wave-function extrapolation algorithm to tackle the Clebsch method’s numerical instabilities near a dynamic interface. By combining the Clebsch wave function’s expressiveness in representing vortical structures and the level-set function’s ability on tracking interfacial dynamics, we can model complex vortex-interface interaction problems that exhibit rich free-surface flow details on a Cartesian grid. We showcase the efficacy of our approach by simulating a wide range of new free-surface flow phenomena that were impractical for previous methods, including horseshoe vortex, sink vortex, bubble rings, and free-surface wake vortices.

A Clebsch method for free-surface vortical flow simulation

Georg Sperl, Rosa M. Sánchez-Banderas, Manwen Li, Chris Wojtan, Miguel A. Otaduy

This paper introduces a methodology for inverse-modeling of yarn-level mechanics of cloth, based on the mechanical response of fabrics in the real world. We compiled a database from physical tests of several different knitted fabrics used in the textile industry. These data span different types of complex knit patterns, yarn compositions, and fabric finishes, and the results demonstrate diverse physical properties like stiffness, nonlinearity, and anisotropy. We then develop a system for approximating these mechanical responses with yarn-level cloth simulation. To do so, we introduce an efficient pipeline for converting between fabric-level data and yarn-level simulation, including a novel swatch-level approximation for speeding up computation, and some small-but-necessary extensions to yarn-level models used in computer graphics.

Estimation of Yarn-Level Simulation Models for Production Fabrics

Marcel Padilla, Oliver Gross, Felix Knoppel, Albert Chern, Ulrich Pinkall, Peter Schroder

Simulation of stellar atmospheres, such as that of our own sun, is a common task in CGI for scientific visualization, movies and games. A fibrous volumetric texture is a visually dominant feature of the solar corona—the plasma that extends from the solar surface into space. These coronal fibers can be modeled as magnetic filaments whose shape is governed by the magnetohydrostatic equation. The magnetic filaments provide a Lagrangian curve representation and their initial configuration can be prescribed by an artist or generated from magnetic flux given as a scalar texture on the sun’s surface. Subsequently the shape of the filaments is determined based on a variational formulation. The output is a visual rendering of the whole sun. We demonstrate the fidelity of our method by comparing the resulting renderings with actual images of our sun’s corona.

Filament Based Plasma

Jerry Hsu, Nghia Truong,Cem Yuksel, Kui Wu

Initializing simulations of deformable objects involves setting the rest state of all internal forces at the rest shape of the object. However, often times the rest shape is not explicitly provided. In its absence, it is common to initialize by treating the given initial shape as the rest shape. This leads to sagging, the undesirable deformation under gravity as soon as the simulation begins. Prior solutions to sagging are limited to specific simulation systems and material models, most of them cannot handle frictional contact, and they require solving expensive global nonlinear optimization problems. We introduce a novel solution to the sagging problem that can be applied to a variety of simulation systems and materials. The key feature of our approach is that we avoid solving a global nonlinear optimization problem by performing the initialization in two stages. First, we use a global linear optimization for static equilibrium. Any nonlinearity of the material definition is handled in the local stage, which solves many small local problems efficiently and in parallel. Notably, our method can properly handle frictional contact orders of magnitude faster than prior work. We show that our approach can be applied to various simulation systems by presenting examples with mass-spring systems, cloth simulations, the finite element method, the material point method, and position-based dynamics.

A General Two-Stage Initialization for Sag-Free Deformable Simulations

Fresh new 2022 SIGGRAPH papers, coming in hot!

• Filament Based Plasma
• Estimation of Yarn-Level Simulation Models for Production Fabrics
• A Clebsch Method for Free-Surface Vortical Flow Simulation
• A Fast Unsmoothed Aggregation Algebraic Multigrid Framework for the Large-Scale Simulation of Incompressible Flow
• Ecoclimates: Climate-Response Modeling of Vegetation
• DiffCloth: Differentiable Cloth Simulation with Dry Frictional Contact
• A General Two-Stage Initialization for Sag-Free Deformable Simulations
• Contact-Centric Deformation Learning
• Covector Fluids
• Simulation and Optimization of Magnetoelastic Thin Shells
• A Moving Eulerian-Lagrangian Particle Method for Thin Film and Foam Simulation
• A GPU-Based Multilevel Additive Schwarz Preconditioner for Cloth and Deformable Body Simulation
• A Unified Newton Barrier Method for Multibody Dynamics
• The Power Particle-In-Cell Method
• Penetration-free Projective Dynamics on the GPU
• Affine Body Dynamics: Fast, Stable & Intersection-free Simulation of Stiff Materials
• Energetically Consistent Inelasticity for Optimization Time Integration
• Adaptive Rigidification of Elastic Solids
• Efficient Kinetic Simulation of Two-Phase Flows
• Go Green: General Regularized Green’s Functions for Elasticity
• Guided Bubbles and Wet Foam for Realistic Whitewater Simulation
• Implicit Neural Representation for Physics-driven Actuated Soft Bodies
• Loki: A Unified Multiphysics Simulation Framework for Production
• Unified Many-Worlds Browsing of Arbitrary Physics-Based Animations
• VEMPIC: Particle-in-polyhedron Fluid Simulation for Intricate Solid Boundaries
• Analytically Integratable Zero-Restlength Springs for Capturing Dynamic Modes Unrepresented by Quasistatic Neural Networks
• Compact Poisson Filters for Fast Fluid Simulation
• Automatic Quantization for Physics-based Simulation
• True Seams: Modeling Seams in Digital Garments
  

TOG:

• An Efficient B-Spline Lagrangian/Eulerian Method for Compressible Flow, Shock Waves, and Fracturing Solids
• Fine Wrinkling on Coarsely Meshed Thin Shells
• Simulating Brittle Fracture With Material Points
• Physics-based Combustion Simulation

Dan Koschier, Jan Bender, Barbara Solenthaler, Matthias Teschner

Throughout the past decades, the graphics community has spent major resources on the research and development of physics simulators on the mission to computer-generate behaviors achieving outstanding visual effects or to make the virtual world indistinguishable from reality. The variety and impact of recent research based on Smoothed Particle Hydrodynamics (SPH) demonstrates the concept’s importance as one of the most versatile tools for the simulation of fluids and solids. With this survey, we offer an overview of the developments and still-active research on physics simulation methodologies based on SPH that has not been addressed in previous SPH surveys. Following an introduction about typical SPH discretization techniques, we provide an overview over the most used incompressibility solvers and present novel insights regarding their relation and conditional equivalence. The survey further covers recent advances in implicit and particle-based boundary handling and sampling techniques. While SPH is best known in the context of fluid simulation we discuss modern concepts to augment the range of simulatable physical characteristics including turbulence, highly viscous matter, deformable solids, as well as rigid body contact handling. Besides the purely numerical approaches, simulation techniques aided by machine learning are on the rise. Thus, the survey discusses recent data-driven approaches and the impact of differentiable solvers on artist control. Finally, we provide context for discussion by outlining existing problems and opportunities to open up new research directions.

A Survey on SPH Methods in Computer Graphics

Qiang Chen, Tingsong Lu, Yang Tong, Guoliang Luo, Xiaogang Jin, Zhigang Deng

As one of ubiquitous insects on the earth, butterflies are also widely-known
for inspiring thrill resonance with their elegant and peculiar flights. However, realistically modeling and simulating butterfly flights, in particular, for real-time graphics and animation applications, remains an under-explored problem. In this paper we propose an efficient and practical model to simulate butterfly flights. Specifically, we first model a butterfly with parametric maneuvering functions, including wing-abdomen interaction. Then, we simulate dynamic maneuvering control of the butterfly through our force-based model that includes both the aerodynamics force and the vortex force. Through many simulation experiments and comparisons, we demonstrate that our method can efficiently simulate realistic butterfly flight motions in various real-world settings.

A Practical Model for Realistic Butterfly Flight Simulation

Teseo Schneider, Yixin Hu, Xifeng Gao, Jeremie Dumas, Denis Zorin, Daniele Panozzo

The Finite Element Method (FEM) is widely used to solve discrete Partial Differential Equations (PDEs) in engineering and graphics applications. The popularity of FEM led to the development of a large family of variants, most of which require a tetrahedral or hexahedral mesh to construct the basis. While the theoretical properties of FEM basis (such as convergence rate, stability, etc.) are well understood under specific assumptions on the mesh quality, their practical performance, influenced both by the choice of the basis construction and quality of mesh generation, have not been systematically documented for large collections of automatically meshed 3D geometries. We introduce a set of benchmark problems involving most commonly solved elliptic PDEs, starting from simple cases with an analytical solution, moving to commonly used test problem setups, and using manufactured solutions for thousands of real-world, automatically meshed geometries. For all these cases, we use state-of-the-art meshing tools to create both tetrahedral and hexahedral meshes, and compare the performance of different element types for common elliptic PDEs.
The goal of this benchmark is to enable comparison of complete FEM pipelines, from mesh generation to algebraic solver, and exploration of relative impact of different factors on the overall system performance.

A Large-Scale Comparison of Tetrahedral and Hexahedral Elements for Solving Elliptic PDEs with the Finite Element Method