Physics-Based Animation

Bin Wang, Longhua Wu, Kangkang Yin, Uri Ascher, Libin Liu, Hui Huang

We present a data-driven method for the deformation capture and physics-based modeling of soft deformable objects. Our framework enables both realistic motion reconstruction and synthesis of virtual soft object models in response to user stimulation. Low cost depth sensors are used for the deformation capture, and we do not insist on any force-displacement measurements which are commonly required by previous work, thus making the capturing a cheap and convenient process. A physics-based probabilistic tracking method is employed to increase the tracking robustness to noise, occlusions, fast movements and large deformations. The deformation estimation task that includes the reference shape, material elasticity parameters and damping coefficient is then formulated and solved as a spacetime optimization problem, aiming at matching the simulated trajectories with the tracked ones. The optimized deformation parameters are used in turn to make the physics-based tracking results more accurate, consequently improving the deformation estimation itself. Numerical experiments demonstrate that our physics-based deformation tracking and deformation parameter optimization can be unified and made complementary to each other. The obtained optimal deformation parameters can yield high quality animations for various soft models.

Deformation Capture and Modeling of Soft Objects

Bo Zhu, Minjae Lee, Ed Quigley, Ronald Fedkiw

We present a novel method to simulate codimensional nonNewtonian fluids on simplicial complexes. Our method extends previous work for codimensional incompressible flow to various types of non-Newtonian fluids including both shear thinning and thickening, Bingham plastics, and elastoplastics. We propose a novel time integration scheme for semi-implicitly treating elasticity, which when combined with a semi-implicit method for variable viscosity alleviates the need for small time steps. Furthermore, we propose an improved treatment of viscosity on the rims of thin fluid sheets that allows us to capture their elusive, visually appealing twisting motion. In order to simulate complex phenomena such as the mixing of colored paint, we adopt a multiple level set framework and propose a discretization on simplicial complexes that facilitates the tracking of material interfaces across codimensions. We demonstrate the efficacy of our approach by simulating a wide variety of non-Newtonian fluid phenomena exhibiting various codimensional features.

Co-Dimensional Non-Newtonian Fluids

Nuttapong Chentanez, Matthias Mueller, Miles Macklin, Tae-Yong Kim

We present a novel explicit surface tracking method. Its main advantage over existing approaches is the fact that it is both completely grid-free and fast which makes it ideal for the use in large unbounded domains. A further advantage is that its running time is less sensitive to temporal variations of the input mesh than existing approaches. In terms of performance, the method provides a good trade-off point between speed and quality. The main idea behind our approach to handle topological changes is to delete all overlapping triangles and to fill or join the resulting holes in a robust and efficient way while guaranteeing that the output mesh is both manifold and without boundary. We demonstrate the flexibility, speed and quality of our method in various applications such as Eulerian and Lagrangian liquid simulations and the simulation of solids under large plastic deformations.

Fast Grid-Free Surface Tracking

Matthias Mueller, Nuttapong Chentanez, Tae-Yong Kim, Miles Macklin

We propose a new method for both collision detection and collision response geared towards handling complex deformable objects in close contact. Our method does not miss collision events between time steps and solves the challenging problem of untangling automatically and robustly. It is conceptually simple and straight forward to parallelize due to the regularity of the algorithm. The main idea is to tessellate the air between objects once before the simulation and by considering one unilateral constraint per element that prevents its inversion during the simulation. If large relative rotations and translations are present in the simulation, an additional dynamic mesh optimization step is needed to prevent mesh locking. This step is fast in 2D and allows the simulation of arbitrary scenes. Because mesh optimization is expensive in 3D, however, the method is best suited for the subclass of 3D scenarios in which relative motions are limited. This subclass contains two important problems, namely the simulation of multi-layered clothing and tissue on animated characters.

Air Meshes for Robust Collision Handling

Ryoichi Ando, Nils Thuerey, Chris Wojtan

This paper presents a liquid simulation technique that enforces the incompressibility condition using a stream function solve instead of a pressure projection. Previous methods have used stream function techniques for the simulation of detailed single-phase flows, but a formulation for liquid simulation has proved elusive in part due to the free surface boundary conditions. In this paper, we introduce a stream function approach to liquid simulations with novel boundary conditions for free surfaces, solid obstacles, and solid-fluid coupling. Although our approach increases the dimension of the linear system necessary to enforce incompressibility, it provides interesting and surprising benefits. First, the resulting flow is guaranteed to be divergence-free regardless of the accuracy of the solve. Second, our free-surface boundary conditions guarantee divergence-free motion even in the un-simulated air phase, which enables two-phase flow simulation by only computing a single phase. We implemented this method using a variant of FLIP simulation which only samples particles within a narrow band of the liquid surface, and we illustrate the effectiveness of our method for detailed two-phase flow simulations with complex boundaries, detailed bubble interactions, and two-way solid-fluid coupling.

A Stream Function Solver for Liquid Simulations

Fang Da, Christopher Batty, Chris Wojtan, Eitan Grinspun

Simulating the delightful dynamics of soap films, bubbles, and foams has traditionally required the use of a fully three-dimensional many-phase Navier-Stokes solver, even though their visual appearance is completely dominated by the thin liquid surface. We depart from earlier work on soap bubbles and foams by noting that their dynamics are naturally described by a Lagrangian vortex sheet model in which circulation is the primary variable. This leads us to derive a novel circulation-preserving surface-only discretization of foam dynamics driven by surface tension on a non-manifold triangle mesh. We represent the surface using a mesh-based multimaterial surface tracker which supports complex bubble topology changes, and evolve the surface according to the ambient air flow induced by a scalar circulation field stored on the mesh. Surface tension forces give rise to a simple update rule for circulation, even at non-manifold Plateau borders, based on a discrete measure of signed scalar mean curvature. We further incorporate vertex constraints to enable the interaction of soap films with wires. The result is a method that is at once simple, robust, and efficient, yet able to capture an array of soap films behaviors including foam rearrangement, catenoid collapse, blowing bubbles, and double bubbles being pulled apart.

Double Bubbles Sans Toil and Trouble: Discrete Circulation-Preserving Vortex Sheets for Soap Films and Foams

Yun Teng, Mark Meyer, Tony DeRose, Theodore Kim

Subspace deformable body simulations can be very fast, but can behave unrealistically when behaviors outside the prescribed subspace, such as novel external collisions, are encountered. We address this limitation by presenting a fast, flexible new method that allows full space computation to be activated in the neighborhood of novel events while the rest of the body still computes in a subspace. We achieve this using a method we call subspace condensation, a variant on the classic static condensation precomputation. However, instead of a precomputation, we use the speed of subspace methods to perform the condensation at every frame. This approach allows the full space regions to be specified arbitrarily at runtime, and forms a natural two-way coupling with the subspace regions. While condensation is usually only applicable to linear materials, the speed of our technique enables its application to non-linear materials as well. We show the effectiveness of our approach by applying it to a variety of articulated character scenarios.

Subspace Condensation: Full Space Adaptivity for Subspace Deformations

Xinxin Zhang, Robert Bridson, Chen Greif

Most visual effects fluid solvers use a time-splitting approach where velocity is first advected in the flow, then projected to be incompressible with pressure. Even if a highly accurate advection scheme is used, the self-advection step typically transfers some kinetic energy from divergence-free modes into divergent modes, which are then projected out by pressure, losing energy noticeably for large time steps. Instead of taking smaller time steps or using significantly more complex time integration, we propose a new scheme called IVOCK (Integrated Vorticity of Convective Kinematics) which cheaply captures much of what is lost in self-advection by identifying it as a violation of the vorticity equation. We measure vorticity on the grid before and after advection, taking into account vortex stretching, and use a cheap multigrid V-cycle approximation to a vector potential whose curl will correct the vorticity error. IVOCK works independently of the advection scheme (we present examples with various semi-Lagrangian methods and FLIP), works independently of how boundary conditions are applied (it just corrects error in advection, leaving pressure etc.\ to take care of boundaries and other forces), and other solver parameters (we provide smoke, fire, and water examples). For 10~25% extra computation time per step much larger steps can be used, while producing detailed vorticial structures and convincing turbulence that are lost without correction.

Restoring the Missing Vortices in Advection-Projection Fluid Solvers

Fernando de Goes, Corentin Wallez, Jin Huang, Dmitry Pavlov, Mathieu Desbrun

This paper introduces a new particle-based approach to incompressible fluid simulation. We depart from previous Lagrangian methods by considering fluid particles no longer purely as material points, but also as volumetric parcels that partition the fluid domain. The fluid motion is described as a time series of well-shaped power diagrams (hence the name power particles), offering evenly spaced particles and accurate pressure computations. As a result, we circumvent the typical excess damping arising from kernel-based evaluations of internal forces or density without having recourse to auxiliary Eulerian grids. The versatility of our solver is demonstrated by the simulation of multiphase flows and free surfaces.

Power Particles: An incompressible fluid solver based on power diagrams

It’s my favourite time of the year. Stay tuned for more simulation paper links as they appear. As usual, see Ke-Sen Huang’s page for the full list.

• A Stream Function Solver for Liquid Simulations
• Power Particles: An incompressible fluid solver based on power diagrams
• Double Bubbles Sans Toil and Trouble: Discrete Circulation-Preserving Vortex Sheets for Soap Films and Foams
• Air Meshes for Robust Collision Handling
• Fast Grid-Free Surface Tracking
• Deformation Capture and Modeling of Soft Objects
• Restoring the Missing Vortices in Advection Projection Fluid Solvers
• Subspace Condensation: Full Space Adaptivity for Subspace Deformations
• Co-Dimensional Non-Newtonian Fluids
• The Affine Particle-In-Cell Method
• Simulating Rigid Body Fracture with Surface Meshes
• GRIDiron: An interactive authoring and cognitive training foundation for reconstructive plastic surgery procedures
• High-Resolution Brittle Fracture Simulation with Boundary Elements
• Stable Constrained Dynamics
• Data-Driven Finite elements for Geometry and Material Design
• LazyFluids: Appearance Transfer for Fluid Animations
• Nonlinear Material Design Using Principal Stretches
• OmniAD: Data-Driven Omni-Directional Aerodynamics
• An Implicit Viscosity Formulation for SPH Fluids
• Fluid-Volume Modeling from Sparse Multi-View Images by Appearance Transfer
• Biomechanical Simulation and Control of Hands and Tendinous Systems
• A Perceptual Control Space for Garment Simulation
• Dyna: A Model of Dynamic Human Shape in Motion

TOG papers:

• Water Wave Animation via Wavefront Parameter Interpolation
• Realistic Biomechanical Simulation and Control of Human Swimming
• Real-Time Nonlinear Shape Interpolation
• Robust Simulation of Sparsely Sampled Thin Features in SPH-Based Free Surface Flows