Pose-Space Subspace Dynamics

Hongyi Xu, Jernej Barbic

We enrich character animations with secondary soft-tissue Finite Element Method (FEM) dynamics computed under arbitrary rigged or skeletal motion. Our method optionally incorporates pose-space deformation (PSD). It runs at milliseconds per frame for complex characters, and fits directly into standard character animation pipelines. Our simulation method does not require any skin data capture; hence, it can be applied to humans, animals, and arbitrary (real-world or fictional) characters. In standard model reduction of three-dimensional nonlinear solid elastic models, one builds a reduced model around a single pose, typically the rest configuration. We demonstrate how to perform multi-model reduction of Finite Element Method (FEM) nonlinear elasticity, where separate reduced models are precomputed around a representative set of object poses, and then combined at runtime into a single fast dynamic system, using subspace interpolation. While time-varying reduction has been demonstrated before for offline applications, our method is fast and suitable for hard real-time applications in games and virtual reality. Our method supports self-contact, which we achieve by computing linear modes and derivatives under contact constraints.

Pose-Space Subspace Dynamics

Posted by christopherbatty on April 24th, 2016 | Comments Off on Pose-Space Subspace Dynamics Posted in Deformables

Physics-Driven Pattern Adjustment for Direct 3D Garment Editing

Aric Bartle, Alla Sheffer, Vladimir G. Kim, Danny Kaufman, Nicholas Vining, Floraine Berthouzoz

Designers frequently reuse existing designs as a starting point for creating new garments. In order to apply garment modifications, which the designer envisions in 3D, existing tools require meticulous manual editing of 2D patterns. These 2D edits need to account both for the envisioned geometric changes in the 3D shape, as well as for various physical factors that affect the look of the draped garment. We propose a new framework that allows designers to directly apply the changes they envision in 3D space; and creates the 2D patterns that replicate this envisioned target geometry when lifted into 3D via a physical draping simulation. Our framework removes the need for laborious and knowledge-intensive manual 2D edits and allows users to effortlessly mix existing garment designs as well as adjust for garment length and fit. Following each user specified editing operation we first compute a target 3D garment shape, one that maximally preserves the input garment’s style–its proportions, fit and shape–subject to the modifications specified by the user. We then automatically compute 2D patterns that recreate the target garment shape when draped around the input mannequin within a user-selected simulation environment. To generate these patterns, we propose a fixed-point optimization scheme that compensates for the deformation due to the physical forces affecting the drape and is independent of the underlying simulation tool used. Our experiments show that this method quickly and reliably converges to patterns that, under simulation, form the desired target look, and works well with different black-box physical simulators. We demonstrate a range of edited and resimulated garments, and further validate our approach via expert and amateur critique, and comparisons to alternative solutions.

Physics-Driven Pattern Adjustment for Direct 3D Garment Editing

Posted by christopherbatty on April 23rd, 2016 | Comments Off on Physics-Driven Pattern Adjustment for Direct 3D Garment Editing Posted in Cloth, Deformables

Adaptive Skinning for Interactive Hair-Solid Simulation

Menglei Chai, Changxi Zheng and Kun Zhou

Reduced hair models have proven successful for interactively simulating a full head of hair strands, building upon a fundamental assumption that only a small set of guide hairs are needed for explicit simulation, and the rest of the hair move coherently and thus can be interpolated using guide hairs. Unfortunately, hair-solid interactions is a pathological case for traditional reduced hair models, as the motion coherence between hair strands can be arbitrarily broken by interacting with solids. In this paper, we propose an adaptive hair skinning method for interactive hair simulation with hair-solid collisions. We precompute many eligible sets of guide hairs and the corresponding interpolation relationships that are represented using a compact strand-based hair skinning model. At runtime, we simulate only guide hairs; for interpolating every other hair, we adaptively choose its guide hairs, taking into account motion coherence and potential hair-solid collisions. Further, we introduce a two-way collision correction algorithm to allow sparsely sampled guide hairs to resolve collisions with solids that can have small geometric features. Our method enables interactive simulation of more than 150K hair strands interacting with complex solid objects, using 400 guide hairs. We demonstrate the efficiency and robustness of the method with various hairstyles and user-controlled arbitrary hair-solid interactions.

Adaptive Skinning for Interactive Hair-Solid Simulation

Posted by christopherbatty on April 23rd, 2016 | Comments Off on Adaptive Skinning for Interactive Hair-Solid Simulation Posted in Hair/Rods/Rope

Artist-Directed Dynamics for 2D Animation

Yunfei Bai, Danny M. Kaufman, C.Karen Liu, Jovan Popović

Animation artists enjoy the benefits of simulation but do not want to be held back by its constraints. Artist-directed dynamics seeks to resolve this need with a unified method that combines simulation with classical keyframing techniques. The combination of these approaches improves upon both extremes: simulation becomes more customizable and keyframing becomes more automatic. Examining our system in the context of the twelve fundamental animation principles reveals that it stands out for its treatment of exaggeration and appeal. Our system accommodates abrupt jumps, large plastic deformations, and makes it easy to reuse carefully crafted animations.

Artist-Directed Dynamics for 2D Animation

Posted by christopherbatty on April 23rd, 2016 | Comments Off on Artist-Directed Dynamics for 2D Animation Posted in Deformables

Surface-Only Liquids

Fang Da, David Hahn, Christopher Batty, Chris Wojtan, Eitan Grinspun

We propose a novel surface-only technique for simulating incompressible, inviscid and uniform-density liquids with surface tension in three dimensions. The liquid surface is captured by a triangle mesh on which a Lagrangian velocity field is stored. Because advection of the velocity field may violate the incompressibility condition, we devise an orthogonal projection technique to remove the divergence while requiring the evaluation of only two boundary integrals. The forces of surface tension, gravity, and solid contact are all treated by a boundary element solve, allowing us to perform detailed simulations of a wide range of liquid phenomena, including waterbells, droplet and jet collisions, fluid chains, and crown splashes.

Surface-Only Liquids

Posted by christopherbatty on April 21st, 2016 | Comments Off on Surface-Only Liquids Posted in Fluids

Drucker-Prager Elastoplasticity for Sand Animation

Gergely Klar, Theodore Gast, Andre Pradhana, Chuyuan Fu, Craig Schroeder, Chenfanfu Jiang, Joseph Teran

We simulate sand dynamics using an elastoplastic, continuum assumption. We demonstrate that the Drucker-Prager plastic flow model combined with a Hencky-strain-based hyperelasticity accurately recreates a wide range of visual sand phenomena with moderate computational expense. We use the Material Point Method (MPM) to discretize the governing equations for its natural treatment of contact, topological change and history dependent constitutive relations. The Drucker-Prager model naturally represents the frictional relation between shear and normal stresses through a yield stress criterion. We develop a stress projection algorithm used for enforcing this condition with a non-associative flow rule that works naturally with both implicit and explicit time integration. We demonstrate the efficacy of our approach on examples undergoing large deformation, collisions and topological changes necessary for producing modern visual effects.

Drucker-Prager Elastoplasticity for Sand Animation

Posted by christopherbatty on April 20th, 2016 | Comments Off on Drucker-Prager Elastoplasticity for Sand Animation Posted in Fluids

Resolving Fluid Boundary Layers with Particle Strength Exchange and Weak Adaptivity

Xinxin Zhang, Minchen Li, Robert Bridson

Most fluid scenarios in graphics have a high Reynolds number, where viscosity is dominated by inertial effects, thus most solvers drop viscosity altogether: numerical damping from coarse grids is generally stronger than physical viscosity while resembling it in character. However, viscosity remains crucial near solid boundaries, in the boundary layer, to a large extent determining the look of the flow as a function of Reynolds number. Typical graphics simulations do not resolve boundary layer dynamics, so their look is determined mostly by numerical errors with the given grid size and time step, rather than physical parameters. We introduce two complementary techniques to capture boundary layer dynamics, bringing more physical control and predictability. We extend the FLIP particle-grid method with viscous particle strength exchange[Rivoalen and Huberson 2001] to better transfer momentum at solid boundaries, dubbed VFLIP. We also introduce Weakly Higher Resolution Regional Projection (WHIRP), a cheap and simple way to increase grid resolution where important by overlaying high resolution grids on the global coarse grid.

Resolving Fluid Boundary Layers with Particle Strength Exchange and Weak Adaptivity

Posted by christopherbatty on April 20th, 2016 | Comments Off on Resolving Fluid Boundary Layers with Particle Strength Exchange and Weak Adaptivity Posted in Fluids

SIGGRAPH 2016

SIGGRAPH 2016 papers:

 

TOG presentations at SIGGRAPH 2016:

Posted by christopherbatty on April 4th, 2016 | Comments Off on SIGGRAPH 2016 Posted in Uncategorized

Ebb: A DSL for Physical Simulation on CPUs and GPUs

Gilbert Bernstein, Chinmayee Shah, Crystal Lemire, Zachery DeVito, Matthew Fisher, Philip Levis, Pat Hanrahan

Designing programming environments for physical simulation is challenging because simulations rely on diverse algorithms and geometric domains. These challenges are compounded when we try to run efficiently on heterogeneous parallel architectures. We present Ebb, a domain-specific language (DSL) for simulation, that runs efficiently on both CPUs and GPUs. Unlike previous DSLs, Ebb uses a three-layer architecture to separate (1) simulation code, (2) definition of data structures for geometric domains, and (3) runtimes supporting parallel architectures. Different geometric domains are implemented as libraries that use a common, unified, relational data model. By structuring the simulation framework in this way, programmers implementing simulations can focus on the physics and algorithms for each simulation without worrying about their implementation on parallel computers. Because the geometric domain libraries are all implemented using a common runtime based on relations, new geometric domains can be added as needed, without specifying the details of memory management, mapping to different parallel architectures, or having to expand the runtime’s interface. We evaluate Ebb by comparing it to several widely used simulations, demonstrating comparable performance to hand-written GPU code where available, and surpassing existing CPU performance optimizations by up to 9x when no GPU code exists.

Ebb: A DSL for Physical Simulation on CPUs and GPUs

Posted by christopherbatty on April 4th, 2016 | Comments Off on Ebb: A DSL for Physical Simulation on CPUs and GPUs Posted in Deformables, Fluids

Boundary detection in particle-based fluids

Marcos Sandim, Douglas Cedrim, Luis Gustavo Nonato, Paulo Pagliosa, and Afonso Paiva

This paper presents a novel method to detect free-surfaces on particle-based volume representation. In contrast to most particle-based free-surface detection methods, which perform the surface identification based on physical and geometrical properties derived from the underlying fluid flow simulation, the proposed approach only demands the spatial location of the particles to properly recognize surface particles, avoiding even the use of kernels. Boundary particles are identified through a Hidden Point Removal (HPR) operator used for visibility test. Our method is very simple, fast, easy to implement and robust to changes in the distribution of particles, even when facing large deformation of the free-surface. A set of comparisons against state-of-the-art boundary detection methods show the effectiveness of our approach. The good performance of our method is also attested in the context of fluid flow simulation involving free-surface, mainly when using level-sets for rendering purposes.

Boundary detection in particle-based fluids

Posted by christopherbatty on April 2nd, 2016 | Comments Off on Boundary detection in particle-based fluids Posted in Uncategorized
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