Physics-Based Animation

Duo Li, Shinjiro Sueda, Debanga R. Neog, Dinesh K. Pai

We present a novel approach to simulating thin hyperelastic skin. Real human skin is only a few millimeters thick. It can stretch and slide over underlying body structures such as muscles, bones, and tendons, revealing rich details of a moving character. Simulating such skin is challenging because it is in close contact with the body and shares its geometry. Despite major advances in simulating elastodynamics of cloth and soft bodies for computer graphics, such methods are difficult to use for simulating thin skin due to the need to deal with non-conforming meshes, collision detection, and contact response. We propose a novel Eulerian representation of skin that avoids all the difficulties of constraining the skin to lie on the body surface by working directly on the surface itself. Skin is modeled as a 2D hyperelastic membrane with arbitrary topology, which makes it easy to cover an entire character or object. Unlike most Eulerian simulations, we do not require a regular grid and can use triangular meshes to model body and skin geometry. The method is easy to implement, and can use low resolution meshes to animate high resolution details stored in texture-like maps. Skin movement is driven by the animation of body shape prescribed by an artist or by another simulation, and so it can be easily added as a post-processing stage to an existing animation pipeline. We provide several examples simulating human and animal skin, and skin-tight clothes.

Thin Skin Elastodynamics

Saket Patkar, Mridul Aanjaneya, Dimitriy Karpman, Ronald Fedkiw

A Hybrid Lagrangian-Eulerian Formulation for Bubble Generation and Dynamics

R. Elliot English, Linhai Qiu, Yue Yu, Ronald Fedkiw

We introduce a new method for large scale water simulation using Chimera grid embedding, which discretizes space with overlapping Cartesian grids that translate and rotate in order to decompose the domain into different regions of interest with varying spatial resolutions. Grids can track both fluid features and solid objects, allowing for dynamic spatial adaptivity without remeshing or repartitioning the domain. We solve the inviscid incompressible NavierStokes equations with an arbitrary-Lagrangian-Eulerian style semiLagrangian advection scheme and a monolithic SPD Poisson solver. We modify the particle level set method in order to adapt it to Chimera grids including particle treatment across grid boundaries with disparate cell sizes, and strategies to deal with locality in the implementation of the level set and fast marching algorithms. We use a local Voronoi mesh construction to solve for pressure and address a number of issues that arise with the treatment of the velocity near the interface. The resulting method is highly scalable on distributed parallel architectures with minimal communication costs.

Chimera Grids for Water Simulation

Theodore Kim, John Delaney

We present a new subspace integration method that is capable of efficiently adding and subtracting dynamics from an existing high-resolution fluid simulation. We show how to analyze the results of an existing high-resolution simulation, discover an efficient reduced approximation, and use it to quickly “re-simulate” novel variations of the original dynamics. Prior subspace methods have had difficulty re-simulating the original input dynamics because they lack efficient means of handling semi-Lagrangian advection methods. We show that multi-dimensional cubature schemes can be applied to this and other advection methods, such as MacCormack advection. The remaining pressure and diffusion stages can be written as a single matrix-vector multiply, so as with previous subspace methods, no matrix inversion is needed at runtime. We additionally propose a novel importance sampling-based fitting algorithm that asymptotically accelerates the precomputation stage, and show that the Iterated Orthogonal Projection method can be used to elegantly incorporate moving internal boundaries into a subspace simulation. In addition to efficiently producing variations of the original input, our method can produce novel, abstract fluid motions that we have not seen from any other solver.

Subspace Fluid Re-Simulation

Olivier Remillard, Paul G. Kry

We present a new technique for simulating high resolution surface wrinkling deformations of composite objects consisting of a soft interior and a harder skin. We combine high resolution thin shells with coarse finite element lattices and define frequency based constraints that allow the formation of wrinkles with properties matching those predicted by the physical parameters of the composite object. Our two-way coupled model produces the expected wrinkling behavior without the computational expense of a large number of volumetric elements to model deformations under the surface. We use C1 quadratic shape functions for the interior deformations, allowing very coarse resolutions to model the overall global deformation efficiently, while avoiding visual artifacts of wrinkling at discretization boundaries. We demonstrate that our model produces wrinkle wavelengths that match both theoretical predictions and high resolution volumetric simulations. We also show example applications in simulating wrinkles on passive objects, such as furniture, and for wrinkles on faces in character animation.

Embedded Thin Shells for Wrinkle Simulation

Better late than never!

• A Novel Projection Technique with Detail Capture and Shape Correction for Smoke Simulation
  
• A Particle-Grid Method for Opaque Ice Formation
• Geometry-Aware Volume-of-Fluid Method

STAR report:

• Position-based Methods for the Simulation of Solid Objects in Computer Graphics

The SCA 2013 program is up. Physics animation papers…

Full papers:

• Chimera Grids for Water Simulation
• Integrated Model for Surface Effects in SPH-based Fluid Simulation
• A Level Set Method for Ductile Fracture
• Artistic Simulation of Curly Hair
• Efficient Simulation of Secondary Motion in Rig-Space
• A Hybrid Lagrangian-Eulerian Formulation for Bubble Generation and Dynamics

Short papers:

• Fast Deformation of Volume Data Using Tetrahedral Mesh Rasterization
• Accurate Surface Embedding for GPU-based Quadratic Finite Elements
• Rod Constraints for Simplified Ragdolls

Daniel Weber, Jan Bender, Markus Schnoes, Andre Stork and Dieter Fellner

We present GPU data structures and algorithms to efficiently solve sparse linear systems which are typically required in simulations of multibody systems and deformable bodies. Thereby, we introduce an efficient sparse matrix data structure that can handle arbitrary sparsity patterns and outperforms current state-of-the-art implementations for sparse matrix vector multiplication. Moreover, an efficient method to construct global matrices on the GPU is presented where hundreds of thousands of individual element contributions are assembled in a few milliseconds. A finite element based method for the simulation of deformable solids as well as an impulse-based method for rigid bodies are introduced in order to demonstrate the advantages of the novel data structures and algorithms. These applications share the characteristic that a major computational effort consists of building and solving systems of linear equations in every time step. Our solving method results in a speed-up factor of up to 13 in comparison to other GPU methods.

Efficient GPU data structures and methods to solve sparse linear systems in dynamics applications

Shengfeng He, Rynson W. H. Lau

Capturing fine details of turbulence on a coarse grid is one of the main tasks in real-time fluid simulation. Existing methods for doing this have various limitations. In this paper, we propose a new turbulence method that uses a refined Second Vorticity Confinement method, referred to as Robust Second Vorticity Confinement, and a synthesis scheme to create highly turbulent effects from coarse grid. The new technique is sufficiently stable to efficiently produce highly turbulent flows, while allowing intuitive control of vortical structures. Second Vorticity Confinement captures and defines the vortical features of turbulence on a coarse grid. However, due to the stability problem, it cannot be used to produce highly turbulent flows. In this work, we propose a robust formulation to improve the stability problem by making the positive diffusion term to vary with helicity adaptively. In addition, we also employ our new method to procedurally synthesize the high resolution flow fields. As shown in our results, this approach produces stable high resolution turbulence very efficiently.

Synthetic Controllable Turbulence Using Robust Second Vorticity Confinement

Romain Casati, Florence Bertails-Descoubes

Thin elastic filaments in real world such as vine tendrils, hair ringlets or curled ribbons often depict a very smooth, curved shape that low-order rod models — e.g., segment-based rods — fail to reproduce accurately and compactly. In this paper, we push forward the investigation of high-order models for thin, inextensible elastic rods by building the dynamics of a G2-continuous piecewise 3D clothoid: a smooth space curve with piecewise affine curvature. With the aim of precisely integrating the rod kinematic problem, for which no closed-form solution exists, we introduce a dedicated integration scheme based on power series expansions. It turns out that our algorithm reaches machine precision orders of magnitude faster compared to classical numerical integrators. This property, nicely preserved under simple algebraic and differential operations, allows us to compute all spatial terms of the rod kinematics and dynamics in both an efficient and accurate way. Combined with a semi-implicit time-stepping scheme, our method leads to the efficient and robust simulation of arbitrary curly filaments that exhibit rich, visually pleasing configurations and motion. Our approach was successfully applied to generate various scenarios such as the unwinding of a curled ribbon as well as the aesthetic animation of spiral-like hair or the fascinating growth of twining plants.

Super Space Clothoids