SCA 11: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/425

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Asynchronous Integration with Phantom Meshes
(The Eurographics Association, 2011) Harmony, David; Zhou, Qingnan; Zorin, Denis; A. Bargteil and M. van de Panne
Asynchronous variational integration of layered contact models provides a framework for robust collision handling, correct physical behavior, and guaranteed eventual resolution of even the most difficult contact problems. Yet, even for low-contact scenarios, this approach is significantly slower compared to its less robust alternatives- often due to handling of stiff elastic forces in an explicit framework. We propose a method that retains the guarantees, but allows for variational implicit integration of some of the forces, while maintaining asynchronous integration needed for contact handling. Our method uses phantom meshes for calculations with stiff forces, which are then coupled to the original mesh through constraints. We use the augmented discrete Lagrangian of theconstrained system to derive a variational integrator with the desired conservation properties
Biomechanically-Inspired Motion Path Editing
(The Eurographics Association, 2011) Lockwood, Noah; Singh, Karan; A. Bargteil and M. van de Panne
We present a system for interactive kinematic editing of motion paths and timing that employs various biomechanical observations to augment and restrict the edited motion. Realistic path manipulations are enforced by restricting user interaction to handles identified along a motion path using motion extrema. An as-rigid-as-possibledeformation technique modified specifically for use on motion paths is used to deform the path to satisfy the usermanipulated handle positions. After all motion poses have been adjusted to satisfy the new path, an automatic timewarping step modifies the timing of the new motion to preserve the timing qualities of the original motion.This timewarp is based on biomechanical heuristics relating velocity to stride length and path curvature, as well as the preservation of acceleration for ballistic motion. We show that our system can be used to quickly and easily modify a variety of locomotive motions, and can accurately reproduce recorded motions that were not used during the editing process.
Content Retargeting Using Parameter-Parallel Facial Layers
(The Eurographics Association, 2011) Kholgade, Natasha; Matthews, Iain; Sheikh, Yaser; A. Bargteil and M. van de Panne
Facial motion retargeting approaches often transfer expressions by establishing correspondences between shared units of motion, such as action units, or spatial correspondences of landmarks between the source actor and target character faces. When the actor and character are structurally dissimilar, shared units of motion or spatiallandmarks may not exist, and subtle styles of performance may differ. We present a method to deconstruct the content of an actor's facial expression into three parameter-parallel layers using a composition function, transfer the content to equivalent parameter-parallel layers for the character, and reconstruct the character's expression using the same composition function. Our algorithm uses the same parameter-parallel layered model of facial expression for both the actor and character, separating the content of facial expressions into emotion, speech, and eye-blink layers. Facial motion in each layer is embedded in simplicial bases, each of which encodes semantically significant configurations of the face. We show the transfer of facial motion capture and video-based tracking of the eyes and mouth of an actor to a number of faces with dissimilar facial structure and expressive disposition.
Controllable Hand Deformation from Sparse Examples with Rich Details
(The Eurographics Association, 2011) Huang, Haoda; Zhao, Ling; Yin, KangKang; Qi, Yue; Yu, Yizhou; Tong, Xin; A. Bargteil and M. van de Panne
Recent advances in laser scanning technology have made it possible to faithfully scan a real object with tiny geometric details, such as pores and wrinkles. However, a faithful digital model should not only capture static details of the real counterpart but also be able to reproduce the deformed versions of such details. In this paper, we develop a data-driven model that has two components respectively accommodating smooth large-scale deformations and high-resolution deformable details. Large-scale deformations are based on a nonlinear mapping between sparse control points and bone transformations. A global mapping, however, would fail to synthesize realistic geometries from sparse examples, for highly-deformable models with a large range of motion. The key is to train a collection of mappings defined over regions locally in both the geometry and the pose space. Deformable fine-scale details are generated from a second nonlinear mapping between the control points and per-vertex displacements. We apply our modeling scheme to scanned human hand models. Experiments show that our deformation models, learned from extremely sparse training data, are effective and robust in synthesizing highly-deformable modelswith rich fine features, for keyframe animation as well as performance-driven animation. We also compare our results with those obtained by alternative techniques.
A Data-driven Appearance Model for Human Fatigue
(The Eurographics Association, 2011) Jr., Joseph T. Kider; Pollock, Kaitlin; Safonova, Alla; A. Bargteil and M. van de Panne
Humans become visibly tired during physical activity. After a set of squats, jumping jacks or walking up a flight of stairs, individuals start to pant, sweat, loose their balance, and flush. Simulating these physiological changes due to exertion and exhaustion on an animated character greatly enhances a motion's realism. These fatigue factors depend on the mechanical, physical, and biochemical function states of the human body. The difficulty ofsimulating fatigue for character animation is due in part to the complex anatomy of the human body. We present a multi-modal capturing technique for acquiring synchronized biosignal data and motion capture data to enhance character animation. The fatigue model utilizes an anatomically derived model of the human body that includes a torso, organs, face, and rigged body. This model is then driven by biosignal output. Our animations show the wide range of exhaustion behaviors synthesized from real biological data output. We demonstrate the fatigue model by augmenting standard motion capture with exhaustion effects to produce more realistic appearance changes during three exercise examples. We compare the fatigue model with both simple procedural methods and a dense markerset data capture of exercise motions.
Element-Wise Mixed Implicit-Explicit Integration for Stable Dynamic Simulation of Deformable Objects
(The Eurographics Association, 2011) Fierz, B.; Spillmann, J.; Harders, M.; A. Bargteil and M. van de Panne
In order to evolve a deformable object in time, the underlying equations of motion have to be numerically integrated. This is commonly done by employing either an explicit or an implicit integration scheme. While explicit methods are only stable for small time steps, implicit methods are unconditionally stable. In this paper, we present a novel methodology to combine explicit and implicit linear integration approaches, based on element-wise stabilityconsiderations. First, we detect the ill-shaped simulation elements which hinder the stable explicit integration of the element nodes as a pre-computation step. These nodes are then simulated implicitly, while the remaining parts of the mesh are explicitly integrated. As a consequence, larger integration time steps than in purely explicit methods are possible, while the computation time per step is smaller than in purely implicit integration. Duringmodifications such as cutting or fracturing, only newly created or modified elements need to be reevaluated, thus making the technique usable in real-time simulations. In addition, our method reduces problems due to numerical dissipation.
Facial Cartography: Interactive Scan Correspondence
(The Eurographics Association, 2011) Wilson, Cyrus A.; Alexander, Oleg; Tunwattanapong, Borom; Ghosh, Pieter PeersAbhijeet; Busch, Jay; Hartholt, Arno; Debevec, Paul; A. Bargteil and M. van de Panne
We present a semi-automatic technique for computing surface correspondences between 3D facial scans in different expressions, such that scan data can be mapped into a common domain for facial animation. The technique can accurately correspond high-resolution scans of widely differing expressions without requiring intermediate posesequences such that they can be used, together with reflectance maps, to create high-quality blendshape-based facial animation. We optimize correspondences through a combination of Image, Shape, and Internal forces, as well as Directable forces to allow a user to interactively guide and refine the solution. Key to our method is a novel representation, called an Active Visage, that balances the advantages of both deformable templates and correspondencecomputation in a 2D canonical domain. We show that our semi-automatic technique achieves more robust results than automated correspondence alone, and is more precise than is practical with unaided manual input.
Graph-based Fire Synthesis
(The Eurographics Association, 2011) Zhang, Yubo; Correa, Carlos D.; Ma, Kwan-Liu; A. Bargteil and M. van de Panne
We present a novel graph-based data-driven technique for cost-effective fire modeling. This technique allows composing long animation sequences using a small number of short simulations. While traditional techniques such as motion graphs and motion blending work well for character motion synthesis, they cannot be trivially applied to fluids to produce results with physically consistent properties which are crucial to the visual appearance offluids. Motivated by the motion graph technique used in character animations, we introduce a new type of graph which can be applied to create various fire phenomena. Each graph node consists of a group of compact spatialtemporal flow pathlines instead of a set of volumetric state fields. Consequently, achieving smooth transitions between discontinuous graph nodes for modeling turbulent fires becomes feasible and computationally efficient.The synthesized particle flow results allow direct particle controls which is much more flexible than a full volumetric representation of the simulation output. The accompanying video shows the versatility and potential power of this new technique for synthesizing realtime complex fire at the quality comparable to production animations.
Human Motion Reconstruction from Force Sensors
(The Eurographics Association, 2011) Ha, Sehoon; Bai, Yunfei; Liu, C. Karen; A. Bargteil and M. van de Panne
Consumer-grade, real-time motion capture devices are becoming commonplace in every household, thanks to the recent development in depth-camera technologies. We introduce a new approach to capturing and reconstructing freeform, full-body human motion using force sensors, supplementary to existing, consumer-grade mocap systems. Our algorithm exploits the dynamic aspects of human movement, such as linear and angular momentum, to providekey information for full-body motion reconstruction. Using two pressure sensing platforms (Wii Balance Board) and a hand tracking device, we demonstrate that human motion can be largely reconstructed from ground reaction forces along with a small amount of arm movement information.
Hybrid Smoothed Particle Hydrodynamics
(The Eurographics Association, 2011) Raveendrany, Karthik; Wojtanz, Chris; Turk, Greg; A. Bargteil and M. van de Panne
We present a new algorithm for enforcing incompressibility for Smoothed Particle Hydrodynamics (SPH) by preserving uniform density across the domain. We propose a hybrid method that uses a Poisson solve on a coarse grid to enforce a divergence free velocity field, followed by a local density correction of the particles. This avoids typical grid artifacts and maintains the Lagrangian nature of SPH by directly transferring pressures onto particles.Our method can be easily integrated with existing SPH techniques such as the incompressible PCISPH method as well as weakly compressible SPH by adding an additional force term. We show that this hybrid method accelerates convergence towards uniform density and permits a significantly larger time step compared to earlier approaches while producing similar results. We demonstrate our approach in a variety of scenarios with significant pressuregradients such as splashing liquids.
A Level-set Method for Skinning Animated Particle Data
(The Eurographics Association, 2011) Bhatacharya, Haimasree; Gao, Yue; Bargteil, Adam; A. Bargteil and M. van de Panne
In this paper, we present a straightforward, easy to implement method for particle skinning-generating surfaces from animated particle data. We cast the problem in terms of constrained optimization and solve the optimization using a level-set approach. The optimization seeks to minimize the thin-plate energy of the surface, while stayingbetween surfaces defined by the union of spheres centered at the particles. Our approach skins each frame independently while preserving the temporal coherence of the underlying particle animation. Thus, it is well-suited for environments where particle skinning is treated as a post-process, with each frame generated in parallel. We demonstrate our method on data generated by a variety of fluid simulation techniques and simple particle systems.
Mass and Momentum Conservation for Fluid Simulation
(The Eurographics Association, 2011) Lentine, Michael; Aanjaneya, Mridul; Fedkiw, Ronald; A. Bargteil and M. van de Panne
Momentum conservation has long been used as a design principle for solid simulation (e.g. collisions between rigid bodies, mass-spring elastic and damping forces, etc.), yet it has not been widely used for fluid simulation. In fact, semi-Lagrangian advection does not conserve momentum, but is still regularly used as a bread and butter method for fluid simulation. In this paper, we propose a modification to the semi-Lagrangian method in order to make it fully conserve momentum. While methods of this type have been proposed earlier in the omputational physics literature, they are not necessarily appropriate for coarse grids, large time steps or inviscid flows, all of which are common in graphics applications. In addition, we show that the commonly used vorticity confinement turbulence model can be modified to exactly conserve momentum as well. We provide a number of examples that illustrate the benefits of this new approach, both in conserving fluid momentum and passively advected scalars such as smoke density. In particular, we show that our new method is amenable to efficient smoke simulation with one time step per frame, whereas the traditional non-conservative semi-Lagrangian method experiences serious artifacts when run with these large time steps, especially when object interaction is considered.
Mathematical Foundation of the Optimization-based Fluid Animation Method
(The Eurographics Association, 2011) Erleben, Kenny; Misztal, Marek Krzysztof; Bæren, J. Andreas; A. Bargteil and M. van de Panne
We present the mathematical foundation of a fluid animation method for unstructured meshes. Key contributions not previously treated are the extension to include diffusion forces and higher order terms of non-linear force approximations. In our discretization we apply a fractional step method to be able to handle advection in a numerically simple Lagrangian approach. Following this a finite element method is used for the remaining terms of the fractional step method. The key to deriving a discretization for the diffusion forces lies inrestating the momentum equations in terms of a Newtonian stress tensor. Rather than applying a straightforward temporal finite difference method followed by a projection method to enforce incompressibility as done in the stable fluids method, the last step of the fractional step method is rewritten as an optimization problem to make it easy to incorporate non-linear force terms such as surface tension.
A Multigrid Fluid Pressure SolverHandling Separating Solid Boundary Conditions
(The Eurographics Association, 2011) Chentanez, Nuttapong; Müller, Matthias; A. Bargteil and M. van de Panne
We present a multigrid method for solving the linear complementarity problem (LCP) resulting from discretizing the Poisson equation subject to separating solid boundary conditions in an Eulerian liquid simulation's pressure projection step. The method requires only a few small changes to a multigrid solver for linear systems. Our generalized solver is fast enough to handle 3D liquid simulations with separating boundary conditions in practical domain sizes. Previous methods could only handle relatively small 2D domains in reasonable time because they used expensive quadratic programming (QP) solvers. We demonstrate our technique in several practical scenarios in which the omission of separating boundary conditions results in disturbing artifacts of liquid sticking to walls.
Optimization for Sag-Free Simulations
(The Eurographics Association, 2011) Twigg, Christopher D.; Kacic-Alesic, Zoran; A. Bargteil and M. van de Panne
A common problem during the first few seconds of a cloth, hair, or flesh simulation is that the mesh sags under gravity, which can undo the work of hours of careful modeling. The typical response of increasing the stiffness of the mesh is unsatisfactory as it increases the computational cost of simulation and adversely impacts the quality of the resulting motion. Modelers are accustomed to creating geometry as it is found in the real world, whichalready includes the effect of gravity. We propose a fast and effective approach for optimizing parameters such as spring rest lengths so that the artistically modeled shape represents the equilibrium after the mesh has settled under gravity. This eliminates sagging, preserves the quality of motion, and is intuitive for the artists.
A Particle-based Method for Preserving Fluid Sheets
(The Eurographics Association, 2011) Ando, Ryoichi; Tsuruno, Reiji; A. Bargteil and M. van de Panne
We present a new particle-based method that explicitly preserves thin fluid sheets for animating liquids. Our primary contribution is a meshless particle-based framework that splits at thin points and collapses at dense points to prevent the breakup of liquid. In contrast to existing surface tracking methods, the proposed framework does notsuffer from numerical diffusion or tangles, and robustly handles topology changes by the meshless representation. As the underlying fluid model, we use Fluid-Implicit-Particle (FLIP) with weak spring forces to generate smooth particle-based liquid animation that maintains an even spatial particle distribution in the presence of eddying or inertial motions. The thin features are detected by examining stretches of distributions of neighboring particles by performing Principle Component Analysis (PCA), which is used to reconstruct thin surfaces with anisotropic kernels. Our algorithm is intuitively implemented, easy to parallelize and capable of producing visually complex thin liquid animations.
Perceptual evaluation of footskate cleanup
(The Eurographics Association, 2011) Pra ák, Martin; Hoyet, Ludovic; O'Sullivan, Carol; A. Bargteil and M. van de Panne
When animating virtual humans for real-time applications such as games and virtual reality, animation systems often have to edit motions in order to be responsive. In many cases, contacts between the feet and the ground are not (or cannot be) properly enforced, resulting in a disturbing artifact know as footsliding or footskate. In this paper, we explore the perceptibility of this error and show that participants can perceive even very low levelsof footsliding (<21mm in most conditions). We then explore the visual fidelity of animations where footskate has been cleaned up using two different methods. We found that corrected animations were always preferred to those with footsliding, irrespective of the extent of the correction required. We also determined that a simple approach of lengthening limbs was preferred to a more complex approach using IK fixes and trajectory smoothing.
Physics-based Character Skinningusing Multi-Domain Subspace Deformations
(The Eurographics Association, 2011) Kimy, Theodore; James, Doug L.; A. Bargteil and M. van de Panne
We propose a domain-decomposition method to simulate articulated deformable characters entirely within a subspace framework. The method supports quasistatic and dynamic deformations, nonlinear kinematics and materials, and can achieve interactive time-stepping rates. To avoid artificial rigidity, or locking, associated with coupling low-rank domain models together with hard constraints, we employ penalty-based coupling forces. Themulti-domain subspace integrator can simulate deformations efficiently, and exploits efficient subspace-only evaluation of constraint forces between rotated domains using a novel Fast Sandwich Transform (FST). Examples are presented for articulated characters with quasistatic and dynamic deformations, and interactive performance with hundreds of fully coupled modes. Using our method, we have observed speedups of between three and four orders of magnitude over full-rank, unreduced simulations.
Practical Color-Based Motion Capture
(The Eurographics Association, 2011) Wang, Robert; Paris, Sylvain; Popovi, Jovan; A. Bargteil and M. van de Panne
Motion capture systems have been widely used for high quality content creation and virtual reality but are rarely used in consumer applications due to their price and setup cost. In this paper, we propose a motion capture system built from commodity components that can be deployed in a matter of minutes. Our approach uses one or more webcams and a color shirt to track the upper-body at interactive rates. We describe a robust color calibration systemthat enables our color-based tracking to work against cluttered backgrounds and under multiple illuminants. We demonstrate our system in several real-world indoor and outdoor settings.
Preview-based Sampling for Controlling Gaseous Simulations
(The Eurographics Association, 2011) Huangy, Ruoguan; Melekz, Zeki; Keyser, John; A. Bargteil and M. van de Panne
In this work, we describe an automated method for directing the control of a high resolution gaseous fluid simulation based on the results of a lower resolution preview simulation. Small variations in accuracy between low and high resolution grids can lead to divergent simulations, which is problematic for those wanting to achieve a desired behavior. Our goal is to provide a simple method for ensuring that the high resolution simulation matcheskey properties from the lower resolution simulation. We first let a user specify a fast, coarse simulation that will be used for guidance. Our automated method samples the data to be matched at various positions and scales in the simulation, or allows the user to identify key portions of the simulation to maintain. During the high resolution simulation, a matching process ensures that the properties sampled from the low resolution simulation are maintained. This matching process keeps the different resolution simulations aligned even for complex systems, and can ensure consistency of not only the velocity field, but also advected scalar values. Because the final simulation is naturally similar to the preview simulation, only minor controlling adjustments are needed, allowing a simplercontrol method than that used in prior keyframing approaches.

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