SCA 11: Eurographics/SIGGRAPH Symposium on Computer Animation

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

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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.
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.
Scenario Space:Characterizing Coverage, Quality, and Failure of SteeringAlgorithms
(The Eurographics Association, 2011) Kapadia, Mubbasir; Wang, Matt; Singh, Shawn; Reinman, Glenn; Faloutsos, Petros; A. Bargteil and M. van de Panne
Navigation and steering in complex dynamically changing environments is a challenging research problem, and a fundamental aspect of immersive virtual worlds. While there exist a wide variety of approaches for navigation and steering, there is no definitive solution for evaluating and analyzing steering algorithms. Evaluating a steering algorithm involves two major challenges: (a) characterizing and generating the space of possible scenarios thatthe algorithm must solve, and (b) defining evaluation criteria (metrics) and applying them to the solution. In this paper, we address both of these challenges. First, we characterize and analyze the complete space of steering scenarios that an agent may encounter in dynamic situations. Then, we propose the representative scenario space and a sampling method that can generate subsets of the representative space with good statistical properties. Wealso propose a new set of metrics and a statistically robust approach to determining the coverage and the quality of a steering algorithm in this space. We demonstrate the effectiveness of our approach on three state of the art techniques. Our results show that these methods can only solve 60% of the scenarios in the representative scenariospace.
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 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.
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.
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.
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.
Real-time Facial Animation from Live Video Tracking
(The Eurographics Association, 2011) Rhee, Taehyun; Hwang, Youngkyoo; Kim, James Dokyoon; Kim, Changyeong; A. Bargteil and M. van de Panne
This paper describes a complete pipe-line of a practical system for producing real-time facial expressions of a 3D virtual avatar controlled by an actor's live performances. The system handles practical challenges arising from markerless expression captures from a single conventional video camera. For robust tracking, a localized algorithm constrained by belief propagation is applied to the upper face, and an appearance matching techniqueusing a parameterized generic face model is exploited for lower face and head pose tracking. The captured expression features then transferred to high dimensional 3D animation controls using our facial expression space which is a structure-preserving map between two algebraic structures. The transferred animation controls drive facial animation of a 3D avatar while optimizing the smoothness of the face mesh. An example-based face deformation technique produces non-linear local detail deformations on the avatar that are not captured in the movement of the animation controls.
A Puppet Interface for Retrieval of Motion Capture Data
(The Eurographics Association, 2011) Numaguchi, Naoki; Nakazawa, Atsushi; Shiratori, Takaaki; Hodgins, Jessica K.; A. Bargteil and M. van de Panne
Intuitive and efficient retrieval of motion capture data is essential for effective use of motion capture databases. In this paper, we describe a system that allows the user to retrieve a particular sequence by performing an approximation of the motion with an instrumented puppet. This interface is intuitive because both adults and children have experience playacting with puppets and toys to express particular behaviors or to tell stories with style and emotion. The puppet has 17 degrees of freedom and can therefore represent a variety of motions. We develop a novel similarity metric between puppet and human motion by computing the reconstruction errors of the puppet motion in the latent space of the human motion and those of the human motion in the latent space of the puppet motion. This metric works even for relatively large databases. We conducted a user study of the system and subjects could find the desired motion with reasonable accuracy from a database consisting of everyday, exercise, and acrobatic behaviors.
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.
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.
Spacetime Vertex Constraints for Dynamically-based Adaptation of Motion-Captured Animation
(The Eurographics Association, 2011) O'Brien, C.; Dingliana, J.; Collins, S.; A. Bargteil and M. van de Panne
We present a novel technique for editing motion captured animation. Our iterative solver produces physicallyplausible adaptated animations that satisfy alterations in foot and hand contact placement with the animated character's surroundings. The technique uses a system of particles to represent the poses and mass distribution of the character at sampled frames of the animation. Constraints between the vertices within each frame enforce theskeletal structure, including joint limits. Novel constraints extending over vertices in several frames enforce the aggregate dynamics of the character, as well as features such as joint acceleration smoothness. We demonstrate adaptation of several animations to altered foot and hand placement.
SPH Granular Flow with Friction and Cohesion
(The Eurographics Association, 2011) Alduán, Iván; Otaduy, Miguel A.; A. Bargteil and M. van de Panne
Combining mechanical properties of solids and fluids, granular materials pose important challenges for the design of algorithms for realistic animation. In this paper, we present a simulation algorithm based on smoothed particle hydrodynamics (SPH) that succeeds in modeling important features of the behavior of granular materials. These features are unilateral incompressibility, friction and cohesion. We extend an existing unilateral incompressibility formulation to be added at almost no effort to an existing SPH-based algorithm for fluids. The main advantages of this extension are the ease of implementation, the lack of grid artifacts, and the simple two-way coupling with other objects. Our friction and cohesion models can also be incorporated in a seamless manner in the overall SPHsimulation algorithm.
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.
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.
A Simple Finite Volume Method for Adaptive Viscous Liquids
(The Eurographics Association, 2011) Batty, Christopher; Houston, Ben; A. Bargteil and M. van de Panne
We present the first spatially adaptive Eulerian fluid animation method to support challenging viscous liquid effects such as folding, coiling, and variable viscosity. We propose a tetrahedral node-based embedded finite volume method for fluid viscosity, adapted from popular techniques for Lagrangian deformable objects. Applied in an Eulerian fashion with implicit integration, this scheme stably and efficiently supports high viscosity fluids while yielding symmetric positive definite linear systems. To integrate this scheme into standard tetrahedral meshbased fluid simulators, which store normal velocities on faces rather than velocity vectors at nodes, we offer two methods to reconcile these representations. The first incorporates a mapping between different degrees offreedom into the viscosity solve itself. The second uses a FLIP-like approach to transfer velocity data between nodes and faces before and after the linear solve. The former offers tighter coupling by enabling the linear solver to act directly on the face velocities of the staggered mesh, while the latter provides a sparser linear system and a simpler implementation. We demonstrate the effectiveness of our approach with animations of spatially varying viscosity, realistic rotational motion, and viscous liquid buckling and coiling.
Real-Time Classification of Dance Gesturesfrom Skeleton Animation
(The Eurographics Association, 2011) Raptis, Michalis; Kirovski, Darko; Hoppe, Hugues; A. Bargteil and M. van de Panne
We present a real-time gesture classification system for skeletal wireframe motion. Its key components include an angular representation of the skeleton designed for recognition robustness under noisy input, a cascaded correlation-based classifier for multivariate time-series data, and a distance metric based on dynamic timewarping to evaluate the difference in motion between an acquired gesture and an oracle for the matching gesture. While the first and last tools are generic in nature and could be applied to any gesture-matching scenario, the classifier is conceived based on the assumption that the input motion adheres to a known, canonical time-base: a musical beat. On a benchmark comprising 28 gesture classes, hundreds of gesture instances recorded using the XBOX Kinect platform and performed by dozens of subjects for each gesture class, our classifier has an average accuracy of 96:9%, for approximately 4-second skeletal motion recordings. This accuracy is remarkable given the input noise from the real-time depth sensor.
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.

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