SCA 11: Eurographics/SIGGRAPH Symposium on Computer Animation

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

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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.
Simulating Heterogeneous Crowd BehaviorsUsing Personality Trait Theory
(The Eurographics Association, 2011) Guy, Stephen J.; Kim, Sujeong; Lin, Ming C.; Manocha, Dinesh; A. Bargteil and M. van de Panne
We present a new technique to generate heterogeneous crowd behaviors using personality trait theory. Our formulation is based on adopting results of a user study to derive a mapping from crowd simulation parameters to the perceived behaviors of agents in computer-generated crowd simulations. We also derive a linear mapping between simulation parameters and personality descriptors corresponding to the well-established Eysenck Threefactor personality model. Furthermore, we propose a novel two-dimensional factorization of perceived personality in crowds based on a statistical analysis of the user study results. Finally, we demonstrate that our mappings and factorizations can be used to generate heterogeneous crowd behaviors in different settings.
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.
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.
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.
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 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.
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.
Robust Real-Time Deformation of Incompressible Surface Meshes
(The Eurographics Association, 2011) Diziol, R.; Bender, J.; Bayer, D.; A. Bargteil and M. van de Panne
We introduce an efficient technique for robustly simulating incompressible objects with thousands of elements in real-time. Instead of considering a tetrahedral model, commonly used to simulate volumetric bodies, we simply use their surfaces. Not requiring hundreds or even thousands of elements in the interior of the object enables us to simulate more elements on the surface, resulting in high quality deformations at low computation costs. Theelasticity of the objects is robustly simulated with a geometrically motivated shape matching approach which is extended by a fast summation technique for arbitrary triangle meshes suitable for an efficient parallel computation on the GPU. Moreover, we present an oscillation-free and collision-aware volume constraint, purely based on the surface of the incompressible body. The novel heuristic we propose in our approach enables us to conserve the volume, both globally and locally. Our volume constraint is not limited to the shape matching method and can be used with any method simulating the elasticity of an object. We present several examples which demonstrate high quality volume conserving deformations and compare the run-times of our CPU implementation, as well as our GPU implementation with similar methods.
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.
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.
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.
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
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.
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.
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.
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.
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.

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