SCA 12: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/424

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Smoke Sheets for Graph-Structured Vortex Filaments
(The Eurographics Association, 2012) Barnat, Alfred; Pollard, Nancy S.; Jehee Lee and Paul Kry
Smoke is one of the core phenomena which fluid simulation techniques in computer graphics have attempted to capture. It is both well understood mathematically and important in lending realism to computer generated effects. In an attempt to overcome the diffusion inherent to Eulerian grid-based simulators, a technique has recently been developed which represents velocity using a sparse set of vortex filaments. This has the advantage of providing an easily understandable and controllable model for fluid velocity, but is computationally expensive because each filament affects the fluid velocity over an unbounded region of the simulation space. We present an alternative to existing techniques which merge adjacent filament rings, instead allowing filaments to form arbitrary structures, and we develop a new set of reconnection criteria to take advantage of this filament graph. To complement this technique, we also introduce a method for smoke surface tracking and rendering designed to minimize the number of sample points without introducing excessive diffusion or blurring. Though this representation lends itself to straightforward real-time rendering, we also present a method which renders the thin sheets and curls of smoke as diffuse volumes using any GPU capable of supporting geometry shaders.
Efficient Collision Detection for Brittle Fracture
(The Eurographics Association, 2012) Glondu, Loeiz; Schvartzman, Sara C.; Marchal, Maud; Dumont, Georges; Otaduy, Miguel A.; Jehee Lee and Paul Kry
In complex scenes with many objects, collision detection plays a key role in the simulation performance. This is particularly true for fracture simulation, where multiple new objects are dynamically created. In this paper, we present novel algorithms and data structures for collision detection in real-time brittle fracture simulations. We build on a combination of well-known efficient data structures, namely distance fields and sphere trees, making our algorithm easy to integrate on existing simulation engines. We propose novel methods to construct these data structures, such that they can be efficiently updated upon fracture events and integrated in a simple yet effective self-adapting contact selection algorithm. Altogether, we drastically reduce the cost of both collision detection and collision response. We have evaluated our global solution for collision detection on challenging scenarios, achieving high frame rates suited for hard real-time applications such as video games or haptics. Our solution opens promising perspectives for complex brittle fracture simulations involving many dynamically created objects.
Component-based Locomotion Composition
(The Eurographics Association, 2012) Kim, Yejin; Neff, Michael; Jehee Lee and Paul Kry
When generating locomotion, it is particularly challenging to adjust the motion's style. This paper introduces a component-based system for human locomotion composition that drives off a set of example locomotion clips. The distinctive style of each example is analyzed in the form of sub-motion components decomposed from separate body parts via independent component analysis (ICA). During the synthesis process, we use these components as combinatorial ingredients to generate new locomotion sequences that are stylistically different from the example set. Our system is designed for novice users who do not have much knowledge of important locomotion properties, such as the correlations throughout the body. Thus, the proposed system analyzes the examples in a unsupervised manner and synthesizes an output locomotion from a small number of control parameters. Our experimental results show that the system can generate physically plausible locomotion in a desired style at interactive speed.
Tiling Motion Patches
(The Eurographics Association, 2012) Kim, Manmyung; Hwang, Youngseok; Hyun, Kyunglyul; Lee, Jehee; Jehee Lee and Paul Kry
Simulating multiple character interaction is challenging because character actions must be carefully coordinated to align their spatial locations and synchronized with each other. We present an algorithm to create a dense crowd of virtual characters interacting with each other. The interaction may involve physical contacts, such as hand shaking, hugging, and carrying a heavy object collaboratively. We address the problem by collecting deformable motion patches, each of which describes an episode of multiple interacting characters, and tiling them spatially and temporally. The tiling of motion patches generates a seamless simulation of virtual characters interacting with each other in a non-trivial manner. Our tiling algorithm uses a combination of stochastic sampling and deterministic search to address the discrete and continuous aspects of the tiling problem. Our tiling algorithm made it possible to automatically generate highly-complex animation of multiple interacting characters. We achieved the level of complexity far beyond the current state-of-the-art animation techniques could generate, in terms of the diversity of human behaviors and the spatial/temporal density of interpersonal interactions.
Principal Geodesic Dynamics
(The Eurographics Association, 2012) Tournier, Maxime; Reveret, Lionel; Jehee Lee and Paul Kry
This paper presents a new integration of a data-driven approach using dimension reduction and a physicallybased simulation for real-time character animation. We exploit Lie group statistical analysis techniques (Principal Geodesic Analysis, PGA) to approximate the pose manifold of a motion capture sequence by a reduced set of pose geodesics. We integrate this kinematic parametrization into a physically-based animation approach of virtual characters, by using the PGA-reduced parametrization directly as generalized coordinates of a Lagrangian formulation of mechanics. In order to achieve real-time without sacrificing stability, we derive an explicit time integrator by approximating existing variational integrators. Finally, we test our approach in task-space motion control. By formulating both physical simulation and inverse kinematics time stepping schemes as two quadratic programs, we propose a features-based control algorithm that interpolates between the two metrics. This allows for an intuitive trade-off between realistic physical simulation and controllable kinematic manipulation.
Energetically Consistent Invertible Elasticity
(The Eurographics Association, 2012) Stomakhin, Alexey; Howes, Russell; Schroeder, Craig; Teran, Joseph M.; Jehee Lee and Paul Kry
We provide a smooth extension of arbitrary isotropic hyperelastic energy density functions to inverted configurations. This extension is designed to improve robustness for elasticity simulations with ex- tremely large deformations and is analogous to the extension given to the first Piola-Kirchoff stress in [ITF04]. We show that our energy-based approach is significantly more robust to large deformations than the first Piola-Kirchoff fix. Furthermore, we show that the robustness and stability of a hyper- elastic model can be predicted from a characteristic contour, which we call its primary contour. The extension to inverted configurations is defined via extrapolation from a convex threshold surface that lies in the uninverted portion of the principal stretches space. The extended hyperelastic energy den- sity yields continuous stress and unambiguous stress derivatives in all inverted configurations, unlike in [TSIF05]. We show that our invertible energy-density-based approach outperforms the popular hy- perelastic corotated model, and we also show how to use the primary contour methodology to improve the robustness of this model to large deformations.
Linear-Time Smoke Animation with Vortex Sheet Meshes
(The Eurographics Association, 2012) Brochu, Tyson; Keeler, Todd; Bridson, Robert; Jehee Lee and Paul Kry
We present the first quality physics-based smoke animation method which runs in time approximately linear in the size of the rendered two-dimensional visual detail. Our fundamental representation is a closed triangle mesh surface dividing space between clear air and a uniformly smoky region, on which we compute vortex sheet dynamics to accurately solve inviscid buoyant flow. We handle arbitrary moving no-stick solid boundaries and by default handle an infinite domain. The simulation itself runs in time linear to the number of triangles thanks to the use of a well-conditioned integral equation treatment together with a Fast Multipole Method for linear-time summations, providing excellent performance. Basic zero-albedo smoke rendering, with embedded solids, is easy to implement for interactive rates, and the mesh output can also serve as an extremely compact and detailed input to more sophisticated volume rendering.
Enriching Coarse Interactive Elastic Objects with High-Resolution Data-Driven Deformations
(The Eurographics Association, 2012) Seiler, Martin; Spillmann, Jonas; Harders, Matthias; Jehee Lee and Paul Kry
Efficient approximate deformation models allow to interactively simulate elastic objects. However, these approaches usually cannot reproduce the complex deformation behavior governed by geometric and material nonlinearities. In addition, objects having slender shapes require dense simulation meshes, which necessitates additional computational effort. We propose an approach where a dynamic interactive coarse simulation is enriched with details stemming from a more accurate quasi-static simulation in a data-driven way. While the coarse simulation is based on a low-resolution (low-res) mesh and a fast linear deformation model the accurate simulation employs a quasi-static non-linear deformation model at a higher mesh resolution (high-res). We pre-compute pairs of low-res mesh deformations and corresponding high-res details by applying a series of training interactions on both the coarse and the accurate model. At run-time, we only run the coarse simulation and correlate the current state to the training states. Subsequently, we blend detail data in order to obtain a spatio-temporally smooth displacement field that we super-impose on the surface skin, resulting in a plausible display of the non-linearly deformed object at real-time rates. We present examples from both computer animation and medical simulation.
Finger Walking: Motion Editing with Contact-Based Hand Performance
(The Eurographics Association, 2012) Lockwood, Noah; Singh, Karan; Jehee Lee and Paul Kry
We present a system for generating full-body animations from the performance on a touch-sensitive tabletop of ''finger walking'', where two fingers are used to pantomime leg movements. A user study was conducted to explore how users can communicate full-body motion using their hands, which concluded that finger walking is a naturally-chosen and comfortable performance method. Based on contact data recorded during this study, the properties of a variety of performed locomotion types were analyzed to determine which motion parameters are most reliable and expressive for the purpose of generating corresponding full-body animations. Based on this analysis, a compact set of motion features was developed for classifying the locomotion type of a finger performance. A prototype interactive animation system was implemented to generate full-body animations of a known locomotion type from finger walking by estimating the motion path of a finger performance, and editing the path of a corresponding animation to match. The classification accuracy and output animation quality of this system was evaluated in a second user study, demonstrating that satisfying full-body animations can be reliably generated from finger performances.
Combining Marker-based Mocap and RGB-D Camera for Acquiring High-fidelity Hand Motion Data
(The Eurographics Association, 2012) Zhao, Wenping; Chai, Jinxiang; Xu, Ying-Qing; Jehee Lee and Paul Kry
Motion capture data has been pivotal to the success of creating realistic animation for human characters. There are a number of public full-body motion databases available, but large and heterogeneous databases for hand articulations are not available. In this paper, we introduce a novel acquisition framework for acquiring a wide range of high-fidelity hand motion data. Our key idea is to leverage marker position data recorded by a twelvecamera optical motion capture system and RGB/Depth data obtained from a single Microsoft Kinect camera. We formulate the hand motion capture problem in a nonlinear optimization framework by maximizing consistency between the reconstructed motion and observed measurement. We introduce an efficient optimization technique to estimate the optimal hand pose that best matches observed data.We have demonstrated the power and effectiveness of our system by capturing a wide variety of delicate hand articulations, even in case of significant self-occlusion.
Evaluating the Plausibility of Edited Throwing Animations
(The Eurographics Association, 2012) Vicovaro, Michele; Hoyet, Ludovic; Burigana, Luigi; O'Sullivan, Carol; Jehee Lee and Paul Kry
Animation budget constraints during the development of a game often call for the use of a limited set of generic motions. Editing operations are thus generally required to animate virtual characters with a sufficient level of variety. Evaluating the perceptual plausibility of edited animations can therefore contribute greatly towards producing visually plausible animations. In this paper we study observers' sensitivity to manipulations of overarm and underarm biological throwing animations. In our first experiment, we used Dynamic Time Warping to edit the biological throwing motions, and modified the release velocity of the ball accordingly. We found that observers are more tolerant to speeding up of the original throwing motion than to slowing down, and that slowed down underarm throws are perceived as particularly unnatural. In our second experiment, we modified separately horizontal and vertical components of the release velocity of the ball, while leaving the motion of the thrower unchanged. We found that observers are more sensitive to manipulations of the horizontal component in overarm throws, and of the vertical component in underarm throws. As in the first experiment, we found that observers are most disturbed by decreases in the velocity of the ball in underarm throws. Our results provide valuable insights for developers of games and VR applications by specifying thresholds for the perceptual plausibility of throwing manipulations.
Interactive Steering of Mesh Animations
(The Eurographics Association, 2012) Vögele, Anna; Hermann, Max; Krüger, Björn; Klein, Reinhard; Jehee Lee and Paul Kry
Creating geometrically detailed mesh animations is an involved and resource-intensive process in digital content creation. In this work we present a method to rapidly combine available sparse motion capture data with existing mesh sequences to produce a large variety of new animations. The key idea is to model shape changes correlated to the pose of the animated object via a part-based statistical shape model. We observe that compact linear models suffice for a segmentation into nearly rigid parts. The same segmentation further guides the parameterization of the pose which is learned in conjunction with the marker movement. Besides the inherent high geometric detail, further benefits of the presented method arise from its robustness against errors in segmentation and pose parameterization. Due to efficiency of both learning and synthesis phase, our model allows to interactively steer virtual avatars based on few markers extracted from video data or input devices like the Kinect sensor.
Mass-Conserving Eulerian Liquid Simulation
(The Eurographics Association, 2012) Chentanez, Nuttapong; Müller, Matthias; Jehee Lee and Paul Kry
We present a GPU friendly, Eulerian, free surface fluid simulation method that conserves mass locally and globally without the use of Lagrangian components. Local mass conservation prevents small scale details of the free surface from disappearing, a problem that plagues many previous approaches, while global mass conservation ensures that the total volume of the liquid does not decrease over time. Our method handles moving solid boundaries as well as cells that are partially filled with solids. Due to its stability, it allows the use of large time steps which makes it suitable for both off-line and real-time applications. We achieve this by using density based surface tracking with a novel, unconditionally stable, conservative advection scheme and a novel interface sharpening method. While our approach conserves mass, volume loss is still possible but only temporarily. With constant mass, local volume loss causes a local increase of the density used for surface tracking which we detect and correct over time. We also propose a density post-processing method to reveal sub-grid details of the liquid surface.We show the effectiveness of the proposed method in several practical examples all running either at interactive rates or in real-time.
Precomputed Motion Maps for Unstructured Motion Capture
(The Eurographics Association, 2012) Mahmudi, Mentar; Kallmann, Marcelo; Jehee Lee and Paul Kry
We present in this paper a solution for extracting high-quality motions from unstructured motion capture databases at interactive rates. The proposed solution is based on automatically-built motion graphs, and offers two key contributions. First, we show how precomputed expansion trees (or motion maps) coupled with new heuristics and backtracking techniques are able to significantly improve the time taken to search for motions satisfying user constraints. Second, we show that when feature-based transitions are employed for constructing the underlying motion graph, the connectivity of motion maps is greatly increased, allowing the overall method to perform search and synthesis at interactive frame rates. We demonstrate the effectiveness of our approach with the problem of extracting path-following motions around obstacles from a motion graph structure at interactive performances.
Efficient Composition for Virtual Camera Control
(The Eurographics Association, 2012) Lino, Christophe; Christie, Marc; Jehee Lee and Paul Kry
Automatically positioning a virtual camera in a 3D environment given the specification of visual properties to be satisfied (on-screen layout of subjects, vantage angles, visibility) is a complex and challenging problem. Most approaches tackle the problem by expressing visual properties as constraints or functions to optimize, and rely on computationally expensive search techniques to explore the solution space. We show here how to express and solve the exact on-screen positioning of two or three subjects using a simple and very efficient technique. We express the solution space for each couple of subjects as a 2D manifold surface. We demonstrate how to use this manifold surface to solve Blinn's spacecraft problem with a straightforward algebraic approach. We extend the solution to three subjects and we show how to cast the complex 6D optimization problem tackled by most contributions in the field in a simple 2D optimization on the manifold surface by pruning large portions of the search space. The result is a robust and very efficient technique which finds a wide range of applications in virtual camera control and more generally in computer graphics.
Cloning Crowd Motions
(The Eurographics Association, 2012) Li, Yi; Christie, Marc; Siret, Orianne; Kulpa, Richard; Pettré, Julien; Jehee Lee and Paul Kry
This paper introduces a method to clone crowd motion data. Our goal is to efficiently animate large crowds from existing examples of motions of groups of characters by applying an enhanced copy and paste technique on them. Specifically, we address spatial and temporal continuity problems to enable animation of significantly larger crowds than our initial data. We animate many characters from the few examples with no limitation on duration. Moreover, our animation technique answers the needs of real-time applications through a technique of linear complexity. Therefore, it is significantly more efficient than any existing crowd simulation-based technique, and in addition, we ensure a predictable level of realism for animations. We provide virtual population designers and animators with a powerful framework which (i) enables them to clone crowd motion examples while preserving the complexity and the aspect of group motion and (ii) is able to animate large-scale crowds in real-time. Our contribution is the formulation of the cloning problem as a double search problem. Firstly, we search for almost periodic portions of crowd motion data through the available examples. Secondly, we search for almost symmetries between the conditions at the limits of these portions in order to interconnect them. The result of our searches is a set of crowd patches that contain portions of example data that can be used to compose large and endless animations. Through several examples prepared from real crowd motion data, we demonstrate the advantageous properties of our approach as well as identify its potential for future developments.
Physically Plausible Simulation for Character Animation
(The Eurographics Association, 2012) Levine, Sergey; Popovic, Jovan; Jehee Lee and Paul Kry
Artist-created animated characters can exhibit stylized, engaging behavior, but require considerable effort to construct, while interactive applications require numerous motions and variations to create a dynamic, believable character. This paper describes a method for generating some of these variations automatically: given a stream of poses, our method simulates plausible responses to physical disturbances and environmental variations. Our quasi-physical simulation accounts for the dynamics of the character and surrounding objects, but does not require the motion to be physically valid, making it suitable for both realistic and stylized, cartoony motions. It further does not require any preprocessing, allowing it to run as an online filter that transforms the output of any real-time animation system. Our prototype runs at 50 Hz, on bipeds and quadrupeds with over 50 degrees of freedom, and generates plausible variations for walking, running, hopping, crawling, rolling, cartwheeling, and other motions.
Learning Motion Controllers with Adaptive Depth Perception
(The Eurographics Association, 2012) Lo, Wan-Yen; Knaus, Claude; Zwicker, Matthias; Jehee Lee and Paul Kry
We present a novel approach to real-time character animation that allows a character to move autonomously based on vision input. By allowing the character to ''see'' the environment directly using depth perception, we can skip the manual design phase of parameterizing the state space in a reinforcement learning framework. In previous work, this is done manually since finding a minimal set of parameters for describing a character's environment is crucial for efficient learning. Learning from raw vision input, however, suffers from the ''curse of dimensionality'', which we avoid by introducing a hierarchical state model and a novel regression algorithm. We demonstrate that our controllers allow a character to navigate and survive in environments containing arbitrarily shaped obstacles, which is hard to achieve with existing reinforcement learning frameworks.
Quaternion Space Sparse Decomposition for Motion Compression and Retrieval
(The Eurographics Association, 2012) Zhu, Mingyang; Sun, Huaijiang; Deng, Zhigang; Jehee Lee and Paul Kry
Quaternion has become one of the most widely used representations for rotational transformations in 3D graphics for decades. Due to the sparse nature of human motion in both the spatial domain and the temporal domain, an unexplored yet challenging research problem is how to directly represent intrinsically sparse human motion data in quaternion space. In this paper we propose a novel quaternion space sparse decomposition (QSSD) model that decomposes human rotational motion data into two meaningful parts (namely, the dictionary part and the weight part) with the sparseness constraint on the weight part. Specifically, a linear combination (addition) operation in Euclidean space is equivalently modeled as a quaternion multiplication operation, and the weight of linear combination is modeled as a power operation on quaternion. Besides validations of the robustness, convergence, and accuracy of the QSSD model, we also demonstrate its two selected applications: human motion data compression and content-based human motion retrieval. Through numerous experiments and quantitative comparisons, we demonstrate that the QSSD-based approaches can soundly outperform existing state-of-the-art human motion compression and retrieval approaches.
Simple Data-Driven Control for Simulated Bipeds
(The Eurographics Association, 2012) Geijtenbeek, T.; Pronost, Nicolas; Stappen, A. F. van der; Jehee Lee and Paul Kry
We present a framework for controlling physics-based bipeds in a simulated environment, based on a variety of reference motions. Unlike existing methods for control based on reference motions, our framework does not require preprocessing of the reference motion, nor does it rely on inverse dynamics or on-line optimization methods for torque computation. It consists of three components: Proportional-Derivative Control to mimic motion characteristics, a specific form of Jacobian Transpose Control for balance control, and Covariance Matrix Adaption for off-line parameter optimization, based on a novel high-level reward function. The framework can easily be implemented using common off-the-shelf physics engines, and generates simulations at approximately 4x realtime on a single core of a modern PC. Our framework advances the state-of-the-art by demonstrating motions of a diversity and dynamic nature previously unseen in comparable methods, including squatting, bowing, kicking, and dancing motions. We also demonstrate its ability to withstand external perturbations and adapt to changes in character morphology.

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