SCA 04: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/432

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Synchronization for dynamic blending of motions
(The Eurographics Association, 2004) Ménardais, Stéphane; Kulpa, Richard; Multon, Franck; Arnaldi, Bruno; R. Boulic and D. K. Pai
In this paper we present a new real-time synchronization algorithm. In dynamic environments, motions need to be continuously adapted to obtain realistic animations. We propose an advanced time warping algorithm to synchronize such motions. This algorithm uses the sequence of support phases of the motions. It also takes into account the priority associated to each motion. It is based on an algebraic relation to detect incompatible motions and to select elements of the sequence to be enlarged. The resulting time warping function can be non-derivable so it is corrected by using a cardinal spline interpolation. In this paper, we demonstrate that our algorithm always finds at least one solution. This synchronization module is part of a complete animation engine called MKM already used in production.
Image-Based Tomographic Reconstruction of Flames
(The Eurographics Association, 2004) Ihrke, Ivo; Magnor, Marcus; R. Boulic and D. K. Pai
Non-invasively determining the three-dimensional structure of real flames is a challenging task. We present a tomographic method for reconstructing a volumetric model from multiple images of fire. The method is similar to sparse-view computerized tomography and is applicable to static camera setups observing dynamic flames. Using an algebraic reconstruction method, we can restrict the solution space such that a high quality model is obtained from only a small number of camera images. An additional advantage is fast processing of multi-video sequences to generate time-varying models for animation purposes. The resulting three-dimensional fire model is useful for realistic rendering of fire animations, as well as for analyzing gasdynamics of fires.
Modeling and Editing Flows Using Advected Radial Basis Functions
(The Eurographics Association, 2004) Pighin, Frédéric; Cohen, Jonathan M.; Shah, Maurya; R. Boulic and D. K. Pai
Fluid simulations are notoriously difficult to predict and control. As a result, authoring fluid flows often involves a tedious trial and error process. There is to date no convenient way of editing a fluid after it has been simulated. In particular, the Eulerian approach to fluid simulation is not suitable for flow editing since it does not provide a convenient spatio-temporal parameterization of the simulated flows. In this research, we develop a new technique to learn such parameterization. This technique is based on a new representation, the Advected Radial Basis Function. It is a time-varying kernel that models the local properties of the fluid. We describe this representation and demonstrate its use for interactive three-dimensional flow editing.
Simulation Levels of Detail for Plant Motion
(The Eurographics Association, 2004) Beaudoin, Jacob; Keyser, John; R. Boulic and D. K. Pai
In this paper we describe a method for simulating motion of realistically complex plants interactively. We use a precomputation stage to generate the plant structure, along with a set of simulation levels of detail. The levels of detail are made by continuously grouping branches starting from the tips of the branches and working toward the trunk. Grouped branches are simulated as single branches that have similar simulation characteristics to the original branches. During run-time, we traverse the plant and determine the allowable error in the simulation of branch motion. This allows us to choose the appropriate simulation level of detail and we provide smooth transitions from level to level. Our level of detail approach affects only the simulation parameters, allowing geometry to be handled independently. Using this method we can significantly improve simulation times for complex trees.
Decomposing Cloth
(The Eurographics Association, 2004) Boxerman, Eddy; Ascher, Uri; R. Boulic and D. K. Pai
Implicit schemes have become the standard for integrating the equations of motion in cloth simulation. These schemes, however, require the solution of a system representing the entire, fully connected cloth mesh at each time step. In this paper we present techniques that dynamically improve the sparsity of the underlying system, ultimately allowing the mesh to be decomposed into multiple components which can then be solved more efficiently and in parallel. Our techniques include a novel adaptive implicit-explicit (IMEX) scheme which takes advantage of simulation parameters, locally in both space and time, to minimize the coupling of the system. This scheme further directly improves the efficiency of the computation at each time step. Other sparsity improvements are obtained by exploiting the physical model of Choi and Ko (2002), as well as static constraints in the system. In addition, we present a modified preconditioner for the modified preconditioned conjugate gradient (MPCG) technique of Baraff and Witkin (1998), improving its performance by taking constraints into account.
Better with Bubbles: Enhancing the Visual Realism of Simulated Fluid
(The Eurographics Association, 2004) Greenwood, Shannon T.; House, Donald H.; R. Boulic and D. K. Pai
We present a method for including the visual effect of bubbles in a computer graphics fluid simulation, thus enhancing the illusion of realism for a splashing fluid. Previous fluid simulation methods have not included bubbles. Bubble creation is integrated into the particle level-set fluid simulation algorithm. Individual bubbles are approximated by spheres, which form more complex shapes where they intersect. The rendering of bubbles and fluid are integrated to create the appearance of one continuous surface. At the fluid-air boundary, we integrate bubbles whenever level-set marker particles pass from from the outside to the inside of the fluid. Thus, these particles represent air that has become trapped within the fluid surface. In addition, we detect empty pockets within the fluid, that are often formed due to turbulence, and create bubbles within this space. This is an inexpensive way of giving the impression that the air trapped in air pockets has become bubbles. Photo-realistic images of simulation results are rendered with a raytracer that has been enhanced to include caustics, and to handle bubble-bubble interfaces. Comparison of these images with images rendered without bubbles supports our position that the simple addition of bubbles to a fluid simulation greatly enhances visual realism.
Precomputing Avatar Behavior from Human Motion Data
(The Eurographics Association, 2004) Lee, Jehee; Lee, Kang Hoon; R. Boulic and D. K. Pai
Creating controllable, responsive avatars is an important problem in computer games and virtual environments. Recently, large collections of motion capture data have been exploited for increased realism in avatar animation and control. Large motion sets have the advantage of accommodating a broad variety of natural human motion. However, when a motion set is large, the time required to identify an appropriate sequence of motions is the bottleneck for achieving interactive avatar control. In this paper, we present a novel method of precomputing avatar behavior from unlabelled motion data in order to animate and control avatars at minimal runtime cost. Based on dynamic programming, our method finds a control policy that indicates how the avatar should act in any given situation. We demonstrate the effectiveness of our approach through examples that include avatars interacting with each other and with the user.
Invertible Finite Elements for Robust Simulation of Large Deformation
(The Eurographics Association, 2004) Irving, Geoffrey; Teran, Joseph; Fedkiw, Ron; R. Boulic and D. K. Pai
We present an algorithm for the finite element simulation of elastoplastic solids which is capable of robustly and efficiently handling arbitrarily large deformation. In fact, our model remains valid even when large parts of the mesh are inverted. The algorithm is straightforward to implement and can be used with any material constitutive model, and for both volumetric solids and thin shells such as cloth. We also provide a mechanism for controlling plastic deformation, which allows a deformable object to be guided towards a desired final shape without sacri-
System for Authoring Highly Interactive, Personality-Rich Interactive Characters
(The Eurographics Association, 2004) Loyall, A. Bryan; Reilly, W. Scott Neal; Bates, Joseph; Weyhrauch, Peter; R. Boulic and D. K. Pai
We describe an innovative system for authoring expressive, fully autonomous interactive characters. The focus of our work is creating a system to allow rich authoring that captures as much of the artistic intent of the author in procedural form as we can, and that provides automatic support for expressive execution of that content. The system is composed of two parts: (1) a programming language with unusual language features including concurrency, reflection, backtracking, continuously monitored expressions, and a model of emotion, that was created for the expression of interactive self-animating characters; and (2) a motion synthesis system that combines hand-animated motion data with artistically authored procedures for generalizing the motion while preserving the artistic intent. This system has been used to create over a dozen interactive characters, which have been shown at juried venues, as well as being deployed commercially. We describe how artistic qualities important to interactive characters are encoded and supported using this system, and demonstrate the system with an implemented interactive character.
Modeling Deformable Human Hands from Medical Images
(The Eurographics Association, 2004) Kurihara, Tsuneya; Miyata, Natsuki; R. Boulic and D. K. Pai
This paper presents a new method for constructing an example-based deformable human hand model from medical images. Realistic animation of human hands requires good estimates of the joint structure and properly weighted skeleton-driven surface deformation. For this purpose, we propose a method based on medical images of hands in several poses. Our method consists of the following 3 steps: First, using the measured bone shapes, we estimate the link structure (joint rotation centers) and the joint angles of each scan. Second, we construct a mutually consistent polygonal mesh of all the scans. For this purpose, a polygonal mesh of one pose, the base mesh, is deformed using skeletal subspace deformation, and then fitted interactively to the measured meshes from the other scans. Finally, the hand is deformed using a weighted pose space deformation. We demonstrate results of deformable hand models consisting of 100,000 triangle meshes derived from CT scans.
Motion Map: Image-based Retrieval and Segmentation of Motion Data
(The Eurographics Association, 2004) Sakamoto, Yasuhiko; Kuriyama, Shigeru; Kaneko, Toyohisa; R. Boulic and D. K. Pai
Recent proliferation of motion capture systems enables motion data to be saved as an archive system, and the data are usually extracted by selecting an appropriate file by its name or annotation explaining the content of motions. Such semantic-based retrieval, however, is not suited to unstructured files that include many types of elemental motions, due to the difficulty in giving comprehensible annotations. Moreover, expected motion clips are often included as a part of entire sequences, and the data therefore should be manually clipped using some authoring tools. This paper proposes an image-based user interface for retrieving motion data using a self-organizing map for supplying recognizable icons of postures. The postures are used as keys for retrieval, and the desirable segments of the motion data can be accurately extracted by specifying their starting and ending postures. The number of possible motion segments is flexibly controlled by changing the scope of postures used as the keys.
Point Based Animation of Elastic, Plastic and Melting Objects
(The Eurographics Association, 2004) Müller, Matthias; Keiser, Richard; Nealen, Andrew; Pauly, Mark; Gross, Markus; Alexa, Marc; R. Boulic and D. K. Pai
We present a method for modeling and animating a wide spectrum of volumetric objects, with material properties anywhere in the range from stiff elastic to highly plastic. Both the volume and the surface representation are point based, which allows arbitrarily large deviations form the original shape. In contrast to previous point based elasticity in computer graphics, our physical model is derived from continuum mechanics, which allows the specification of common material properties such as Young s Modulus and Poisson s Ratio. In each step, we compute the spatial derivatives of the discrete displacement field using a Moving Least Squares (MLS) procedure. From these derivatives we obtain strains, stresses and elastic forces at each simulated point. We demonstrate how to solve the equations of motion based on these forces, with both explicit and implicit integration schemes. In addition, we propose techniques for modeling and animating a point-sampled surface that dynamically adapts to deformations of the underlying volumetric model.
Breathe Easy: Model and control of simulated respiration for animation
(The Eurographics Association, 2004) Zordan, Victor B.; Celly, Bhrigu; Chiu, Bill; DiLorenzo, Paul C.; R. Boulic and D. K. Pai
Animation of the breath has been largely ignored by the graphics community, even though it is a signature movement of the human body and an indicator for lifelike motion. In this paper, we present an anatomically inspired, physically based model of the human torso for the visual simulation of espiration using a mixed system of rigid and deformable parts. This novel composition of anatomical components is necessary to capture the key characteristics of breathing motion visible in the human trunk because the movement is generated fundamentally through the combination of both rigid bone and soft tissue. We propose a simple anatomically meaningful muscle element based on springs, which is used throughout both actively to drive the motion of the ribs and diaphragm and passively for other muscles like those of the abdomen. In addition, we introduce a straightforward method for preserving incompressible volume in deformable bodies to use in approximating the motion of the gut related to breath. Through the careful construction of this anatomically based torso, control for respiration becomes the generation of periodic contraction signals for a minimal set of two muscle groups. We show the flexibility of ourapproach through the animation of several breathing styles using our system and we verify our results through video and analytical comparisons.
Crowdbrush: Interactive Authoring of Real-time Crowd Scenes
(The Eurographics Association, 2004) Ulicny, Branislav; Ciechomski, Pablo de Heras; Thalmann, Daniel; R. Boulic and D. K. Pai
Recent advances in computer graphics techniques and increasing power of graphics hardware made it possible to display and animate large crowds in real-time. Most of the research efforts have been directed towards improving rendering or behavior control; the question how to author crowd scenes in an efficient way is usually not addressed. We introduce a novel approach to create complex scenes involving thousands of animated individuals in a simple and intuitive way. By employing a brush metaphor, analogous to the tools used in image manipulation programs, we can distribute, modify and control crowd members in real-time with immediate visual feedback. We define concepts of operators and instance properties that allow to create and manage variety in populations of virtual humans. An efficient technique allowing to render up to several thousands of fully three-dimensional polygonal characters with keyframed animations at interactive framerates is presented. The potential of our approach is demonstrated by authoring a scenario of a virtual audience in a theater and a scenario of a pedestrian crowd in a city.
Extended Galilean Invariance for Adaptive Fluid Simulation
(The Eurographics Association, 2004) Shah, Maurya; Cohen, Jonathan M.; Patel, Sanjit; Lee, Penne; Pighin, Frédéric; R. Boulic and D. K. Pai
In an unbounded physical domain, simulating a turbulent fluid on an Eulerian grid is rather tricky. Since it is difficult to predict the motion of the fluid, it is also difficult to guess which computational domain would allow the simulation of the fluid without crossing the computational boundaries. To address this dilemma, we have developed a novel adaptive framework where the simulation grid follows the motion of the flow. Our technique is based on the principle of Galilean Invariance and the culling of simulation cells using a metric derived from continuative boundary conditions. We describe our framework and showcase its advantages over traditional techniques. Timing results and visual comparisons are presented.
Real-time Speech Motion Synthesis from Recorded Motions
(The Eurographics Association, 2004) Cao, Yong; Faloutsos, Petros; Kohler, Eddie; Pighin, Frédéric; R. Boulic and D. K. Pai
Data-driven approaches have been successfully used for realistic visual speech synthesis. However, little effort has been devoted to real-time lip-synching for interactive applications. In particular, algorithms that are based on a graph of motions are notorious for their exponential complexity. In this paper, we present a greedy graph search algorithm that yields vastly superior performance and allows real-time motion synthesis from a large database of motions. The time complexity of the algorithm is linear with respect to the size of an input utterance. In our experiments, the synthesis time for an input sentence of average length is under a second.
Cartoon Textures
(The Eurographics Association, 2004) Juan, Christina de; Bodenheimer, Bobby; R. Boulic and D. K. Pai
In this paper we present a method for creating novel animations from a library of existing two-dimensional cartoon data. Drawing inspiration from the idea of video textures, sequences of similar-looking cartoon data are combined into a user-directed sequence. Starting with a small amount of cartoon data, we employ a method of nonlinear dimensionality reduction to discover a lower-dimensional structure of the data. The user selects a start and end frame and the system traverses this lower-dimensional manifold to re-sequence the data into a new animation. The system can automatically detect when a new sequence has visual discontinuities and may require additional source material.
Momentum-based Parameterization of Dynamic Character Motion
(The Eurographics Association, 2004) Abe, Yeuhi; Liu, C. Karen; Popovic, Zoran; R. Boulic and D. K. Pai
This paper presents a system for rapid editing of highly dynamic motion capture data. At the heart of this system is an optimization algorithm that can transform the captured motion so that it satisfies high-level user constraints while enforcing that the linear and angular momentum of the motion remain physically plausible. Unlike most previous approaches to motion editing, our algorithm does not require pose specification or model reduction, and the user only need specify high-level changes to the input motion. To preserve the dynamic behavior of the input motion, we introduce a spline-based parameterization that matches the linear and angular momentum patterns of the motion capture data. Because our algorithm enables rapid convergence by presenting a good initial state of the optimization, the user can efficiently generate a large number of realistic motions from a single input motion. The algorithm can then populate the dynamic space of motions by simple interpolation, effectively parameterizing the space of realistic motions. We show how this framework can be used to produce an effective interface for rapid creation of dynamic animations, as well as to drive the dynamic motion of a character in real-time.
Animation of Reactive Gaseous Fluids through Chemical Kinetics
(The Eurographics Association, 2004) Ihm, Insung; Kang, Byungkwon; Cha, Deukhyun; R. Boulic and D. K. Pai
Although chemically reactive fluids may be used effectively to increase the reality of visual effects, little work has been done with the general modeling of chemical reactions in computer animation. In this paper, we attempt to extend an established, physically based fluid simulation technique to handle reactive gaseous fluids. The proposed technique exploits the theory of chemical kinetics to account for a variety of chemical reactions that are frequently found in everyday life. In extending the existing fluid simulation method, we introduce a new set of physically motivated control parameters that allow an animator to control intuitively the behavior of reactive fluids. Our method is straightforward to implement, and is flexible enough to create various interesting visual effects including explosions and catalysis. We demonstrate the effectiveness of our new simulation technique by generating several animation examples with user control.
Enriching a Motion Collection by Transplanting Limbs
(The Eurographics Association, 2004) Ikemoto, Leslie; Forsyth, David A.; R. Boulic and D. K. Pai
This paper describes a method that can significantly increase the size of a collection of motion observations by cutting limbs from one motion sequence and attaching them to another. Not all such transplants are successful, because correlations across the body are a significant feature of human motion. The method uses randomized search based around a set of rules to generate transplants that are (a) likely to be successful and (b) likely to enrich the existing motion collection. The resulting frames are annotated by a classifier to tell whether they look like human motion or not. We evaluate the method by obtaining motion demands from an application, synthesizing motions to meet those demands, and then scoring the synthesized motions. Motions synthesized using transplants are generally somewhat better than those synthesized without using transplants, because transplanting generates many frames quite close to the original frames, so that it is easier for the motion synthesis process to find a good path in the motion graph. Furthermore, we show classifier errors tend to have relatively little impact in practice. Finally, we show that transplanted motion data can be used to synthesize motions of a group coordinated in space and time without producing motions that share frames.

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