SCA 04: Eurographics/SIGGRAPH Symposium on Computer Animation
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Item Animal gaits from video(The Eurographics Association, 2004) Favreau, Laurent; Reveret, Lionel; Depraz, Christine; Cani, Marie-Paule; R. Boulic and D. K. PaiWe present a method for animating 3D models of animals from existing live video sequences such as wild life documentaries. Videos are first segmented into binary images on which Principal Component Analysis (PCA) is applied. The time-varying coordinates of the images in the PCA space are then used to generate 3D animation. This is done through interpolation with Radial Basis Functions (RBF) of 3D pose examples associated with a small set of key-images extracted from the video. In addition to this processing pipeline, our main contributions are: an automatic method for selecting the best set of key-images for which the designer will need to provide 3D pose examples. This method saves user time and effort since there is no more need for manual selection within the video and then trials and errors in the choice of key-images and 3D pose examples. As another contribution, we propose a simple algorithm based on PCA images to resolve 3D pose prediction ambiguities. These ambiguities are inherent to many animal gaits when only monocular view is available. The method is first evaluated on sequences of synthetic images of animal gaits, for which full 3D data is available. We achieve a good quality reconstruction of the input 3D motion from a single video sequence of its 2D rendering. We then illustrate the method by reconstructing animal gaits from live video of wild life documentaries. Key words: Animation from Motion/Video Data, Interpolation Keyframing, Intuitive Interfaces for Animation.Item Animation and Control of Breaking Waves(The Eurographics Association, 2004) Mihalef, Viorel; Metaxas, Dimitris; Sussman, Mark; R. Boulic and D. K. PaiControlling fluids is still an open and challenging problem in fluid animation. In this paper we develop a novel fluid animation control approach and we present its application to controlling breaking waves. In our Slice Method framework an animator defines the shape of a breaking wave at a desired moment in its evolution based on a library of breaking waves. Our system computes then the subsequent dynamics with the aid of a 3D Navier-Stokes solver. The wave dynamics previous to the moment the animator exerts control can also be generated based on the wave library. The animator is thus enabled to obtain a full animation of a breaking wave while controlling the shape and the timing of the breaking. An additional advantage of the method is that it provides a significantly faster method for obtaining the full 3D breaking wave evolution compared to starting the simulation at an early stage and using solely the 3D Navier-Stokes equations. We present a series of 2D and 3D breaking wave animations to demonstrate the power of the method.Item Animation of Reactive Gaseous Fluids through Chemical Kinetics(The Eurographics Association, 2004) Ihm, Insung; Kang, Byungkwon; Cha, Deukhyun; R. Boulic and D. K. PaiAlthough 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.Item Autonomous Behaviors for Interactive Vehicle Animations(The Eurographics Association, 2004) Go, Jared; Vu, Thuc; Kuffner, James; R. Boulic and D. K. PaiWe present a method for synthesizing animations of autonomous space, water, and land-based vehicles in games or other interactive simulations. Controlling the motion of such vehicles to achieve a desirable behavior is difficult due to the constraints imposed by the system dynamics. We combine real-time path planning and a simplified physics model to automatically compute control actions to drive a vehicle from an input state to desirable output states based on a behavior cost function. Both offline trajectory preprocessing and online search are used to build an animation framework suitable for interactive vehicle simulations. We demonstrate synthesized animations of spacecraft performing a variety of autonomous behaviors, including Seek, Pursue, Avoid, Avoid Collision, and Flee. We also explore several enhancements to the basic planning algorithm and examine the resulting tradeoffs in runtime performance and quality of the generated motion.Item 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. PaiWe 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.Item 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. PaiAnimation 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.Item Cartoon Textures(The Eurographics Association, 2004) Juan, Christina de; Bodenheimer, Bobby; R. Boulic and D. K. PaiIn 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.Item Collision Between Deformable Objects Using Fast-Marching on Tetrahedral Models(The Eurographics Association, 2004) Marchal, Damien; Aubert, Fabrice; Chaillou, Christophe; R. Boulic and D. K. PaiThis paper presents an approach to handling collision between deformable objects using tetrahedral decomposition. The tetrahedral volumetric model is often used to simulate deformable objects that handle cuts and splits. Interaction between such objects in a complex environment is still an open problem in interactive simulation. This paper is mainly focused on obtaining a fast computation of a reliable penalty response. The method consists in using an approximated distance map to compute a penalty based response. We propose to compute the distances to the boundary using a modified 'Closest Point' algorithm derived from Fast Marching. The presented algorithm, inspired by the [FL01], has the advantage of computing rapidly the 'Closest Point' in the volumetric tetrahedral mesh without any use of an additional computation grid. From the resulting distance map a response is computed using a new "segment-in-object" response that offers more reliable results than the "point-in-object" generally used in previous works. Using this collision model, simulation at interactive rate can be considered in an environment composed of objects that can be deformed and cut.Item Computing the Duration of Motion Transitions: An Empirical Approach(The Eurographics Association, 2004) Wang, Jing; Bodenheimer, Bobby; R. Boulic and D. K. PaiThis paper develops methods for determining a visually appealing length for a motion transition, i.e., a segue between two sequences of character animation. Motion transitions are an important component in generating compelling animation streams in virtual environments and computer games. For reasons of efficiency and speed, linear interpolation is often used as the transition method, where the motion is blended between specified start and end frames. The blend length of a transition using this technique is critical to the visual appearance of the motion. Two methods for determining an optimal blend length for such transitions are presented. These methods are suited to different types of motion. They are empirically evaluated through user studies. For the motions tested, we find (1) that visually pleasing transitions can be generated using our optimal blend lengths without further tuning of the blending parameters; and (2), that users prefer these methods over a generic fixed-length blend.Item 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. PaiRecent 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.Item Decomposing Cloth(The Eurographics Association, 2004) Boxerman, Eddy; Ascher, Uri; R. Boulic and D. K. PaiImplicit 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.Item Directable Photorealistic Liquids(The Eurographics Association, 2004) Rasmussen, Nick; Enright, Doug; Nguyen, Duc; Marino, Sebastian; Sumner, Nigel; Geiger, Willi; Hoon, Samir; Fedkiw, Ron; R. Boulic and D. K. PaiWe present a method for the directable animation of photorealistic liquids using the particle level set method to obtain smooth, visually pleasing complex liquid surfaces. We also provide for a degree of control common to particle-only based simulation techniques. A variety of directable liquid primitive variables, including the isosurface value, velocity, and viscosity, can be set throughout the liquid. Interaction of thin liquid sheets with immersed rigid bodies is improved with newly proposed object-liquid boundary conditions. Efficient calculation of largescale animations is supported via a multiple grid pipelined flow method and a novel moving grid windowing technique. In addition, we propose a few significant algorithmic enhancements to the basic liquid simulation algorithm to provide for the smooth merging of liquid drops, allow for the efficient calculation of high viscosity liquids, and ensure the proper treatment of isolated free liquid pockets surrounded by controlled liquid regions.Item Enriching a Motion Collection by Transplanting Limbs(The Eurographics Association, 2004) Ikemoto, Leslie; Forsyth, David A.; R. Boulic and D. K. PaiThis 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.Item Evaluating Motion Graphs for Character Navigation(The Eurographics Association, 2004) Reitsma, Paul S. A.; Pollard, Nancy S.; R. Boulic and D. K. PaiRealistic and directable humanlike characters are an ongoing goal in animation. Motion graph data structures hold much promise for achieving this goal. However, the quality of the results obtained from a motion graph may not be easy to predict from the input motion segments. This paper introduces the idea of assessing a data structure such as a motion graph for its utility in a particular application. We focus on navigation tasks and define metrics for evaluating expected path quality and coverage for a given environment. One key to evaluating a motion graph for navigation tasks is to first embed it into the environment in a way that captures all possible paths that might result from 'playing back' the motion graph within that environment. This paper describes an algorithm for accomplishing this embedding that preserves the flexibility of the original motion graph. We use the metrics defined in this paper to compare motion datasets and to highlight areas where these datasets could be improved.Item Example-Based Control of Human Motion(The Eurographics Association, 2004) Hsu, Eugene; Gentry, Sommer; Popovic, Jovan; R. Boulic and D. K. PaiIn human motion control applications, the mapping between a control specification and an appropriate target motion often defies an explicit encoding.We present a method that allows such a mapping to be defined by example, given that the control specification is recorded motion. Our method begins by building a database of semantically meaningful instances of the mapping, each of which is represented by synchronized segments of control and target motion. A dynamic programming algorithm can then be used to interpret an input control specification in terms of mapping instances. This interpretation induces a sequence of target segments from the database, which is concatenated to create the appropriate target motion. We evaluate our method on two examples of indirect control. In the first, we synthesize a walking human character that follows a sampled trajectory. In the second, we generate a synthetic partner for a dancer whose motion is acquired through motion capture.Item 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. PaiIn 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.Item Finding Paths for Coherent Groups using Clearance(The Eurographics Association, 2004) Kamphuis, Arno; Overmars, Mark H.; R. Boulic and D. K. PaiVirtual environment are often populated with moving units and the paths for these units should be planned. When multiple units need to exhibit coherent behavior in a cluttered environment, current techniques often fail, i.e. the resulting paths for the units in the group lack the coherence required. In this paper, we propose a novel approach to motion planning for coherent groups of units. The method presented uses a path for a single unit, called the backbone path, which can be generated by any motion planner. This backbone path is extended to a corridor using the clearance along the path. The units can move freely inside this corridor. By limiting the width of this corridor, and the extent along the corridor where the units can move to, the approach guarantees coherence of the group. Experiments show that the generated paths exhibit group coherence as required, like passing on the same side of obstacles and waiting for fellow group mates to catch up. Performance measurements show that the approach is capable of generating the paths in real-time. In our implementation, the method requires just a few percent of the processor time for groups consisting of up to 100 units.Item Flow Tiles(The Eurographics Association, 2004) Chenney, Stephen; R. Boulic and D. K. PaiWe present flow tiles, a novel technique for representing and designing velocity fields. Unlike existing procedural flow generators, tiling offers a natural user interface for field design. Tilings can be constructed to meet a wide variety of external and internal boundary conditions, making them suitable for inclusion in larger environments. Tiles offer memory savings through the re-use of prototypical elements. Each flow tile contains a small field and many tiles can be combined to produce large flows. The corners and edges of tiles are constructed to ensure continuity across boundaries between tiles. In addition, all our tiles and the resulting tiling are divergence-free and hence suitable for representing a range of effects. We discuss issues that arise in designing flow tiles, algorithms for creating tilings, and three applications: a crowd on city streets, a river flowing between banks, and swirling fog. The first two applications use stationary fields, while the latter demonstrates a dynamic field.Item A Hybrid Algorithm for Modeling Ice Formation(The Eurographics Association, 2004) Kim, Theodore; Henson, Michael; Lin, Ming C.; R. Boulic and D. K. PaiWe present a novel algorithm that simulates ice formation. Motivated by the physical process of ice growth, we develop a novel hybrid algorithm by synthesizing three techniques: diffusion limited aggregation, phase field methods, and stable fluid solvers. Each technique maps to one of the three stages of solidification. The visual realism of the resulting algorithm appears to surpass that of each technique alone, particularly in animations of freezing. In addition, we present a faster, simplified phase field method, as well as a unified parameterization that enables artistic manipulation of the simulation. We illustrate the results on arbitrary 3D surfaces.Item Image-Based Tomographic Reconstruction of Flames(The Eurographics Association, 2004) Ihrke, Ivo; Magnor, Marcus; R. Boulic and D. K. PaiNon-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.