SCA 04: Eurographics/SIGGRAPH Symposium on Computer Animation

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

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Evaluating Motion Graphs for Character Navigation
(The Eurographics Association, 2004) Reitsma, Paul S. A.; Pollard, Nancy S.; R. Boulic and D. K. Pai
Realistic 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.
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.
Flow Tiles
(The Eurographics Association, 2004) Chenney, Stephen; R. Boulic and D. K. Pai
We 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.
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.
Example-Based Control of Human Motion
(The Eurographics Association, 2004) Hsu, Eugene; Gentry, Sommer; Popovic, Jovan; R. Boulic and D. K. Pai
In 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.
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.
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.
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.
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.
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.
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.
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.
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.
Interactive Motion Deformation with Prioritized Constraints
(The Eurographics Association, 2004) Callennec, Benoît Le; Boulic, Ronan; R. Boulic and D. K. Pai
In this paper, we present an interactive motion deformation method to modify animations so that they satisfy a set of prioritized constraints. Our approach successfully handles the problem of retargetting, adjusting a motion, as well as adding significant changes to preexisting animations. We introduce the concept of prioritized constraints to avoid tweaking issues for competing constraints. Each frame is individually and smoothly adjusted to enforce a set of prioritized constraints. The iterative construction of the solution channels the convergence through intermediate solutions, enforcing the highest prioritized constraints first. In addition, we propose a new, simple formulation to control the position of the center of mass so that the resulting motions are physically plausible. Finally, we demonstrate that our method can address a wide range of motion editing problems.
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.
Autonomous Behaviors for Interactive Vehicle Animations
(The Eurographics Association, 2004) Go, Jared; Vu, Thuc; Kuffner, James; R. Boulic and D. K. Pai
We 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.
Quasi-Rigid Objects in Contact
(The Eurographics Association, 2004) Pauly, Mark; Pai, Dinesh K.; Guibas, Leonidas J.; R. Boulic and D. K. Pai
We investigate techniques for modeling contact between quasi-rigid objects ? solids that undergo modest deformation in the vicinity of a contact, while the overall object still preserves its basic shape. The quasi-rigid model combines the benefits of rigid body models for dynamic simulation and the benefits of deformable models for resolving contacts and producing visible deformations. We argue that point cloud surface representations are advantageous for modeling rapidly varying, wide area contacts. Using multi-level computations based on point primitives, we obtain a scalable system that efficiently handles complex contact configurations, even for high-resolution models obtained from laser range scans. Our method computes consistent and realistic contact surfacesand traction distributions, which are useful in many applications.
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.
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.
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. Pai
This 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.

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