SCA: Eurographics/SIGGRAPH Symposium on Computer Animation

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A 2-Stages Locomotion Planner for Digital Actors
(The Eurographics Association, 2003) Pettré, Julien; Laumond, Jean-Paul; Siméon, Thierry; D. Breen and M. Lin
This paper presents a solution to the locomotion planning problem for digital actors. The solution is based both on probabilistic motion planning and on motion capture blending and warping. The paper describes the various components of our solution, from the first path planning to the last animation step. An example illustrates the progression of the animation construction all along the presentation.
A Sketching Interface for Articulated Figure Animation
(The Eurographics Association, 2003) Davis, James; Agrawala, Maneesh; Chuang, Erika; Popovic, Zoran; Salesin, David; D. Breen and M. Lin
We introduce a new interface for rapidly creating 3D articulated figure animation, from 2D sketches of the character in the desired key frame poses. Since the exact 3D animation corresponding to a set of 2D drawings is ambiguous we first reconstruct the possible 3D configurations and then apply a set of constraints and assumptions to present the user with the most likely 3D pose. The user can refine this candidate pose by choosing among alternate poses proposed by the system. This interface is supported by pose reconstruction and optimization methods specifically designed to work with imprecise hand drawn figures. Our system provides a simple, intuitive and fast interface for creating rough animations that leverages our users existing ability to draw. The resulting key framed sequence can be exported to commercial animation packages for interpolation and additional refinement.
Particle-Based Fluid Simulation for Interactive Applications
(The Eurographics Association, 2003) Müller, Matthias; Charypar, David; Gross, Markus; D. Breen and M. Lin
Realistically animated fluids can add substantial realism to interactive applications such as virtual surgery simulators or computer games. In this paper we propose an interactive method based on Smoothed Particle Hydrodynamics (SPH) to simulate fluids with free surfaces. The method is an extension of the SPH-based technique by Desbrun to animate highly deformable bodies. We gear the method towards fluid simulation by deriving the force density fields directly from the Navier-Stokes equation and by adding a term to model surface tension effects. In contrast to Eulerian grid-based approaches, the particle-based approach makes mass conservation equations and convection terms dispensable which reduces the complexity of the simulation. In addition, the particles can directly be used to render the surface of the fluid. We propose methods to track and visualize the free surface using point splatting and marching cubes-based surface reconstruction. Our animation method is fast enough to be used in interactive systems and to allow for user interaction with models consisting of up to 5000 particles.
Finite Volume Methods for the Simulation of Skeletal Muscle
(The Eurographics Association, 2003) Teran, J.; Blemker, S.; Hing, V. Ng Thow; Fedkiw, R.; D. Breen and M. Lin
Since it relies on a geometrical rather than a variational framework, many find the finite volume method (FVM) more intuitive than the finite element method (FEM).We show that the FVM allows one to interpret the stress inside a tetrahedron as a simple 'multidimensional force' pushing on each face. Moreover, this interpretation leads to a heuristic method for calculating the force on each node, which is as simple to implement and comprehend as masses and springs. In the finite volume spirit, we also present a geometric rather than interpolating function definition of strain. We use the FVM and a quasi-incompressible, transversely isotropic, hyperelastic constitutive model to simulate contracting muscle tissue. B-spline solids are used to model fiber directions, and the muscle activation levels are derived from key frame animations.
Interactive Physically Based Solid Dynamics
(The Eurographics Association, 2003) Hauth, M.; Groß, J.; Straßer, W.; D. Breen and M. Lin
The interactive simulation of deformable solids has become a major working area in Computer Graphics. We present a sophisticated material law, better suited for dynamical computations than the standard approaches. As an important example, it is employed to reproduce measured material data from biological soft tissue. We embed it into a state-of-the-art finite element setting employing an adaptive basis. For time integration the use of an explicit stabilized Runge-Kutta method is proposed.
Blowing in the Wind
(The Eurographics Association, 2003) Wei, Xiaoming; Zhao, Ye; Fan, Zhe; Li, Wei; Yoakum-Stover, Suzanne; Kaufman, Arie; D. Breen and M. Lin
We present an approach for simulating the natural dynamics that emerge from the coupling of a flow field to lightweight, mildly deformable objects immersed within it. We model the flow field using a Lattice Boltzmann Model (LBM) extended with a subgrid model and accelerate the computation on commodity graphics hardware to achieve real-time simulations. We demonstrate our approach using soap bubbles and a feather blown by wind fields, yet our approach is general enough to apply to other light-weight objects. The soap bubbles illustrate Fresnel reflection, reveal the dynamics of the unseen flow field in which they travel, and display spherical harmonics in their undulations. The free feather floats and flutters in response to lift and drag forces. Our single bubble simulation allows the user to directly interact with the wind field and thereby influence the dynamics in real time.
Trackable Surfaces
(The Eurographics Association, 2003) Guskov, Igor; Klibanov, Sergey; Bryant, Benjamin; D. Breen and M. Lin
We introduce a novel approach for real-time non-rigid surface acquisition based on tracking quad marked surfaces. The color-identified quad arrangement allows for automatic feature correspondence, tracking initialization, and simplifies 3D reconstruction. We present a prototype implementation of our approach together with several examples of acquired surface motions.
Learning Controls for Blend Shape Based Realistic Facial Animation
(The Eurographics Association, 2003) Joshi, Pushkar; Tien, Wen C.; Desbrun, Mathieu; Pighin, Frédéric; D. Breen and M. Lin
Blend shape animation is the method of choice for keyframe facial animation: a set of blend shapes (key facial expressions) are used to define a linear space of facial expressions. However, in order to capture a significant range of complexity of human expressions, blend shapes need to be segmented into smaller regions where key idiosyncracies of the face being animated are present. Performing this segmentation by hand requires skill and a lot of time. In this paper, we propose an automatic, physically-motivated segmentation that learns the controls and parameters directly from the set of blend shapes. We show the usefulness and efficiency of this technique for both, motion-capture animation and keyframing. We also provide a rendering algorithm to enhance the visual realism of a blend shape model.
Interactive Control of Component-based Morphing
(The Eurographics Association, 2003) Zhao, Yonghong; Ong, Hong-Yang; Tan, Tiow-Seng; Xiao, Yongguan; D. Breen and M. Lin
This paper presents an interactive morphing framework to empower users to conveniently and effectively control the whole morphing process. Although research on mesh morphing has reached a state where most computational problems have been solved in general, the novelty of our framework lies in the integration of global-level and local-level user control through the use of components, and the incorporation of deduction and assistance in user interaction. Given two polygonal meshes, users can choose to specify their requirements either at the global level over components or at the local level within components, whichever is more intuitive. Based on user specifications, the framework proposes several techniques to deduce implied correspondences and add assumed correspondences at both levels. The framework also supports multi-level interpolation control users can operate on a component as a whole or on its individual vertices to specify trajectories. On the whole, in the multi-level componentbased framework, users can choose to specify any number of requirements at each level and the system can complete all other tasks to produce final morphs. Therefore, user control is greatly enhanced and even an amateur can use it to design morphing with ease.
A Scenario Language to orchestrate Virtual World Evolution
(The Eurographics Association, 2003) Devillers, Frédéric; Donikian, Stéphane; D. Breen and M. Lin
Behavioural animation techniques provide autonomous characters with the ability to react credibly in interactive simulations. The direction of these autonomous agents is inherently complex. Typically, simulations evolve according to reactive and cognitive behaviours of autonomous agents. The free flow of actions makes it difficult to precisely control the happening of desired events. In this paper, we propose a scenario language designed to support direction of semi-autonomous characters. This language offers temporal management and character communication tools. It also allows parallelism between scenarios, and a form of competition for the reservation of characters. Seen from the computing angle, this language is generic: in other words, it doesn't make assumptions about the nature of the simulation. Lastly, this language allows a programmer to build scenarios in a variety of different styles ranging from highly directed cinema-like scripts to scenarios which will momentary finely tune free streams of actions.
Flexible Automatic Motion Blending with Registration Curves
(The Eurographics Association, 2003) Kovar, Lucas; Gleicher, Michael; D. Breen and M. Lin
Many motion editing algorithms, including transitioning and multitarget interpolation, can be represented as instances of a more general operation called motion blending. We introduce a novel data structure called a registration curve that expands the class of motions that can be successfully blended without manual input. Registration curves achieve this by automatically determining relationships involving the timing, local coordinate frame, and constraints of the input motions. We show how registration curves improve upon existing automatic blending methods and demonstrate their use in common blending operations.
Simulation of Clothing with Folds and Wrinkles
(The Eurographics Association, 2003) Bridson, R.; Marino, S.; Fedkiw, R.; D. Breen and M. Lin
Clothing is a fundamental part of a character's persona, a key storytelling tool used to convey an intended impression to the audience. Draping, folding, wrinkling, stretching, etc. all convey meaning, and thus each is carefully controlled when filming live actors. When making films with computer simulated cloth, these subtle but important elements must be captured. In this paper we present several methods essential to matching the behavior and look of clothing worn by digital stand-ins to their real world counterparts. Novel contributions include a mixed explicit/ implicit time integration scheme, a physically correct bending model with (potentially) nonzero rest angles for pre-shaping wrinkles, an interface forecasting technique that promotes the development of detail in contact regions, a post-processing method for treating cloth-character collisions that preserves folds and wrinkles, and a dynamic constraint mechanism that helps to control large scale folding. The common goal of all these techniques is to produce a cloth simulation with many folds and wrinkles improving the realism.
Constrained Animation of Flocks
(The Eurographics Association, 2003) Anderson, Matt; McDaniel, Eric; Chenney, Stephen; D. Breen and M. Lin
Group behaviors are widely used in animation, yet it is difficult to impose hard constraints on their behavior. We describe a new technique for the generation of constrained group animations that improves on existing approaches in two ways: the agents in our simulations meet exact constraints at specific times, and our simulations retain the global properties present in unconstrained motion. Users can position constraints on agents' positions at any time in the animation, or constrain the entire group to meet center of mass or shape constraints. Animations are generated in a two stage process. The first step finds an initial set of trajectories that exactly meet the constraints, but which may violate the behavior rules. The second stage samples new animations that maintain the constraints while improving the motion with respect to the underlying behavioral model. We present a range of animations created with our system.
Mapping optical motion capture data to skeletal motion using a physical model
(The Eurographics Association, 2003) Zordan, Victor B.; Horst, Nicholas C. Van Der; D. Breen and M. Lin
Motion capture has become a premiere technique for animation of humanlike characters. To facilitate its use, researchers have focused on the manipulation of data for retargeting, editing, combining, and reusing motion capture libraries. In many of these efforts joint angle plus root trajectories are used as input, although this format requires an inherent mapping from the raw data recorded by many popular motion capture set-ups. In this paper, we propose a novel solution to this mapping problem from 3D marker position data recorded by optical motion capture systems to joint trajectories for a fixed limb-length skeleton using a forward dynamic model. To accomplish the mapping, we attach virtual springs to marker positions located on the appropriate landmarks of a physical simulation and apply resistive torques to the skeleton's joints using a simple controller. For the motion capture samples, joint-angle postures are resolved from the simulation's equilibrium state, based on the internal torques and external forces. Additional constraints, such as foot plants and hand holds, may also be treated as addition forces applied to the system and are a trivial and natural extension to the proposed technique. We present results for our approach as applied to several motion-captured behaviors.
Discrete Shells
(The Eurographics Association, 2003) Grinspun, Eitan; Hirani, Anil N.; Desbrun, Mathieu; Schröder, Peter; D. Breen and M. Lin
In this paper we introduce a discrete shell model describing the behavior of thin flexible structures, such as hats, leaves, and aluminum cans, which are characterized by a curved undeformed configuration. Previously such models required complex continuum mechanics formulations and correspondingly complex algorithms. We show that a simple shell model can be derived geometrically for triangle meshes and implemented quickly by modifying a standard cloth simulator. Our technique convincingly simulates a variety of curved objects with materials ranging from paper to metal, as we demonstrate with several examples including a comparison of a real and simulated falling hat.
Adaptive Wisp Tree - a multiresolution control structure for simulating dynamic clustering in hair motion
(The Eurographics Association, 2003) Bertails, F.; Kim, T-Y.; Cani, M-P.; Neumann, U.; D. Breen and M. Lin
Realistic animation of long human hair is difficult due to the number of hair strands and to the complexity of their interactions. Existing methods remain limited to smooth, uniform, and relatively simple hair motion. We present a powerful adaptive approach to modeling dynamic clustering behavior that characterizes complex long-hair motion. The Adaptive Wisp Tree (AWT) is a novel control structure that approximates the large-scale coherent motion of hair clusters as well as small-scaled variation of individual hair strands. The AWT also aids computation efficiency by identifying regions where visible hair motions are likely to occur. The AWT is coupled with a multiresolution geometry used to define the initial hair model. This combined system produces stable animations that exhibit the natural effects of clustering and mutual hair interaction. Our results show that the method is applicable to a wide variety of hair styles.
Advected Textures
(The Eurographics Association, 2003) Neyret, Fabrice; D. Breen and M. Lin
Game and special effects artists like to rely on textures (image or procedural) to specify the details of surface aspect. In this paper, we address the problem of applying textures to animated fluids. The purpose is to allow artists to increase the details of flowing water, foam, lava, mud, flames, cloud layers, etc. Our first contribution is a new algorithm for advecting textures, which compromises between two contradictory requirements: continuity in space and time and preservation of statistical texture properties. It consist of combining layers of advected (periodically regenerated) parameterizations according to a criterion based on the local accumulated deformation. To correctly achieve this combination, we introduce a way of blending procedural textures while avoiding classical interpolation artifacts. Lastly, we propose a scheme to add and control small scale texture animation amplifying the low resolution simulation. Our results illustrate how these three contributions solve the major visual flaws of textured fluids.
Construction and Animation of Anatomically Based Human Hand Models
(The Eurographics Association, 2003) Albrecht, Irene; Haber, Jörg; Seidel, Hans-Peter; D. Breen and M. Lin
The human hand is a masterpiece of mechanical complexity, able to perform fine motor manipulations and powerful work alike. Designing an animatable human hand model that features the abilities of the archetype created by Nature requires a great deal of anatomical detail to be modeled. In this paper, we present a human hand model with underlying anatomical structure. Animation of the hand model is controlled by muscle contraction values. We employ a physically based hybrid muscle model to convert these contraction values into movement of skin and bones. Pseudo muscles directly control the rotation of bones based on anatomical data and mechanical laws, while geometric muscles deform the skin tissue using a mass-spring system. Thus, resulting animations automatically exhibit anatomically and physically correct finger movements and skin deformations. In addition, we present a deformation technique to create individual hand models from photographs. A radial basis warping function is set up from the correspondence of feature points and applied to the complete structure of the reference hand model, making the deformed hand model instantly animatable.
Geometry Videos: A New Representation for 3D Animations
(The Eurographics Association, 2003) Briceño, Hector M.; Sander, Pedro V.; McMillan, Leonard; Gortler, Steven; Hoppe, Hugues; D. Breen and M. Lin
We present the 'Geometry Video', a new data structure to encode animated meshes. Being able to encode animated meshes in a generic source-independent format allows people to share experiences. Changing the viewpoint allows more interaction than the fixed view supported by 2D video. Geometry videos are based on the 'Geometry Image' mesh representation introduced by Gu et al. 4. Our novel data structure provides a way to treat an animated mesh as a video sequence (i.e., 3D image) and is well suited for network streaming. This representation also offers the possibility of applying and adapting existing mature video processing and compression techniques (such as MPEG encoding) to animated meshes. This paper describes an algorithm to generate geometry videos from animated meshes. The main insight of this paper, is that Geometry Videos re-sample and re-organize the geometry information, in such a way, that it becomes very compressible. They provide a unified and intuitive method for level-of-detail control, both in terms of mesh resolution (by scaling the two spatial dimensions) and of frame rate (by scaling the temporal dimension). Geometry Videos have a very uniform and regular structure. Their resource and computational requirements can be calculated exactly, hence making them also suitable for applications requiring level of service guarantees.
Synthesizing Animatable Body Models with Parameterized Shape Modifications
(The Eurographics Association, 2003) Seo, Hyewon; Cordier, Frederic; Magnenat-Thalmann, Nadia; D. Breen and M. Lin
Based on an existing modeller that can generate realistic and controllable whole-body models, we introduce our modifier synthesizer for obtaining higher level of manipulations of body models by using parameters such as fat percentage and hip-to-waist ratio. Users are assisted in automatically modifying an existing model by controlling the parameters provided. On any synthesized model, the underlying bone and skin structure is properly adjusted, so that the model remains completely animatable using the underlying skeleton. Based on statistical analysis of data models, we demonstrate the use of body attributes as parameters in controlling the shape modification of the body models while maintaining the distinctiveness of the individual as much as possible.

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