SCA 05: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/431

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Motion Modeling for On-Line Locomotion Synthesis
(The Eurographics Association, 2005) Kwon, Taesoo; Shiny, Sung Yong; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
In this paper, we propose an example-based approach to on-line locomotion synthesis. Our approach consists of two parts: motion analysis and motion synthesis. In the motion analysis part, an unlabeled motion sequence is first decomposed into motion segments, exploiting the behavior of the COM (center of mass) trajectory of the performer. Those motion segments are subsequently classified into groups of motion segments such that the same group of motion segments share an identical footstep pattern. Finally, we construct a hierarchical motion transition graph by representing these groups and their connectivity to other groups as nodes and edges, respectively. The coarse level of this graph models locomotive motions and their transitions, and the fine level mainly captures the cyclic nature of locomotive motions. In the motion synthesis part, given a stream of motion specifications in an on-line manner, the motion transition graph is traversed while blending the motion segments to synthesize a motion at a node, one by one, guided by the motion specifications. Our main contributions are the motion labeling scheme and a new motion model, embodied by the hierarchical motion transition graph, which together enable not only artifact-free motion blending but also seamless motion transition.
Spatial Keyframing for Performance-driven Animation
(The Eurographics Association, 2005) Igarashi, T.; Moscovich, T.; Hughes, J. F.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
This paper introduces spatial keyframing, a technique for performance-driven character animation. In traditional temporal keyframing, key poses are defined at specific points in time: i.e., we define a map from a set of key times to the configuration space of the character and then extend this map to the entire timeline by interpolation. By contrast, in spatial keyframing key poses are defined at specific key positions in a 3D space where the character lives; the mapping from the 3D space to the configuration space is again defined by interpolation. The user controls a character by adjusting the position of a control cursor in the 3D space; the pose of the character is given as a blend of nearby key poses. The user thus can make expressive motion in real time and the resulting motion can be recorded and interpreted as an animation sequence. Although similar ideas are present in previous systems, our system is unique in that the designer can quickly design a new set of keyframes from scratch, and make an animation without motion capture data or special input devices. Our technique is especially useful for imaginary characters other than human figures because we do not rely on motion-capture data. We also introduce several applications of the basic idea and give examples showing the expressiveness of the approach.
Video-Based Character Animation
(The Eurographics Association, 2005) Starck, J.; Miller, G.; Hilton, A.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
In this paper we introduce a video-based representation for free viewpoint visualization and motion control of 3D character models created from multiple view video sequences of real people. Previous approaches to videobased rendering provide no control of scene dynamics to manipulate, retarget, and create new 3D content from captured scenes. Here we contribute a new approach, combining image based reconstruction and video-based animation to allow controlled animation of people from captured multiple view video sequences. We represent a character as a motion graph of free viewpoint video motions for animation control. We introduce the use of geometry videos to represent reconstructed scenes of people for free viewpoint video rendering. We describe a novel spherical matching algorithm to derive global surface to surface correspondence in spherical geometry images for motion blending and the construction of seamless transitions between motion sequences. Finally, we demonstrate interactive video-based character animation with real-time rendering and free viewpoint visualization. This approach synthesizes highly realistic character animations with dynamic surface shape and appearance captured from multiple view video of people.
Autonomous Pedestrians
(The Eurographics Association, 2005) Shao, Wei; Terzopoulos, Demetri; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
We address the difficult open problem of emulating the rich complexity of real pedestrians in urban environments. Our artificial life approach integrates motor, perceptual, behavioral, and cognitive components within a model of pedestrians as individuals. Our comprehensive model features innovations in these components, as well as in their combination, yielding results of unprecedented fidelity and complexity for fully autonomous multi-human simulation in a large urban environment. We represent the environment using hierarchical data structures, which efficiently support the perceptual queries of the autonomous pedestrians that drive their behavioral responses and sustain their ability to plan their actions on local and global scales.
An Art-Directed Wrinkle System for CG Character Clothing
(The Eurographics Association, 2005) Cutler, Lawrence D.; Gershbein, Reid; Wang, Xiaohuan Corina; Curtis, Cassidy; Maigret, Erwan; Prasso, Luca; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
We present a kinematic system for creating art-directed wrinkles on costumes for CG characters. This system employs a curve-based method for creating wrinkles on reference poses, which are incorporated into a weighted matching algorithm that generates wrinkle deformations on an animated character. The wrinkle creation tool is intuitive to use and accommodates art direction. The user can easily transfer wrinkle patterns to different characters, costumes, and body types. The algorithm for evaluating wrinkles measures the local stress of a surface and creates weights that are used to interpolate between the reference wrinkle patterns during movement. This algorithm is robust and efficient, and fits well into a large-scale feature-film production environment.
Progressive Multiresolution Meshes for Deforming Surfaces
(The Eurographics Association, 2005) Kircher, Scott; Garland, Michael; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Time-varying surfaces are ubiquitous in movies, games, and scientific applications. For reasons of efficiency and simplicity of formulation, these surfaces are often generated and represented as dense polygonal meshes with static connectivity. As a result, such deforming meshes often have a tremendous surplus of detail, with many more vertices and polygons than necessary for any given frame. An extensive amount of work has addressed the issue of simplifying a static mesh; however, these methods are inadequate for time-varying surfaces when there is a high degree of non-rigid deformation. We thus propose a new multiresolution representation for deforming surfaces that, together with our dynamic improvement scheme, provides high quality surface approximations at any levelof- detail, for all frames of an animation. Our algorithm also gives rise to a new progressive representation for time-varying multiresolution hierarchies, consisting of a base hierarchy for the initial frame and a sequence of update operations for subsequent frames. We demonstrate that this provides a very effective means of extracting static or view-dependent approximations for a deforming mesh over all frames of an animation.
Modeling and Animating Gases with Simulation Features
(The Eurographics Association, 2005) Schpok, Joshua; Dwyer, William; Ebert, David S.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
In modeling natural phenomena, artists often compromise the benefits of direct control for the visual realism of physics-based simulation. For gases, Eulerian simulations traditionally provide realistic results, but a poor representation for artistically shaping the media. In our system, users work with a more intuitive set of continuously extracted features whose manipulation feeds back into the original simulation. This novel approach overcomes common control issues by providing modeling tools to manipulate high-level behavior in Eulerian simulations. We employ techniques in feature extraction, real-time gas simulation, and volume rendering to build an interactive system to sculpt three-dimensional flows.
Helping Hand: An Anatomically Accurate Inverse Dynamics Solution For Unconstrained Hand Motion
(The Eurographics Association, 2005) Tsang, Winnie; Singh, Karan; Fiume, Eugene; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
We present a realistic skeletal musculo-tendon model of the human hand and forearm. The model permits direct forward dynamics simulation, which accurately predicts hand and finger position given a set of muscle activations. We also present a solution to the inverse problem of determining an optimal set of muscle activations to achieve a given pose or motion; muscle fatigue, injury or atrophy can also be specified, yielding different control solutions that favour healthy muscle. As there can be many (or no) solutions to this inverse problem, we demonstrate how the space of possible solutions can be filtered to an optimal representative. Of particular note is the ability of our model to take a wide array of joint interdependence into account for both forward and inverse problems. Given kinematic postures, the model can be used to validate, predict or fill in missing motion and improve coarsely specified motion with anatomic fidelity. Lastly, we address the visualization and understanding of the dynamically changing and spatially compact musculature using various interaction techniques.
Outside-In Anatomy Based Character Rigging
(The Eurographics Association, 2005) Pratscher, Michael; Coleman, Patrick; Laszlo, Joe; Singh, Karan; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
For believable character animation, skin deformation should communicate important deformation effects due to underlying muscle movement. Anatomical models that capture these effects are typically constructed from the inside out. Internal tissue is modeled by hand and a surface skin is attached to, or generated from, the internal structure. This paper presents an outside in approach to anatomical modeling, in which we generate musculature from a predefined structure, which we conform to an artist sculpted skin surface. Motivated by interactive applications, we attach the musculature to an existing control skeleton and apply a novel geometric deformation model to deform the skin surface to capture important muscle motion effects. Musculoskeletal structure can be stored as a template and applied to new character models. We illustrate the methodology, as integrated into a commercial character animation system, with examples driven by both keyframe animation and recorded motion data.
Fast and accurate goal-directed motion synthesis for crowds
(The Eurographics Association, 2005) Sung, Mankyu; Kovar, Lucas; Gleicher, Michael; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
This paper presents a highly efficient motion synthesis algorithm that is well suited for animating large numbers of characters. Given constraints that require characters to be in specific poses, positions, and orientations in specified time intervals, our algorithm synthesizes motions that exactly satisfy these constraints while avoiding inter-character collisions and collisions with the environment. We represent the space of possible actions with a motion graph and use search algorithms to generate motion. To provide a good initial guess for the search, we employ a fast path planner based on probabilistic roadmaps to navigate characters through complex environments. Also, unlike existing algorithms, our search process allows for smooth, continual adjustments to position, orientation, and timing. This allows us both to satisfy constraints precisely and to generate motion much faster than would otherwise be possible.
Morphable model of quadrupeds skeletons for animating 3D animals
(The Eurographics Association, 2005) Reveret, Lionel; Favreau, Laurent; Depraz, Christine; Cani, Marie-Paule; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Skeletons are at the core of 3D character animation. The goal of this work is to design a morphable model of 3D skeleton for four footed animals, controlled by a few intuitive parameters. This model enables the automatic generation of an animation skeleton, ready for character rigging, from a few simple measurements performed on the mesh of the quadruped to animate. Quadruped animals - usually mammals - share similar anatomical structures, but only a skilled animator can easily translate them into a simple skeleton convenient for animation. Our approach for constructing the morphable model thus builds on the statistical learning of reference skeletons designed by an expert animator. This raises the problems of coping with data that includes both translations and rotations, and of avoiding the accumulation of errors due to its hierarchical structure. Our solution relies on a quaternion representation for rotations and the use of a global frame for expressing the skeleton data. We then explore the dimensionality of the space of quadruped skeletons, which yields the extraction of three intuitive parameters for the morphable model, easily measurable on any 3D mesh of a quadruped. We evaluate our method by comparing the predicted skeletons with user-defined ones on one animal example that was not included into the learning database. We finally demonstrate the usability of the morphable skeleton model for animation.
Robust Quasistatic Finite Elements and Flesh Simulation
(The Eurographics Association, 2005) Teran, Joseph; Sifakis, Eftychios; Irving, Geoffrey; Fedkiw, Ronald; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Quasistatic and implicit time integration schemes are typically employed to alleviate the stringent time step restrictions imposed by their explicit counterparts. However, both quasistatic and implicit methods are subject to hidden time step restrictions associated with both the prevention of element inversion and the effects of discontinuous contact forces. Furthermore, although fast iterative solvers typically require a symmetric positive definite global stiffness matrix, a number of factors can lead to indefiniteness such as large jumps in boundary conditions, heavy compression, etc. We present a novel quasistatic algorithm that alleviates geometric and material indefiniteness allowing one to use fast conjugate gradient solvers during Newton-Raphson iteration. Additionally, we robustly compute smooth elastic forces in the presence of highly deformed, inverted elements alleviating artificial time step restrictions typically required to prevent such states. Finally, we propose a novel strategy for treating both collision and self-collision in this context.
Behavior Planning for Character Animation
(The Eurographics Association, 2005) Lau, Manfred; Kuffner, James J.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
This paper explores a behavior planning approach to automatically generate realistic motions for animated characters. Motion clips are abstracted as high-level behaviors and associated with a behavior finite-state machine (FSM) that defines the movement capabilities of a virtual character. During runtime, motion is generated automatically by a planning algorithm that performs a global search of the FSM and computes a sequence of behaviors for the character to reach a user-designated goal position. Our technique can generate interesting animations using a relatively small amount of data, making it attractive for resource-limited game platforms. It also scales efficiently to large motion databases, because the search performance is primarily dependent on the complexity of the behavior FSM rather than on the amount of data. Heuristic cost functions that the planner uses to evaluate candidate motions provide a flexible framework from which an animator can control character preferences for certain types of behavior. We show results of synthesized animations involving up to one hundred human and animal characters planning simultaneously in both static and dynamic environments.
Vortex Fluid for Gaseous Phenomena
(The Eurographics Association, 2005) Park, Sang Il; Kim, Myoung Jun; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
In this paper, we present a method for visual simulation of gaseous phenomena based on the vortex method. This method uses a localized vortex flow as a basic building block and combines those blocks to describe a whole flow field. As a result, we achieve computational efficiency by concentrating only on a localized vorticity region while generating dynamic swirling fluid flows. Based on the Lagrangian framework, we resolve various boundary conditions. By exploiting the panel method, we satisfy the no-through boundary condition in a Lagrangian way. A simple and effective way of handling the no-slip boundary condition is also presented. In treating the no-slip boundary condition, we allow a user to control the roughness of the boundary surface, which further improves visual realism.
Physically Based Rigging for Deformable Characters
(The Eurographics Association, 2005) Capell, Steve; Burkhart, Matthew; Curless, Brian; Duchamp, Tom; Popovic, Zoran; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
In this paper we introduce a framework for instrumenting ( rigging ) characters that are modeled as dynamic elastic bodies, so that their shapes can be controlled by an animator. Because the shape of such a character is determined by physical dynamics, the rigging system cannot simply dictate the shape as in traditional animation. For this reason, we introduce forces as the building blocks of rigging. Rigging forces guide the shape of the character, but are combined with other forces during simulation. Forces have other desirable features: they can be combined easily and simulated at any resolution, and since they are not tightly coupled with the surface geometry, they can be more easily transferred from one model to another. Our framework includes a new pose-dependent linearization scheme for elastic dynamics, which ensures a correspondence between forces and deformations, and at the same time produces plausible results at interactive speeds. We also introduce a novel method of handling collisions around creases.
Particle-based Viscoelastic Fluid Simulation
(The Eurographics Association, 2005) Clavet, Simon; Beaudoin, Philippe; Poulin, Pierre; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
We present a new particle-based method for viscoelastic fluid simulation.We achieve realistic small-scale behavior of substances such as paint or mud as they splash on moving objects. Incompressibility and particle anti-clustering are enforced with a double density relaxation procedure which updates particle positions according to two opposing pressure terms. From this process surface tension effects emerge, enabling drop and filament formation. Elastic and non-linear plastic effects are obtained by adding springs with varying rest length between particles. We also extend the technique to handle interaction between fluid and dynamic objects. Various simulation scenarios are presented including rain drops, fountains, clay manipulation, and floating objects. The method is robust and stable, and can animate splashing behavior at interactive framerates.
Simulation of Smoke based on Vortex Filament Primitives
(The Eurographics Association, 2005) Angelidis, Alexis; Neyret, Fabrice; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
We describe a method that permits the high performance simulation of fluid phenomena such as smoke, with highlevel control for the artist. Our key primitives are vortex filament and vortex ring: vorticity defines a flow as well as velocity does, and for numerous interesting flows such as smoke or explosions this information is very compact and tightly linked to the visual features of the fluid. We treat these vortices as 1D Lagrangian primitives (i.e. connected particles), which permit unbounded fluids and very accurate positioning of the features. The simulation of passive density particles for rendering is totally independent of the fluid animation itself. Thus, the animation can be efficiently simulated, edited and even stored, while the fluid resolution used for rendering can be arbitrarily high. We aim at plausible fluids rather than physical accuracy. For efficiency and stability, we introduce a new formalization of the Biot-Savart law and a modified Biot-Savart kernel. Our model also introduces a hierarchical filament structure for animation LOD, turbulent noise, and an original scheme for density particles.
Pushing People Around
(The Eurographics Association, 2005) Arikan, Okan; Forsyth, David A.; O Brien, James F.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
We present an algorithm for animating characters being pushed by an external source such as a user or a game environment. We start with a collection of motions of a real person responding to being pushed. When a character is pushed, we synthesize new motions by picking a motion from the recorded collection and modifying it so that the character responds to the push from the desired direction and location on its body. Determining the deformation parameters that realistically modify a recorded response motion is difficult. Choosing the response motion that will look best when modified is also non-trivial, especially in real-time. To estimate the envelope of deformation parameters that yield visually plausible modifications of a given motion, and to find the best motion to modify, we introduce an oracle. The oracle is trained using a set of synthesized response motions that are identified by a user as good and bad. Once trained, the oracle can, in real-time, estimate the visual quality of all motions in the collection and required deformation parameters to serve a desired push. Our method performs better than a baseline algorithm of picking the closest response motion in configuration space, because our method can find visually plausible transitions that do not necessarily correspond to similar motions in terms of configuration. Our method can also start with a limited set of recorded motions and modify them so that they can be used to serve different pushes on the upper body.
Group Motion Graphs
(The Eurographics Association, 2005) Lai, Yu-Chi; Chenney, Stephen; Fan, Shao Hua; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
We introduce Group Motion Graphs, a data-driven animation technique for groups of discrete agents, such as flocks, herds, or small crowds. Group Motion Graphs are conceptually similar to motion graphs constructed from motion-capture data, but have some important differences: we assume simulated motion; transition nodes are found by clustering group configurations from the input simulations; and clips to join transitions are explicitly constructed via constrained simulation. Graphs built this way offer known bounds on the trajectories that they generate, making it easier to search for particular output motions. The resulting animations show realistic motion at significantly reduced computational cost compared to simulation, and improved control.
Analyzing the Physical Correctness of Interpolated Human Motion
(The Eurographics Association, 2005) Safonova, Alla; Hodgins, Jessica K.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Two human motions can be linearly interpolated to produce a new motion, giving the animator control over the length of a jump, the speed of walking, or the height of a kick. Over the past ten years, this simple technique has been shown to produce surprisingly natural looking results. In this paper, we analyze the motions produced by this technique for physical correctness and suggest small modifications to the standard interpolation technique that in some circumstances will produce significantly more natural looking motion.

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