SCA 05: Eurographics/SIGGRAPH Symposium on Computer Animation

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

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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.
An Efficient Search Algorithm for Motion Data Using Weighted PCA
(The Eurographics Association, 2005) Forbes, K.; Fiume, E.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Good motion data is costly to create. Such an expense often makes the reuse of motion data through transformation and retargetting a more attractive option than creating new motion from scratch. Reuse requires the ability to search automatically and efficiently a growing corpus of motion data, which remains a difficult open problem. We present a method for quickly searching long, unsegmented motion clips for subregions that most closely match a short query clip. Our search algorithm is based on a weighted PCA-based pose representation that allows for flexible and efficient pose-to-pose distance calculations. We present our pose representation and the details of the search algorithm. We evaluate the performance of a prototype search application using both synthetic and captured motion data. Using these results, we propose ways to improve the application s performance. The results inform a discussion of the algorithm s good scalability characteristics.
Directable Animation of Elastic Objects
(The Eurographics Association, 2005) Kondo, Ryo; Kanai, Takashi; Anjyo, Ken-ichi; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
There is a crucial demand in the computer animation industry to make animations that blend animator-specified expressive motion with physics-based realism. We propose a novel framework to create directable animation of elastically deformable objects. The directable animation is created with animator-specified keyframes and the motion trajectory of the deformable object, while maintaining a plausible realism. Our framework mainly consists of two complementary approaches. The first is a method to control the time-varying geometry of an elastic object, using a loose key-framing technique. In our keyframing, we introduce an FEM-based elastic deformation algorithm that allows us to rearrange the elastic object motion, guided by the shape or pose specified at each keyframe. The second is a motion compensation technique, which allows us to rearrange the physical behavior of elastically deformable objects under a user-specified trajectory. The animation examples demonstrate that our framework provides plausibly realistic deformation animations with greater controllability and usability than existing approaches.
Particle-Based Fluid-Fluid Interaction
(The Eurographics Association, 2005) Müller, Matthias; Solenthaler, Barbara; Keiser, Richard; Gross, Markus; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
The interesting and complex behavior of fluids emerges mainly from interaction processes. While interactions of fluids with static or dynamic solids has caught some attention in computer graphics lately, the mutual interaction of different types of fluids such as air and water or water and wax has received much less attention although these types of interaction are the basis for a variety of important phenomena. In this paper we propose a new technique to model fluid-fluid interaction based on the Smoothed Particle Hydrodynamics (SPH) method. For the simulation of air-water interaction, air particles are generated on the fly only where needed. We also model dynamic phase changes and interface forces. Our technique makes possible the simulation of phenomena such as boiling water, trapped air and the dynamics of a lava lamp.
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.
Adapted Unstructured LBM for Flow Simulation on Curved Surfaces
(The Eurographics Association, 2005) Fan, Z.; Zhao, Y.; Kaufman, A.; He, Y.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Flow motion on curved surfaces of arbitrary topology is an interesting visual effect but a complex dynamics to simulate. In this paper, we introduce a novel and effective way to model such dynamics. We propose a technique that adapts a recently emerged computational fluid dynamics (CFD) model, unstructured lattice Boltzmann model (Unstructured LBM), from the 2D unstructured meshes to the 3D surface meshes. Unlike previous methods in modeling flows on surfaces, which start from the macroscopic point of view and modify the Navier Stokes solvers for the curved surfaces, our method is based on the microscopic kinetic equations for discrete particle distribution functions. All computations on the surface mesh only involve the information within local neighborhoods. This model lends itself the following advantages: (i) simplicity and explicit parallelism in computation, (ii) great capability in handling complex interactions, such as the interactions between flow and boundaries and the interactions of multiple-component fluids; (iii) no need of global surface parameterization which may cause strong distortions; (iv) capability of being applied to meshes with arbitrary connectivity.
Physically Based Grasping Control from Example
(The Eurographics Association, 2005) Pollard, Nancy S.; Zordan, Victor B.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Animated human characters in everyday scenarios must interact with the environment using their hands. Captured human motion can provide a database of realistic examples. However, examples involving contact are difficult to edit and retarget; realism can suffer when a grasp does not appear secure or when an apparent impact does not disturb the hand or the object. Physically based simulations can preserve plausibility through simulating interaction forces. However, such physical models must be driven by a controller, and creating effective controllers for new motion tasks remains a challenge. In this paper, we present a controller for physically based grasping that draws from motion capture data. Our controller explicitly includes passive and active components to uphold compliant yet controllable motion, and it adds compensation for movement of the arm and for gravity to make the behavior of passive and active components less dependent on the dynamics of arm motion. Given a set of motion capture grasp examples, our system solves for all but a small set of parameters for this controller automatically. We demonstrate results for tasks including grasping and two-hand interaction and show that a controller derived from a single motion capture example can be used to form grasps of different object geometries.
Transferable Videorealistic Speech Animation
(The Eurographics Association, 2005) Chang, Yao-Jen; Ezzat, Tony; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Image-based videorealistic speech animation achieves significant visual realism at the cost of the collection of a large 5- to 10-minute video corpus from the specific person to be animated. This requirement hinders its use in broad applications, since a large video corpus for a specific person under a controlled recording setup may not be easily obtained. In this paper, we propose a model transfer and adaptation algorithm which allows for a novel person to be animated using only a small video corpus. The algorithm starts with a multidimensional morphable model (MMM) previously trained from a different speaker with a large corpus, and transfers it to the novel speaker with a much smaller corpus. The algorithm consists of 1) a novel matching-by-synthesis algorithm which semi-automatically selects new MMM prototype images from the new video corpus and 2) a novel gradient descent linear regression algorithm which adapts the MMM phoneme models to the data in the novel video corpus. Encouraging experimental results are presented in which a morphable model trained from a performer with a 10- minute corpus is transferred to a novel person using a 15-second movie clip of him as the adaptation video corpus.
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.
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.
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.
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.
XFluids in Deforming Meshes
(The Eurographics Association, 2005) Feldman, Bryan E.; O Brien, James F.; Klingner, Bryan M.; Goktekin, Tolga G.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
This paper describes a simple modification to an Eulerian fluid simulation that permits the underlying mesh to deform independent of the simulated fluid s motion. The modification consists of a straightforward adaptation of the commonly used semi-Lagrangian advection method to account for the mesh s motion. Because the method does not require more interpolation steps than standard semi-Lagrangian integration, it does not suffer from additional smoothing and requires only the added cost of updating the mesh. By specifying appropriate boundary conditions, mesh boundaries can behave like moving obstacles that act on the fluid resulting in a number of interesting effects. The paper includes several examples that have been computed on moving tetrahedral meshes.
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.
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.
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.
Simulating Complex Hair with Robust Collision Handling
(The Eurographics Association, 2005) Choe, Byoungwon; Choi, Min Gyu; Ko, Hyeong-Seok; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
We present a new framework for simulating dynamic movements of complex hairstyles. The proposed framework, which treats hair as a collection of wisps, includes new approaches to simulating dynamic wisp movements and handling wisp-body collisions and wisp-wisp interactions. For the simulation of wisps, we introduce a new hair dynamics model, a hybrid of the rigid multi-body serial chain and mass-spring models, to formulate the simulation system using an implicit integration method. Consequently, the simulator can impose collision/contact constraints systematically, allowing it to handle wisp-body collisions efficiently without the need for backtracking or subtimestepping. In addition, the simulator handles wisp-wisp collisions based on impulses while taking into account viscous damping and cohesive forces. Experimental results show that the proposed technique can stably simulate hair with intricate geometries while robustly handling wisp-body collisions and wisp-wisp interactions.
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.
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.
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.

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