SCA 05: Eurographics/SIGGRAPH Symposium on Computer Animation

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

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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.
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.
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.
AER: Aesthetic Exploration and Refinement for Expressive Character Animation
(The Eurographics Association, 2005) Neff, Michael; Fiume, Eugene; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Our progress in the problem of making animated characters move expressively has been slow, and it persists in being among the most challenging in computer graphics. Simply attending to the low-level motion control problem, particularly for physically based models, is very difficult. Providing an animator with the tools to imbue character motion with broad expressive qualities is even more ambitious, but it is clear it is a goal to which we must aspire. Part of the problem is simply finding the right language in which to express qualities of motion. Another important issue is that expressive animation often involves many disparate parts of the body, which thwarts bottom-up controller synthesis. We demonstrate progress in this direction through the specification of directed, expressive animation over a limited range of standing movements. A key contribution is that through the use of high-level concepts such as character sketches, actions and properties, which impose different modalities of character behaviour, we are able to create many different animated interpretations of the same script. These tools support both rapid exploration of the aesthetic space and detailed refinement. Basic character actions and properties are distilled from an extensive search in the performing arts literature. We demonstrate how all highlevel constructions for expressive animation can be given a precise semantics that translate into a low-level motion specification that is then simulated either physically or kinematically. Our language and system can act as a bridge across artistic and technical communities to resolve ambiguities regarding the language of motion.We demonstrate our results through an implementation and various examples.
Capture and Synthesis of Insect Motion
(The Eurographics Association, 2005) Gibson, D. P.; Oziem, D. J.; Dalton, C. J.; Campbell, N. W.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
We present an integrated system that enables the capture and synthesis of 3D motions of small scale dynamic creatures, typically insects and arachnids, in order to drive computer generated models. The system consists of a number of stages, initially, the acquisition of a multi-view calibration scene and synchronised video footage of a subject performing some action is carried out. A user guided labelling process, that can be semi-automated using tracking techniques and a 3D point generating algorithm, then enables a full metric calibration and captures the motions of specific points on the subject. The 3D motions extracted, which often come from a limited number of frames of the original footage, are then extended to generate potentially infinitely long, characteristic motion sequences for multiple similar subjects. Finally a novel path following algorithm is used to find optimal path along with coherent motion for synthetic subjects. The result is a system that, from a potentially small number of original multi-view frames, can generate a whole swarm of novel synthetic subjects all moving in a coherent and natural manner. The proposed system has two major advantages over existing systems, 1) that traditional motion capture techniques cannot in general be used for very small subjects and 2) minimal expense and user input is required to generate, complex, high quality, CG animation.
Taming Liquids for Rapidly Changing Targets
(The Eurographics Association, 2005) Shi, Lin; Yu, Yizhou; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Following rapidly changing target objects is a challenging problem in fluid control, especially when the natural fluid motion should be preserved. The fluid should be responsive to the changing configuration of the target and, at the same time, its motion should not be overconstrained. In this paper, we introduce an efficient and effective solution by applying two different external force fields. The first one is a feedback force field which compensates for discrepancies in both shape and velocity. Its shape component is designed to be divergence free so that it can survive the velocity projection step. The second one is the gradient field of a potential function defined by the shape and skeletion of the target object. Our experiments indicate a mixture of these two force fields can achieve desirable and pleasing effects.
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.
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.
Particle-Based Simulation of Granular Materials
(The Eurographics Association, 2005) Bell, Nathan; Yu, Yizhou; Mucha, Peter J.; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Granular materials, such as sand and grains, are ubiquitous. Simulating the 3D dynamic motion of such materials represents a challenging problem in graphics because of their unique physical properties. In this paper we present a simple and effective method for granular material simulation. By incorporating techniques from physical models, our approach describes granular phenomena more faithfully than previous methods. Granular material is represented by a large collection of non-spherical particles which may be in persistent contact. The particles represent discrete elements of the simulated material. One major advantage of using discrete elements is that the topology of particle interaction can evolve freely. As a result, highly dynamic phenomena, such as splashing and avalanches, can be conveniently generated by this meshless approach without sacrificing physical accuracy. We generalize this discrete model to rigid bodies by distributing particles over their surfaces. In this way, two-way coupling between granular materials and rigid bodies is achieved.
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.
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.
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.
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.
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.
Animosaics
(The Eurographics Association, 2005) Smithy, Kaleigh; Liuz, Yunjun; Klein, Allison; D. Terzopoulos and V. Zordan and K. Anjyo and P. Faloutsos
Animated mosaics are a traditional form of stop-motion animation created by arranging and rearranging small objects or tiles from frame to frame. While this animation style is uniquely compelling, the traditional process of manually placing and then moving tiles in each frame is time-consuming and labourious. Recent work has proposed algorithms for static mosaics, but generating temporally coherent mosaic animations has remained open. In addition, previous techniques for temporal coherence allow non-photorealistic primitives to layer, blend, deform, or scale, techniques that are unsuitable for mosaic animations. This paper presents a new approach to temporal coherence and applies this to build a method for creating mosaic animations. Specifically, we characterize temporal coherence as the coordinated movement of groups of primitives. We describe a system for achieving this coordinated movement to create temporally coherent geometric packings of 2D shapes over time. We also show how to create static mosaics comprised of different tile shapes using area-based centroidal Voronoi diagrams
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.
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.
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.
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.
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.

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