SCA: Eurographics/SIGGRAPH Symposium on Computer Animation

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An Evaluation of a Cost Metric for Selecting Transitions between Motion Segments
(The Eurographics Association, 2003) Wang, Jing; Bodenheimer, Bobby; D. Breen and M. Lin
Designing a rich repertoire of behaviors for virtual humans is an important problem for virtual environments and computer games. One approach to designing such a repertoire is to collect motion capture data and pre-process it to form a structure that can be walked in various orders to re-sequence the data in new ways. In such an approach identifying the location of good transition points in the motion stream is critical. In this paper, we evaluate the cost function described by Lee et al.15 for determining such transition points. Lee et al. proposed an original set of weights for their metric. We compute a set of optimal weights for the cost function using a constrained leastsquares technique. The weights are then evaluated in two ways: first, through a cross-validation study and second, through a medium-scale user study. The cross-validation shows that the optimized weights are robust and work for a wide variety of behaviors. The user study demonstrates that the optimized weights select more appealing transition points than the original weights.
Generating Flying Creatures using Body-Brain Co-Evolution
(The Eurographics Association, 2003) Shim, Yoon-Sik; Kim, Chang-Hun; D. Breen and M. Lin
This paper describes a system that produces double-winged flying creatures using body-brain co-evolution without need of complex flapping flight aerodynamics. While artificial life techniques have been used to create a variety of virtual creatures, little work has explored flapping-winged creatures for the difficulty of genetic encoding problem of wings with limited geometric primitives as well as flapping-wing aerodynamics. Despite of the simplicity of system, our result shows aesthetical looking and organic flapping flight locomotions. The restricted list structure is used in genotype encoding for morphological symmetry of creatures and is more easily handled than other data structures. The creatures evolved by this system have two symmetric flapping wings consisting of continuous triangular patches and show various looking and locomotion such as wings of birds, butterflies and bats or even imaginary wings of a dragon and pterosaurs.
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.
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.
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.
Stylizing Motion with Drawings
(The Eurographics Association, 2003) Li, Yin; Gleicher, Michael; Xu, Ying-Qing; Shum, Heung-Yeung; D. Breen and M. Lin
In this paper, we provide a method that injects the expressive shape deformations common in traditional 2D animation into an otherwise rigid 3D motion captured animation. We allow a traditional animator to modify frames in the rendered animation by redrawing the key features such as silhouette curves. These changes are then integrated into the animation. To perform this integration, we divide the changes into those that can be made by altering the skeletal animation, and those that must be made by altering the character's mesh geometry. To propagate mesh changes into other frames, we introduce a new image warping technique that takes into account the character's 3D structure. The resulting technique provides a system where an animator can inject stylization into 3D animation.
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.
A Real-Time Cloud Modeling, Rendering, and Animation System
(The Eurographics Association, 2003) Schpok, Joshua; Simons, Joseph; Ebert, David S.; Hansen, Charles; D. Breen and M. Lin
Modeling and animating complex volumetric natural phenomena, such as clouds, is a difficult task. Most systems are difficult to use, require adjustment of numerous, complex parameters, and are non-interactive. Therefore, we have developed an intuitive, interactive system to artistically model, animate, and render visually convincing volumetric clouds using modern consumer graphics hardware. Our natural, high-level interface models volumetric clouds through the use of qualitative cloud attributes. The animation of the implicit skeletal structures and independent transformation of octaves of noise emulate various environmental conditions. The resulting interactive design, rendering, and animation system produces perceptually convincing volumetric cloud models that can be used in interactive systems or exported for higher quality offline rendering.
An Example-Based Approach for Facial Expression Cloning
(The Eurographics Association, 2003) Pyun, Hyewon; Kim, Yejin; Chae, Wonseok; Kang, Hyung Woo; Shin, Sung Yong; D. Breen and M. Lin
In this paper, we present a novel example-based approach for cloning facial expressions of a source model to a target model while reflecting the characteristic features of the target model in the resulting animation. Our approach comprises three major parts: key-model construction, parameterization, and expression blending. We first present an effective scheme for constructing key-models. Given a set of source example key-models and their corresponding target key-models created by animators, we parameterize the target key-models using the source key-models and predefine the weight functions for the parameterized target key-models based on radial basis functions. In runtime, given an input model with some facial expression, we compute the parameter vector of the corresponding output model, to evaluate the weight values for the target key-models and obtain the output model by blending the target key-models with those weights. The resulting animation preserves the facial expressions of the input model as well as the characteristic features of the target model specified by animators. Our method is not only simple and accurate but also fast enough for various real-time applications such as video games or internet broadcasting.
Estimating Cloth Simulation Parameters from Video
(The Eurographics Association, 2003) Bhat, Kiran S.; Twigg, Christopher D.; Hodgins, Jessica K.; Khosla, Pradeep K.; Popovic, Zoran; Seitz, Steven M.; D. Breen and M. Lin
Cloth simulations are notoriously difficult to tune due to the many parameters that must be adjusted to achieve the look of a particular fabric. In this paper, we present an algorithm for estimating the parameters of a cloth simulation from video data of real fabric. A perceptually motivated metric based on matching between folds is used to compare video of real cloth with simulation. This metric compares two video sequences of cloth and returns a number that measures the differences in their folds. Simulated annealing is used to minimize the frame by frame error between the metric for a given simulation and the real-world footage. To estimate all the cloth parameters, we identify simple static and dynamic calibration experiments that use small swatches of the fabric. To demonstrate the power of this approach, we use our algorithm to find the parameters for four different fabrics. We show the match between the video footage and simulated motion on the calibration experiments, on new video sequences for the swatches, and on a simulation of a full skirt.
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.
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.
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.
Unsupervised Learning for Speech Motion Editing
(The Eurographics Association, 2003) Cao, Yong; Faloutsos, Petros; Pighin, Frédéric; D. Breen and M. Lin
We present a new method for editing speech related facial motions. Our method uses an unsupervised learning technique, Independent Component Analysis (ICA), to extract a set of meaningful parameters without any annotation of the data. With ICA, we are able to solve a blind source separation problem and describe the original data as a linear combination of two sources. One source captures content (speech) and the other captures style (emotion). By manipulating the independent components we can edit the motions in intuitive ways.
Sound-by-Numbers: Motion-Driven Sound Synthesis
(The Eurographics Association, 2003) Cardle, M.; Brooks, S.; Bar-Joseph, Z.; Robinson, P.; D. Breen and M. Lin
We present the first algorithm for automatically generating soundtracks for input animation based on other animations' soundtrack. This technique can greatly simplify the production of soundtracks in computer animation and video by re-targeting existing soundtracks. A segment of source audio is used to train a statistical model which is then used to generate variants of the original audio to fit particular constraints. These constraints can either be specified explicitly by the user in the form of large-scale properties of the sound texture, or determined automatically and semi-automatically by matching similar motion events in a source animation to those in the target animation.
Vision-based Control of 3D Facial Animation
(The Eurographics Association, 2003) Chai, Jin-xiang; Xiao, Jing; Hodgins, Jessica; D. Breen and M. Lin
Controlling and animating the facial expression of a computer-generated 3D character is a difficult problem because the face has many degrees of freedom while most available input devices have few. In this paper, we show that a rich set of lifelike facial actions can be created from a preprocessed motion capture database and that a user can control these actions by acting out the desired motions in front of a video camera. We develop a real-time facial tracking system to extract a small set of animation control parameters from video. Because of the nature of video data, these parameters may be noisy, low-resolution, and contain errors. The system uses the knowledge embedded in motion capture data to translate these low-quality 2D animation control signals into high-quality 3D facial expressions. To adapt the synthesized motion to a new character model, we introduce an efficient expression retargeting technique whose run-time computation is constant independent of the complexity of the character model. We demonstrate the power of this approach through two users who control and animate a wide range of 3D facial expressions of different avatars.
Visual Simulation of Ice Crystal Growth
(The Eurographics Association, 2003) Kim, Theodore; Lin, Ming C.; D. Breen and M. Lin
The beautiful, branching structure of ice is one of the most striking visual phenomena of the winter landscape. Yet there is little study about modeling this effect in computer graphics. In this paper, we present a novel approach for visual simulation of ice growth. We use a numerical simulation technique from computational physics, the "phase field method", and modify it to allow aesthetic manipulation of ice crystal growth. We present acceleration techniques to achieve interactive simulation performance, as well as a novel geometric sharpening algorithm that removes some of the smoothing artifacts from the implicit representation. We have successfully applied this approach to generate ice crystal growth on 3D object surfaces in several scenes.
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.
A Practical Dynamics System
(The Eurographics Association, 2003) Kacic-Alesic, Zoran; Nordenstam, Marcus; Bullock, David; D. Breen and M. Lin
We present an effective production-proven dynamics system. It uses an explicit time differencing method that is efficient, reasonably accurate, conditionally stable, and above all simple to implement. We describe issues related to integration of physically based simulation techniques into an interactive animation system, present a high level description of the architecture of the system, report on techniques that work, and provide observations that may seem obvious, but only in retrospect. Applications include rigid and deformable body dynamics, particle dynamics, and at a basic level, hair and cloth simulation.
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.

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