SCA 06: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/430

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Animation of Open Water Phenomena with coupled Shallow Water and Free Surface Simulations
(The Eurographics Association, 2006) Thürey, Nils; Rüde, Ulrich; Stamminger, Marc; Marie-Paule Cani and James O'Brien
The goal of this paper is to perform simulations that capture fluid effects from small drops up to the propagation of large waves. To achieve this, we present a hybrid simulation method, that couples a two-dimensional shallow water simulation with a full three-dimensional free surface fluid simulation. We explain the approximations imposed by the shallow water model, and how to parametrize it according to the parameters of a 3D simulation. Each simulation is used to initialize double layered boundary conditions for the other one. The area covered by the 2D region can be an order of magnitude larger than the 3D region without significantly effecting the overall computation time. The 3D region can furthermore be easily moved within the 2D region during the course of the simulation. To achieve realistic results we combine our simulation method with a physically based model to generate and animate drops. For their generation we make use of the fluid turbulence model, and animate them with a simplified drag calculation. This allows simulations with relatively low resolutions.
Automatic Splicing for Hand and Body Animations
(The Eurographics Association, 2006) Majkowska, Anna; Zordan, Victor B.; Faloutsos, Petros; Marie-Paule Cani and James O'Brien
We propose a solution to a new problem in animation research: how to use human motion capture data to create character motion with detailed hand gesticulation without the need for the simultaneous capture of hands and the full-body. Occlusion and a difference in scale make it difficult to capture both the detail of the hand movement and unrestricted full-body motion at the same time. With our method, the two can be captured separately and spliced together seamlessly with little or no user input required. The algorithm relies on a novel distance metric derived from research on gestures and uses a two-pass dynamic time warping algorithm to find correspondence between the hand and full-body motions. In addition, we provide a method for supplying user input, useful to animators who want more control over the integrated animation. We show the power of our technique with a variety of common and highly specialized gesticulation examples.
Composition of Complex Optimal Multi-Character Motions
(The Eurographics Association, 2006) Liu, C. Karen; Hertzmann, Aaron; Popovic, Zoran; Marie-Paule Cani and James O'Brien
This paper presents a physics-based method for creating complex multi-character motions from short singlecharacter sequences. We represent multi-character motion synthesis as a spacetime optimization problem where constraints represent the desired character interactions. We extend standard spacetime optimization with a novel timewarp parameterization in order to jointly optimize the motion and the interaction constraints. In addition, we present an optimization algorithm based on block coordinate descent and continuations that can be used to solve large problems multiple characters usually generate. This framework allows us to synthesize multi-character motion drastically different from the input motion. Consequently, a small set of input motion dataset is sufficient to express a wide variety of multi-character motions.
A Consistent Bending Model for Cloth Simulation with Corotational Subdivision Finite Elements
(The Eurographics Association, 2006) Thomaszewski, Bernhard; Wacker, Markus; Straßer, Wolfgang; Marie-Paule Cani and James O'Brien
Wrinkles and folds play an important role in the appearance of real textiles. The way in which they form depends mainly on the bending properties of the specific material type. Existing approaches fail to reliably reproduce characteristic behaviour like folding and buckling for different material types or resolutions. It is therefore crucial for the realistic simulation of cloth to model bending energy in a physically accurate and consistent way. In this paper we present a new method based on a corotational formulation of subdivision finite elements. Due to the non-local nature of the employed subdivision basis functions a C1-continuous displacement field can be defined. In this way, it is possible to use the governing equations of thin shell analysis leading to physically accurate bending behaviour. Using a corotated strain tensor allows the large displacement analysis of cloth while retaining a linear system of equations. Hence, known convergence properties and computational efficiency are preserved while convincing and detailed folding behaviour is obtained in the simualtion.
A Controllable, Fast and Stable Basis for Vortex Based Smoke Simulation
(The Eurographics Association, 2006) Angelidis, Alexis; Neyret, Fabrice; Singh, Karan; Nowrouzezahrai, Derek; Marie-Paule Cani and James O'Brien
We introduce a novel method for describing and controlling a 3D smoke simulation. Using harmonic analysis and principal component analysis, we define an underlying description of the fluid flow that is compact and meaningful to non-expert users. The motion of the smoke can be modified with high level tools, such as animated current curves, attractors and tornadoes. Our simulation is controllable, interactive and stable for arbitrarily long periods of time. The simulation s computational cost increases linearly in the number of motion samples and smoke particles. Our adaptive smoke particle representation conveniently incorporates the surface-like characteristics of real smoke.
Detail-Preserving Fluid Control
(The Eurographics Association, 2006) Thürey, Nils; Keiser, Richard; Pauly, Mark; Rüde, Ulrich; Marie-Paule Cani and James O'Brien
We propose a new fluid control technique that uses scale-dependent force control to preserve small-scale fluid detail. Control particles define local force fields and can be generated automatically from either a physical simulation or a sequence of target shapes. We use a multi-scale decomposition of the velocity field and apply control forces only to the coarse-scale components of the flow. Small-scale detail is thus preserved in a natural way avoiding the artificial viscosity often introduced by force-based control methods. We demonstrate the effectiveness of our method for both Lagrangian and Eulerian fluid simulation environments.
eFASE: Expressive Facial Animation Synthesis and Editing with Phoneme-Isomap Controls
(The Eurographics Association, 2006) Deng, Zhigang; Neumann, Ulrich; Marie-Paule Cani and James O'Brien
This paper presents a novel data-driven system for expressive facial animation synthesis and editing. Given novel phoneme-aligned speech input and its emotion modifiers (specifications), this system automatically generates expressive facial animation by concatenating captured motion data while animators establish constraints and goals. A constrained dynamic programming algorithm is used to search for best-matched captured motion nodes by minimizing a cost function. Users optionally specify hard constraints" (motion-node constraints for expressing phoneme utterances) and soft constraints" (emotion modifiers) to guide the search process. Users can also edit the processed facial motion node database by inserting and deleting motion nodes via a novel phoneme-Isomap interface. Novel facial animation synthesis experiments and objective trajectory comparisons between synthesized facial motion and captured motion demonstrate that this system is effective for producing realistic expressive facial animations.
Fast Arbitrary Splitting of Deforming Objects
(The Eurographics Association, 2006) Steinemann, Denis; Otaduy, Miguel A.; Gross, Markus; Marie-Paule Cani and James O'Brien
We present a novel algorithm for efficiently splitting deformable solids along arbitrary piecewise linear crack surfaces in cutting and fracture simulations. We propose the use of a meshless discretization of the deformation field, and a novel visibility graph for fast update of shape functions in meshless discretizations. We decompose the splitting operation into a first step where we synthesize crack surfaces as triangle meshes, and a second step where we use the newly synthesized surfaces to update the visibility graph, and thus the meshless discretization of the deformation field. The separation of the splitting operation into two steps, along with our novel visibility graph, enables high flexibility and control over the splitting trajectories, provides fast dynamic update of the meshless discretization, and facilitates an easy implementation, making our algorithm scalable, versatile, and suitable for a large range of applications, from computer animation to interactive medical simulation.We present a novel algorithm for efficiently splitting deformable solids along arbitrary piecewise linear crack surfaces in cutting and fracture simulations. We propose the use of a meshless discretization of the deformation field, and a novel visibility graph for fast update of shape functions in meshless discretizations. We decompose the splitting operation into a first step where we synthesize crack surfaces as triangle meshes, and a second step where we use the newly synthesized surfaces to update the visibility graph, and thus the meshless discretization of the deformation field. The separation of the splitting operation into two steps, along with our novel visibility graph, enables high flexibility and control over the splitting trajectories, provides fast dynamic update of the meshless discretization, and facilitates an easy implementation, making our algorithm scalable, versatile, and suitable for a large range of applications, from computer animation to interactive medical simulation.
Fast Simulation of Deformable Models in Contact Using Dynamic Deformation Textures
(The Eurographics Association, 2006) Galoppo, Nico; Otaduy, Miguel A.; Mecklenburg, Paul; Gross, Markus; Lin, Ming C.; Marie-Paule Cani and James O'Brien
We present an efficient algorithm for simulating contacts between deformable bodies with high-resolution surface geometry using dynamic deformation textures, which reformulate the 3D elastoplastic deformation and collision handling on a 2D parametric atlas to reduce the extremely high number of degrees of freedom in such a computa- tionally demanding simulation. We perform proximity queries for deformable bodies using a two-stage algorithm directly on dynamic deformation textures, resulting in output-sensitive collision detection that is independent of the combinatorial complexity of the deforming meshes. We present a robust, parallelizable formulation for computing constraint forces using implicit methods that exploits the structure of the motion equations to achieve highly stable simulation, while taking large time steps with inhomogeneous materials. The dynamic deformation textures can also be used directly for real-time shading and can easily be implemented using SIMD architecture on commodity hardware. We show that our approach, complementing existing pioneering work, offers significant computational advantages on challenging contact scenarios in dynamic simulation of deformable bodies.
Fat Graphs: Constructing an Interactive Character with Continuous Controls
(The Eurographics Association, 2006) Shin, Hyun Joon; Oh, Hyun Seok; Marie-Paule Cani and James O'Brien
This paper proposes a methodology that allows users to control character s motion interactively but continuously. Inspired by the work of Gleicher et al. [GSKJ03], we propose a semi-automatic method to build fat graphs where a node corresponds to a pose and its incoming and outgoing edges represent the motion segments starting from and ending at similar poses. A group of edges is built into a fat edge that parameterizes similar motion segments into a blendable form. Employing the existing motion transition and blending methods, our run-time system allows users to control a character interactively in continuous parameter spaces with conventional input devices such as joysticks and the mice. The capability of the proposed methodology is demonstrated through several applications. Although our method has some limitations on motion repertories and qualities, it can be adapted to a number of real-world applications including video games and virtual reality applications.
Generating Surface Crack Patterns
(The Eurographics Association, 2006) Iben, Hayley N.; O'Brien, James F.; Marie-Paule Cani and James O'Brien
We present a method for generating surface crack patterns that appear in materials such as mud, ceramic glaze, and glass. To model these phenomena, we build upon existing physically based methods. Our algorithm generates cracks from a stress field defined heuristically over a triangle discretization of the surface. The simulation produces cracks by evolving this field over time. The user can control the characteristics and appearance of the cracks using a set of simple parameters. By changing these parameters, we have generated examples similar to a variety of crack patterns found in the real world. We assess the realism of our results by comparison with photographs of realworld examples. Using a physically based approach also enables us to generate animations similar to time-lapse photography.
Geometric, Variational Integrators for Computer Animation
(The Eurographics Association, 2006) Kharevych, Liliya; Yang, Weiwei; Tong, Yiying; Kanso, Eva; Marsden, Jerrold E.; Schröder, Peter; Desbrun, Matthieu; Marie-Paule Cani and James O'Brien
We present a general-purpose numerical scheme for time integration of Lagrangian dynamical systems an important computational tool at the core of most physics-based animation techniques. Several features make this particular time integrator highly desirable for computer animation: it numerically preserves important invariants, such as linear and angular momenta; the symplectic nature of the integrator also guarantees a correct energy behavior, even when dissipation and external forces are added; holonomic constraints can also be enforced quite simply; finally, our simple methodology allows for the design of high-order accurate schemes if needed. Two key properties set the method apart from earlier approaches. First, the nonlinear equations that must be solved during an update step are replaced by a minimization of a novel functional, speeding up time stepping by more than a factor of two in practice. Second, the formulation introduces additional variables that provide key flexibility in the implementation of the method. These properties are achieved using a discrete form of a general variational principle called the Pontryagin-Hamilton principle, expressing time integration in a geometric manner. We demonstrate the applicability of our integrators to the simulation of non-linear elasticity with implementation details.
Hierarchical Retargetting of 2D Motion Fields to the Animation of 3D Plant Models
(The Eurographics Association, 2006) Diener, Julien; Reveret, Lionel; Fiume, Eugene; Marie-Paule Cani and James O'Brien
The complexity of animating trees, shrubs and foliage is an impediment to the efficient and realistic depiction of natural environments. This paper presents an algorithm to extract, from a single video sequence, motion fields of real shrubs under the influence of wind, and to transfer this motion to the animation of complex, synthetic 3D plant models. The extracted motion is retargeted without requiring physical simulation. First, feature tracking is applied to the video footage, allowing the 2D position and velocity of automatically identified features to be clustered. A key contribution of the method is that the hierarchy obtained through statistical clustering can be used to synthesize a 2D hierarchical geometric structure of branches that terminates according to the cut-off threshold of a classification algorithm. This step extracts both the shape and the motion of a hierarchy of features groups that are identified as geometrical branches. The 2D hierarchy is then extended to three dimensions using the estimated spatial distribution of the features within each group. Another key contribution is that this 3D hierarchical structure can be efficiently used as a motion controller to animate any complex 3D model of similar but non-identical plants using a standard skinning algorithm. Thus, a single video source of a moving shrub becomes an input device for a large class of virtual shrubs. We illustrate the results on two examples of shrubs and one outdoor tree. Extensions to other outdoor plants are discussed.
Interactive Animation of Dynamic Manipulation
(The Eurographics Association, 2006) Abe, Yeuhi; Popovic, Jovan; Marie-Paule Cani and James O'Brien
Lifelike animation of object manipulation requires dynamic interaction between animated characters, objects, and their environment. These interactions can be animated automatically with physically based simulations but proper controls are needed to animate characters that move realistically and that accomplish tasks in spite of unexpected disturbances. This paper describes an efficient control algorithm that generates realistic animations by incorporating motion data into task execution. The end result is a versatile system for interactive animation of dynamic manipulation tasks such as lifting, catching, and throwing.
Keyframe Control of Complex Particle Systems Using the Adjoint Method
(The Eurographics Association, 2006) Wojtan, Chris; Mucha, Peter J.; Turk, Greg; Marie-Paule Cani and James O'Brien
Control of physical simulation has become a popular topic in the field of computer graphics. Keyframe control has been applied to simulations of rigid bodies, smoke, liquid, flocks, and finite element-based elastic bodies. In this paper, we create a framework for controlling systems of interacting particles paying special attention to simulations of cloth and flocking behavior. We introduce a novel integrator-swapping approximation in order to apply the adjoint method to linearized implicit schemes appropriate for cloth simulation. This allows the control of cloth while avoiding computationally infeasible derivative calculations. Meanwhile, flocking control using the adjoint method is significantly more efficient than currently-used methods for constraining group behaviors, allowing the controlled simulation of greater numbers of agents in fewer optimization iterations.
Learning a Correlated Model of Identity and Pose-Dependent Body Shape Variation for Real-Time Synthesis
(The Eurographics Association, 2006) Allen, Brett; Curless, Brian; Popovic, Zoran; Hertzmann, Aaron; Marie-Paule Cani and James O'Brien
We present a method for learning a model of human body shape variation from a corpus of 3D range scans. Our model is the first to capture both identity-dependent and pose-dependent shape variation in a correlated fashion, enabling creation of a variety of virtual human characters with realistic and non-linear body deformations that are customized to the individual. Our learning method is robust to irregular sampling in pose-space and identityspace, and also to missing surface data in the examples. Our synthesized character models are based on standard skinning techniques and can be rendered in real time.
Modeling Ice Dynamics As A Thin-Film Stefan Problem
(The Eurographics Association, 2006) Kim, Theodore; Adalsteinsson, David; Lin, Ming C.; Marie-Paule Cani and James O'Brien
Large, 3D ice formations such as icicles exhibit a high degree of geometric and optical complexity. Modeling these features by hand can be a daunting task, so we present a novel physically-based algorithm for simulating this phenomenon. Solidification is usually posed as a so-called Stefan problem , but the problem in its classic form is inappropriate for simulating the ice typically found in a winter scene. We instead use the thin-film variant of the Stefan problem to derive velocity equations for a level set simulation. However, due to the scales involved in the problem, even an adaptive grid level set solver is still insufficient to track the tip of an icicle. Therefore, we derive an analytical solution for the icicle tip and use it to correct the level set simulation. The results appear to be in agreement with experimental data. We also present a physically-based technique for modeling ripples along the ice surface that alleviates the need to explicitly track small-scale geometry. To our knowledge, our approach is the most complete model available, and produces complex visual phenomena that no previous method has been able to capture.
Motion Templates for Automatic Classification and Retrieval of Motion Capture Data
(The Eurographics Association, 2006) Müller, Meinard; Röder, Tido; Marie-Paule Cani and James O'Brien
This paper presents new methods for automatic classification and retrieval of motion capture data facilitating the identification of logically related motions scattered in some database. As the main ingredient, we introduce the concept of motion templates (MTs), by which the essence of an entire class of logically related motions can be captured in an explicit and semantically interpretable matrix representation. The key property of MTs is that the variable aspects of a motion class can be automatically masked out in the comparison with unknown motion data. This facilitates robust and efficient motion retrieval even in the presence of large spatio-temporal variations. Furthermore, we describe how to learn an MT for a specific motion class from a given set of training motions. In our extensive experiments, which are based on several hours of motion data, MTs proved to be a powerful concept for motion annotation and retrieval, yielding accurate results even for highly variable motion classes such as cartwheels, lying down, or throwing motions.
MotionMaster: Authoring and Choreographing Kung-fu Motions by Sketch Drawings
(The Eurographics Association, 2006) Li, Qilei L.; Geng, Weidong D.; Yu, Tao; Shen, Xiao Jie; Lau, Newman; Yu, Gino; Marie-Paule Cani and James O'Brien
Sketch-drawings is an intuitive and comprehensive means of conveying movement ideas in character animation. We proposed a novel sketch-based approach to assisting the authoring and choreographing of Kungfu motions at the early stage of animation creation. Given two human figure sketches corresponding to the initial and closing posture of a Kungfu form, and the trajectory drawings on specific moving joints, MotionMaster can directly rapidprototype the realistic 3D motion sequence by sketch-based motion retrieval and refinement based on a motion database. The animators can then preview and evaluate the recovered motion sequence from any viewing angles. After the 3D motion sequence has been associated with the 2D sketch drawing, the animator can also interactively and iteratively make changes on the 2D sketch drawing, and the system will automatically transfer the 2D changes to the 3D motion data of current interests. It greatly helps the animator focus on the movement idea development during the evolutionary process of building motion data for articulated characters.
Oriented Strands - Dynamics of Stiff Multi-Body System
(The Eurographics Association, 2006) Hadap, Sunil; Marie-Paule Cani and James O'Brien
The simulation of strand like primitives modeled as dynamics of serial branched multi-body chain, albeit a potential reduced coordinate formulation, gives rise to stiff and highly non-linear differential equations. We introduce a recursive, linear time and fully implicit method to solve the stiff dynamical problem arising from such a multi-body system. We augment the merits of the proposed scheme by means of analytical constraints and an elaborate collision response model. We finally discuss a versatile simulation system based on the strand primitive for character dynamics and visual effects. We demonstrate dynamics of ears, braid, long/curly hair and foliage.

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