SCA 06: Eurographics/SIGGRAPH Symposium on Computer Animation

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

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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.
Re-using Traditional Animation: Methods for Semi-Automatic Segmentation and Inbetweening
(The Eurographics Association, 2006) Juan, Christina N. de; Bodenheimer, Bobby; Marie-Paule Cani and James O'Brien
A large body of traditional animation exists that contains characters with poses, expressions, and appeal not easily achievable with modern 3D techniques. To create new uses for this body of animation, this paper presents components of a system that can help incorporate the animation into re-usable libraries. In particular, we discuss two semi-automatic techniques that allow the re-use of traditional animation. First, support vector machines are used to segment cartoon images from their backgrounds for incorporation into an image library, for such applications as re-sequencing. Second, a radial basis function implicit surface modeling technique and a fast nonrigid elastic registration algorithm provide inbetween contours and textures given two key images of traditional animation. Our system is fast, model-free, and requires minimal animator intervention.
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.
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.
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.
Simultaneous Coupling of Fluids and Deformable Bodies
(The Eurographics Association, 2006) Chentanez, Nuttapong; Goktekin, Tolga G.; Feldman, Bryan E.; O'Brien, James F.; Marie-Paule Cani and James O'Brien
This paper presents a method for simulating the two-way interaction between fluids and deformable solids. The fluids are simulated using an incompressible Eulerian formulation where a linear pressure projection on the fluid velocities enforces mass conservation. Similarly, elastic solids are simulated using a semi-implicit integrator implemented as a linear operator applied to the forces acting on the nodes in Lagrangian formulation. The proposed method enforces coupling constraints between the fluid and the elastic systems by combining both the pressure projection and implicit integration steps into one set of simultaneous equations. Because these equations are solved simultaneously the resulting combined system treats closed regions in a physically correct fashion, and has good stability characteristics allowing for relatively large time steps. This general approach is not tied to any particular volume discretization of fluid or solid, and we present results implemented using both regular-grid and tetrahedral simulations.
Precomputed Search Trees: Planning for Interactive Goal-Driven Animation
(The Eurographics Association, 2006) Lau, Manfred; Kuffner, James J.; Marie-Paule Cani and James O'Brien
We present a novel approach for interactively synthesizing motions for characters navigating in complex environments. We focus on the runtime efficiency for motion generation, thereby enabling the interactive animation of a large number of characters simultaneously. The key idea is to precompute search trees of motion clips that can be applied to arbitrary environments. Given a navigation goal relative to a current body position, the best available solution paths and motion sequences can be efficiently extracted during runtime through a series of table lookups. For distant start and goal positions, we first use a fast coarse-level planner to generate a rough path of intermediate sub-goals to guide each iteration of the runtime lookup phase. We demonstrate the efficiency of our technique across a range of examples in an interactive application with multiple autonomous characters navigating in dynamic environments. Each character responds in real-time to arbitrary user changes to the environment obstacles or navigation goals. The runtime phase is more than two orders of magnitude faster than existing planning methods or traditional motion synthesis techniques. Our technique is not only useful for autonomous motion generation in games, virtual reality, and interactive simulations, but also for animating massive crowds of characters offline for special effects in movies.
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.
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.
"Walk to here": A Voice Driven Animation System
(The Eurographics Association, 2006) Wang, Zhijin; Panne, Michiel van de; Marie-Paule Cani and James O'Brien
We present a novel interface for directing the actions of computer animated characters and camera movements. Our system takes spoken input in combination with mouse pointing to generate desired character animation based on motion capture data. The aim is to achieve a more natural animation interface by supporting the types of dialogue and pointing that might be used when one person is explaining a desired motion to another person. We compare our voice-driven system with a button-driven animation interface that has equivalent capabilities. An informal user study indicates that for the test scenarios, the voice-user interface (VUI) is faster than an equivalent graphical user interface (GUI). Potential applications include storyboarding for film or theatre, directing characters in video games, and scene reconstruction.
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.
Simulation of Bubbles
(The Eurographics Association, 2006) Zheng, Wen; Yong, Jun-Hai; Paul, Jean-Claude; Marie-Paule Cani and James O'Brien
We present a novel framework based on a continuous fluid simulator for general simulation of realistic bubbles, with which we can handle as many significant dynamic bubble effects as possible. To capture nature of the very thin liquid film of bubbles, we have developed a regional level set method allowing multi-manifold interface tracking. The regional level set method is based on the definitions of regional distance and its five operators, which makes it very easy to implement. We can reconstruct an implicit surface of liquid film with arbitrary thickness from the representation of regional level set functions. To overcome the numerical instability caused by surface tension, we exploit a new semi-implicit surface tension model which is unconditionally stable and makes the simulation of surface tension dominated phenomena much more efficient. An approximated film thickness evolution model is proposed to control the bubble s lifecycle. All these new techniques combine into a general framework that can produce various realistic dynamic effects of bubbles.
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.
Physics Based Boiling Simulation
(The Eurographics Association, 2006) Mihalef, Viorel; Unlusu, Betul; Metaxas, Dimitris; Sussman, Mark; Hussaini, M. Y.; Marie-Paule Cani and James O'Brien
In order to animate complex fluid motion, computer animators have to rely on simulation systems that automatically generate the dynamics in a physics based manner. We focus in this paper on the phenomenon of boiling, which, due to its complex formulation and physics, has seen very little work done in the graphics field. We propose a new Eulerian method that couples gas and liquid with variable temperature and with a mass transfer mechanism, and we present its application to simulating boiling phenomena. Our philosophy is using physics based models to obtain visually rich animations that mirror their real life counterparts, including phenomena of increased circulation in the mass of liquid, roiling boil, nucleation seeding on solid boundaries.
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.
Robust Kinematic Constraint Detection for Motion Data
(The Eurographics Association, 2006) Callennec, Benoît Le; Boulic, Ronan; Marie-Paule Cani and James O'Brien
Motion capture data is now widely available to create realistic character animation. However, it is di?cult to reuse without any additional information. For this reason, annotating motion data with kinematic constraints is a clever step to ease further operations such as blending or motion editing. Unfortunately, prior automatic methods prove to be unreliable for noisy data and/or lack genericity. In this paper, we present a method for detecting kinematic constraints for motion data. It detects when an object (or an end-e?ector) is stationary in space or is rotating around an axis or a point. Our method is fast, generic and may be used on any kind of objects in the scene. Furthermore, it is robust to highly noisy data as we detect and reject aberrant data by using a least median of squares (LMedS) method. We demonstrate the accuracy of our method in various motion editing contexts.
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.
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.
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.
Physics-Based Motion Retiming
(The Eurographics Association, 2006) McCann, Jim; Pollard, Nancy S.; Srinivasa, Siddhartha; Marie-Paule Cani and James O'Brien
By changing only the playback timing of a motion sequence, an animator can achieve a variety of effects that alter our perception of an event. In some scenarios, it may be important to consider physical properties of the motion when retiming (e.g., to preserve physical plausibility). However, existing retiming solutions can be quite time consuming when physical parameters are considered. This paper presents an interactive method for creating optimal motion retimings that takes into account physically based constraints and objective functions. We achieve fast performance through a precomputation phase where constraints are projected into the two-dimensional space of velocities and accelerations along the input motion path. Unlike previous approaches, our precomputation technique allows for rapid computation of plausible contact forces that result from retiming, and it also accommodates changing physical parameters. We demonstrate our approach by creating physically plausible results for changes in motion duration, manipulations of the gravity vector, and modifications of character limb masses.

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