SCA 06: Eurographics/SIGGRAPH Symposium on Computer Animation

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

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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.
Segment-Based Human Motion Compression
(The Eurographics Association, 2006) Liu, Guodong; McMillan, Leonard; Marie-Paule Cani and James O'Brien
As more and more human motion data are becoming widely used to animate computer graphics figures in many applications, the growing need for compact storage and fast transmission makes it imperative to compress motion data. We propose a data-driven method for efficient compression of human motion sequences by exploiting both spatial and temporal coherences of the data. We first segment a motion sequence into subsequences such that the poses within a subsequence lie near a low dimensional linear space. We then compress each segment using principal component analysis. Our method achieves further compression by storing only the key frames projections to the principal component space and interpolating the other frames in-between via spline functions. The experimental results show that our method can achieve significant compression rate with low reconstruction errors.
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.
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.
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.
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.
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.
Path-based Control of Smoke Simulations
(The Eurographics Association, 2006) Kim, Yootai; Machiraju, Raghu; Thompson, David; Marie-Paule Cani and James O'Brien
In this paper, we propose a novel path-based control method for generating realistic smoke animations. Our method allows an animator to specify a 3D curve for the smoke to follow. Path control is then achieved using a linear (closed) feedback loop to match the velocity field obtained from a 3D flow simulation with a target velocity field. The target velocity field can be generated in a variety of ways and may include the small scale swirling motion characteristic of turbulent flows. We provide several examples of complex smoke paths to demonstrate the efficacy of our approach.
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.
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.
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.
A Texture Synthesis Method for Liquid Animations
(The Eurographics Association, 2006) Bargteil, Adam W.; Sin, Funshing; Michaels, Jonathan E.; Goktekin, Tolga G.; O'Brien, James F.; Marie-Paule Cani and James O'Brien
In this paper we present a method for synthesizing textures on animated liquid surfaces generated by a physically based fluid simulation system. Rather than advecting texture coordinates on the surface, our algorithm synthesizes a new texture for every frame using an optimization procedure which attempts to match the surface texture to an input sample texture. By synthesizing a new texture for every frame, our method is able to overcome the discontinuities and distortions of an advected parameterization. We achieve temporal coherence by initializing the surface texture with color values advected from the surface at the previous frame and including these colors in the energy function used during optimization.
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.
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.
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.
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.
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.
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.

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