
SESSION: Fluids  particle methods 



A particlebased method for preserving fluid sheets 

Ryoichi Ando,
Reiji Tsuruno


Pages: 716 

doi>10.1145/2019406.2019408 

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We present a new particlebased method that explicitly preserves thin fluid sheets for animating liquids. Our primary contribution is a meshless particlebased framework that splits at thin points and collapses at dense points to prevent the breakup ...
We present a new particlebased method that explicitly preserves thin fluid sheets for animating liquids. Our primary contribution is a meshless particlebased framework that splits at thin points and collapses at dense points to prevent the breakup of liquid. In contrast to existing surface tracking methods, the proposed framework does not suffer from numerical diffusion or tangles, and robustly handles topology changes by the meshless representation. As the underlying fluid model, we use FluidImplicitParticle (FLIP) with weak spring forces to generate smooth particlebased liquid animation that maintains an even spatial particle distribution in the presence of eddying or inertial motions. The thin features are detected by examining stretches of distributions of neighboring particles by performing Principle Component Analysis (PCA), which is used to reconstruct thin surfaces with anisotropic kernels. Our algorithm is intuitively implemented, easy to parallelize and capable of producing visually complex thin liquid animations. expand


A levelset method for skinning animated particle data 

Haimasree Bhatacharya,
Yue Gao,
Adam Bargteil


Pages: 1724 

doi>10.1145/2019406.2019409 

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In this paper, we present a straightforward, easy to implement method for particle skinninggenerating surfaces from animated particle data. We cast the problem in terms of constrained optimization and solve the optimization using a levelset approach. ...
In this paper, we present a straightforward, easy to implement method for particle skinninggenerating surfaces from animated particle data. We cast the problem in terms of constrained optimization and solve the optimization using a levelset approach. The optimization seeks to minimize the thinplate energy of the surface, while staying between surfaces defined by the union of spheres centered at the particles. Our approach skins each frame independently while preserving the temporal coherence of the underlying particle animation. Thus, it is wellsuited for environments where particle skinning is treated as a postprocess, with each frame generated in parallel. We demonstrate our method on data generated by a variety of fluid simulation techniques and simple particle systems. expand


SPH granular flow with friction and cohesion 

Iván Alduán,
Miguel A. Otaduy


Pages: 2532 

doi>10.1145/2019406.2019410 

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Combining mechanical properties of solids and fluids, granular materials pose important challenges for the design of algorithms for realistic animation. In this paper, we present a simulation algorithm based on smoothed particle hydrodynamics (SPH) that ...
Combining mechanical properties of solids and fluids, granular materials pose important challenges for the design of algorithms for realistic animation. In this paper, we present a simulation algorithm based on smoothed particle hydrodynamics (SPH) that succeeds in modeling important features of the behavior of granular materials. These features are unilateral incompressibility, friction and cohesion. We extend an existing unilateral incompressibility formulation to be added at almost no effort to an existing SPHbased algorithm for fluids. The main advantages of this extension are the ease of implementation, the lack of grid artifacts, and the simple twoway coupling with other objects. Our friction and cohesion models can also be incorporated in a seamless manner in the overall SPH simulation algorithm. expand


Hybrid smoothed particle hydrodynamics 

Karthik Raveendran,
Chris Wojtan,
Greg Turk


Pages: 3342 

doi>10.1145/2019406.2019411 

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We present a new algorithm for enforcing incompressibility for Smoothed Particle Hydrodynamics (SPH) by preserving uniform density across the domain. We propose a hybrid method that uses a Poisson solve on a coarse grid to enforce a divergence free velocity ...
We present a new algorithm for enforcing incompressibility for Smoothed Particle Hydrodynamics (SPH) by preserving uniform density across the domain. We propose a hybrid method that uses a Poisson solve on a coarse grid to enforce a divergence free velocity field, followed by a local density correction of the particles. This avoids typical grid artifacts and maintains the Lagrangian nature of SPH by directly transferring pressures onto particles. Our method can be easily integrated with existing SPH techniques such as the incompressible PCISPH method as well as weakly compressible SPH by adding an additional force term. We show that this hybrid method accelerates convergence towards uniform density and permits a significantly larger time step compared to earlier approaches while producing similar results. We demonstrate our approach in a variety of scenarios with significant pressure gradients such as splashing liquids. expand


SESSION: Agents, crowds, and skinning 



Simulating heterogeneous crowd behaviors using personality trait theory 

Stephen J. Guy,
Sujeong Kim,
Ming C. Lin,
Dinesh Manocha


Pages: 4352 

doi>10.1145/2019406.2019413 

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We present a new technique to generate heterogeneous crowd behaviors using personality trait theory. Our formulation is based on adopting results of a user study to derive a mapping from crowd simulation parameters to the perceived behaviors of agents ...
We present a new technique to generate heterogeneous crowd behaviors using personality trait theory. Our formulation is based on adopting results of a user study to derive a mapping from crowd simulation parameters to the perceived behaviors of agents in computergenerated crowd simulations. We also derive a linear mapping between simulation parameters and personality descriptors corresponding to the wellestablished Eysenck Threefactor personality model. Furthermore, we propose a novel twodimensional factorization of perceived personality in crowds based on a statistical analysis of the user study results. Finally, we demonstrate that our mappings and factorizations can be used to generate heterogeneous crowd behaviors in different settings. expand


Scenario space: characterizing coverage, quality, and failure of steering algorithms 

Mubbasir Kapadia,
Matt Wang,
Shawn Singh,
Glenn Reinman,
Petros Faloutsos


Pages: 5362 

doi>10.1145/2019406.2019414 

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Navigation and steering in complex dynamically changing environments is a challenging research problem, and a fundamental aspect of immersive virtual worlds. While there exist a wide variety of approaches for navigation and steering, there is no definitive ...
Navigation and steering in complex dynamically changing environments is a challenging research problem, and a fundamental aspect of immersive virtual worlds. While there exist a wide variety of approaches for navigation and steering, there is no definitive solution for evaluating and analyzing steering algorithms. Evaluating a steering algorithm involves two major challenges: (a) characterizing and generating the space of possible scenarios that the algorithm must solve, and (b) defining evaluation criteria (metrics) and applying them to the solution. In this paper, we address both of these challenges. First, we characterize and analyze the complete space of steering scenarios that an agent may encounter in dynamic situations. Then, we propose the representative scenario space and a sampling method that can generate subsets of the representative space with good statistical properties. We also propose a new set of metrics and a statistically robust approach to determining the coverage and the quality of a steering algorithm in this space. We demonstrate the effectiveness of our approach on three state of the art techniques. Our results show that these methods can only solve 60% of the scenarios in the representative scenario space. expand


Physicsbased character skinning using multidomain subspace deformations 

Theodore Kim,
Doug L. James


Pages: 6372 

doi>10.1145/2019406.2019415 

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We propose a domaindecomposition method to simulate articulated deformable characters entirely within a subspace framework. The method supports quasistatic and dynamic deformations, nonlinear kinematics and materials, and can achieve interactive timestepping ...
We propose a domaindecomposition method to simulate articulated deformable characters entirely within a subspace framework. The method supports quasistatic and dynamic deformations, nonlinear kinematics and materials, and can achieve interactive timestepping rates. To avoid artificial rigidity, or "locking," associated with coupling lowrank domain models together with hard constraints, we employ penaltybased coupling forces. The multidomain subspace integrator can simulate deformations efficiently, and exploits efficient subspaceonly evaluation of constraint forces between rotated domains using a novel Fast Sandwich Transform (FST). Examples are presented for articulated characters with quasistatic and dynamic deformations, and interactive performance with hundreds of fully coupled modes. Using our method, we have observed speedups of between three and four orders of magnitude over fullrank, unreduced simulations. expand


Controllable hand deformation from sparse examples with rich details 

Haoda Huang,
Ling Zhao,
KangKang Yin,
Yue Qi,
Yizhou Yu,
Xin Tong


Pages: 7382 

doi>10.1145/2019406.2019416 

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Recent advances in laser scanning technology have made it possible to faithfully scan a real object with tiny geometric details, such as pores and wrinkles. However, a faithful digital model should not only capture static details of the real counterpart ...
Recent advances in laser scanning technology have made it possible to faithfully scan a real object with tiny geometric details, such as pores and wrinkles. However, a faithful digital model should not only capture static details of the real counterpart but also be able to reproduce the deformed versions of such details. In this paper, we develop a datadriven model that has two components respectively accommodating smooth largescale deformations and highresolution deformable details. Largescale deformations are based on a nonlinear mapping between sparse control points and bone transformations. A global mapping, however, would fail to synthesize realistic geometries from sparse examples, for highlydeformable models with a large range of motion. The key is to train a collection of mappings defined over regions locally in both the geometry and the pose space. Deformable finescale details are generated from a second nonlinear mapping between the control points and pervertex displacements. We apply our modeling scheme to scanned human hand models. Experiments show that our deformation models, learned from extremely sparse training data, are effective and robust in synthesizing highlydeformable models with rich fine features, for keyframe animation as well as performancedriven animation. We also compare our results with those obtained by alternative techniques. expand


SESSION: Fluids  mathematical formulation 



A multigrid fluid pressure solver handling separating solid boundary conditions 

Nuttapong Chentanez,
Matthias Müller


Pages: 8390 

doi>10.1145/2019406.2019418 

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We present a multigrid method for solving the linear complementarity problem (LCP) resulting from discretizing the Poisson equation subject to separating solid boundary conditions in an Eulerian liquid simulation's pressure projection step. The method ...
We present a multigrid method for solving the linear complementarity problem (LCP) resulting from discretizing the Poisson equation subject to separating solid boundary conditions in an Eulerian liquid simulation's pressure projection step. The method requires only a few small changes to a multigrid solver for linear systems. Our generalized solver is fast enough to handle 3D liquid simulations with separating boundary conditions in practical domain sizes. Previous methods could only handle relatively small 2D domains in reasonable time because they used expensive quadratic programming (QP) solvers. We demonstrate our technique in several practical scenarios in which the omission of separating boundary conditions results in disturbing artifacts of liquid sticking to walls. expand


Mass and momentum conservation for fluid simulation 

Michael Lentine,
Mridul Aanjaneya,
Ronald Fedkiw


Pages: 91100 

doi>10.1145/2019406.2019419 

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Momentum conservation has long been used as a design principle for solid simulation (e.g. collisions between rigid bodies, massspring elastic and damping forces, etc.), yet it has not been widely used for fluid simulation. In fact, semiLagrangian advection ...
Momentum conservation has long been used as a design principle for solid simulation (e.g. collisions between rigid bodies, massspring elastic and damping forces, etc.), yet it has not been widely used for fluid simulation. In fact, semiLagrangian advection does not conserve momentum, but is still regularly used as a bread and butter method for fluid simulation. In this paper, we propose a modification to the semiLagrangian method in order to make it fully conserve momentum. While methods of this type have been proposed earlier in the computational physics literature, they are not necessarily appropriate for coarse grids, large time steps or inviscid flows, all of which are common in graphics applications. In addition, we show that the commonly used vorticity confinement turbulence model can be modified to exactly conserve momentum as well. We provide a number of examples that illustrate the benefits of this new approach, both in conserving fluid momentum and passively advected scalars such as smoke density. In particular, we show that our new method is amenable to efficient smoke simulation with one time step per frame, whereas the traditional nonconservative semiLagrangian method experiences serious artifacts when run with these large time steps, especially when object interaction is considered. expand


Mathematical foundation of the optimizationbased fluid animation method 

Kenny Erleben,
Marek Krzysztof Misztal,
J. Andreas Bærentzen


Pages: 101110 

doi>10.1145/2019406.2019420 

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We present the mathematical foundation of a fluid animation method for unstructured meshes. Key contributions not previously treated are the extension to include diffusion forces and higher order terms of nonlinear force approximations. In our discretization ...
We present the mathematical foundation of a fluid animation method for unstructured meshes. Key contributions not previously treated are the extension to include diffusion forces and higher order terms of nonlinear force approximations. In our discretization we apply a fractional step method to be able to handle advection in a numerically simple Lagrangian approach. Following this a finite element method is used for the remaining terms of the fractional step method. The key to deriving a discretization for the diffusion forces lies in restating the momentum equations in terms of a Newtonian stress tensor. Rather than applying a straightforward temporal finite difference method followed by a projection method to enforce incompressibility as done in the stable fluids method, the last step of the fractional step method is rewritten as an optimization problem to make it easy to incorporate nonlinear force terms such as surface tension. expand


A simple finite volume method for adaptive viscous liquids 

Christopher Batty,
Ben Houston


Pages: 111118 

doi>10.1145/2019406.2019421 

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We present the first spatially adaptive Eulerian fluid animation method to support challenging viscous liquid effects such as folding, coiling, and variable viscosity. We propose a tetrahedral nodebased embedded finite volume method for fluid viscosity, ...
We present the first spatially adaptive Eulerian fluid animation method to support challenging viscous liquid effects such as folding, coiling, and variable viscosity. We propose a tetrahedral nodebased embedded finite volume method for fluid viscosity, adapted from popular techniques for Lagrangian deformable objects. Applied in an Eulerian fashion with implicit integration, this scheme stably and efficiently supports high viscosity fluids while yielding symmetric positive definite linear systems. To integrate this scheme into standard tetrahedral meshbased fluid simulators, which store normal velocities on faces rather than velocity vectors at nodes, we offer two methods to reconcile these representations. The first incorporates a mapping between different degrees of freedom into the viscosity solve itself. The second uses a FLIPlike approach to transfer velocity data between nodes and faces before and after the linear solve. The former offers tighter coupling by enabling the linear solver to act directly on the face velocities of the staggered mesh, while the latter provides a sparser linear system and a simpler implementation. We demonstrate the effectiveness of our approach with animations of spatially varying viscosity, realistic rotational motion, and viscous liquid buckling and coiling. expand


SESSION: Motion capture 



A datadriven appearance model for human fatigue 

Joseph T. Kider, Jr.,
Kaitlin Pollock,
Alla Safonova


Pages: 119128 

doi>10.1145/2019406.2019423 

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Humans become visibly tired during physical activity. After a set of squats, jumping jacks or walking up a flight of stairs, individuals start to pant, sweat, loose their balance, and flush. Simulating these physiological changes due to exertion and ...
Humans become visibly tired during physical activity. After a set of squats, jumping jacks or walking up a flight of stairs, individuals start to pant, sweat, loose their balance, and flush. Simulating these physiological changes due to exertion and exhaustion on an animated character greatly enhances a motion's realism. These fatigue factors depend on the mechanical, physical, and biochemical function states of the human body. The difficulty of simulating fatigue for character animation is due in part to the complex anatomy of the human body. We present a multimodal capturing technique for acquiring synchronized biosignal data and motion capture data to enhance character animation. The fatigue model utilizes an anatomically derived model of the human body that includes a torso, organs, face, and rigged body. This model is then driven by biosignal output. Our animations show the wide range of exhaustion behaviors synthesized from real biological data output. We demonstrate the fatigue model by augmenting standard motion capture with exhaustion effects to produce more realistic appearance changes during three exercise examples. We compare the fatigue model with both simple procedural methods and a dense marker set data capture of exercise motions. expand


Human motion reconstruction from force sensors 

Sehoon Ha,
Yunfei Bai,
C. Karen Liu


Pages: 129138 

doi>10.1145/2019406.2019424 

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Consumergrade, realtime motion capture devices are becoming commonplace in every household, thanks to the recent development in depthcamera technologies. We introduce a new approach to capturing and reconstructing freeform, fullbody human motion ...
Consumergrade, realtime motion capture devices are becoming commonplace in every household, thanks to the recent development in depthcamera technologies. We introduce a new approach to capturing and reconstructing freeform, fullbody human motion using force sensors, supplementary to existing, consumergrade mocap systems. Our algorithm exploits the dynamic aspects of human movement, such as linear and angular momentum, to provide key information for fullbody motion reconstruction. Using two pressure sensing platforms (Wii Balance Board) and a hand tracking device, we demonstrate that human motion can be largely reconstructed from ground reaction forces along with a small amount of arm movement information. expand


Practical colorbased motion capture 

Robert Wang,
Sylvain Paris,
Jovan Popović


Pages: 139146 

doi>10.1145/2019406.2019425 

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Motion capture systems have been widely used for high quality content creation and virtual reality but are rarely used in consumer applications due to their price and setup cost. In this paper, we propose a motion capture system built from commodity ...
Motion capture systems have been widely used for high quality content creation and virtual reality but are rarely used in consumer applications due to their price and setup cost. In this paper, we propose a motion capture system built from commodity components that can be deployed in a matter of minutes. Our approach uses one or more webcams and a color shirt to track the upperbody at interactive rates. We describe a robust color calibration system that enables our colorbased tracking to work against cluttered backgrounds and under multiple illuminants. We demonstrate our system in several realworld indoor and outdoor settings. expand


Realtime classification of dance gestures from skeleton animation 

Michalis Raptis,
Darko Kirovski,
Hugues Hoppe


Pages: 147156 

doi>10.1145/2019406.2019426 

Full text: PDF


We present a realtime gesture classification system for skeletal wireframe motion. Its key components include an angular representation of the skeleton designed for recognition robustness under noisy input, a cascaded correlationbased classifier for ...
We present a realtime gesture classification system for skeletal wireframe motion. Its key components include an angular representation of the skeleton designed for recognition robustness under noisy input, a cascaded correlationbased classifier for multivariate timeseries data, and a distance metric based on dynamic timewarping to evaluate the difference in motion between an acquired gesture and an oracle for the matching gesture. While the first and last tools are generic in nature and could be applied to any gesturematching scenario, the classifier is conceived based on the assumption that the input motion adheres to a known, canonical timebase: a musical beat. On a benchmark comprising 28 gesture classes, hundreds of gesture instances recorded using the XBOX Kinect platform and performed by dozens of subjects for each gesture class, our classifier has an average accuracy of 96:9%, for approximately 4second skeletal motion recordings. This accuracy is remarkable given the input noise from the realtime depth sensor. expand


A puppet interface for retrieval of motion capture data 

Naoki Numaguchi,
Atsushi Nakazawa,
Takaaki Shiratori,
Jessica K. Hodgins


Pages: 157166 

doi>10.1145/2019406.2019427 

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Intuitive and efficient retrieval of motion capture data is essential for effective use of motion capture databases. In this paper, we describe a system that allows the user to retrieve a particular sequence by performing an approximation of the motion ...
Intuitive and efficient retrieval of motion capture data is essential for effective use of motion capture databases. In this paper, we describe a system that allows the user to retrieve a particular sequence by performing an approximation of the motion with an instrumented puppet. This interface is intuitive because both adults and children have experience playacting with puppets and toys to express particular behaviors or to tell stories with style and emotion. The puppet has 17 degrees of freedom and can therefore represent a variety of motions. We develop a novel similarity metric between puppet and human motion by computing the reconstruction errors of the puppet motion in the latent space of the human motion and those of the human motion in the latent space of the puppet motion. This metric works even for relatively large databases. We conducted a user study of the system and subjects could find the desired motion with reasonable accuracy from a database consisting of everyday, exercise, and acrobatic behaviors. expand


SESSION: Fluids  control and procedural modeling 



Procedural fluid modeling of explosion phenomena based on physical properties 

Genichi Kawada,
Takashi Kanai


Pages: 167176 

doi>10.1145/2019406.2019429 

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We propose a method to procedurally model the fluid flows of explosion phenomena by taking physical properties into account. Explosion flows are always quite difficult to control, because they easily disturb each other and change rapidly. With this method, ...
We propose a method to procedurally model the fluid flows of explosion phenomena by taking physical properties into account. Explosion flows are always quite difficult to control, because they easily disturb each other and change rapidly. With this method, the target flows are described by control paths, and the propagation flows are controlled by following these paths. We consider the physical properties, which are the propagations of the pressure generated by the ignition, the detonation state caused by the pressure and the fuel combustions. Velocity, density, temperature and pressure fields are generated procedurally, and the fluid flows are computed from these four fields based on gridbased fluid simulations. Using this method, we can achieve a fluid motion that closely resembles one generated solely through simulation. This method realizes the modeling of flows controlled frame by frame and follows the flow's physical properties. expand


Previewbased sampling for controlling gaseous simulations 

Ruoguan Huang,
Zeki Melek,
John Keyser


Pages: 177186 

doi>10.1145/2019406.2019430 

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In this work, we describe an automated method for directing the control of a high resolution gaseous fluid simulation based on the results of a lower resolution preview simulation. Small variations in accuracy between low and high resolution grids can ...
In this work, we describe an automated method for directing the control of a high resolution gaseous fluid simulation based on the results of a lower resolution preview simulation. Small variations in accuracy between low and high resolution grids can lead to divergent simulations, which is problematic for those wanting to achieve a desired behavior. Our goal is to provide a simple method for ensuring that the high resolution simulation matches key properties from the lower resolution simulation. We first let a user specify a fast, coarse simulation that will be used for guidance. Our automated method samples the data to be matched at various positions and scales in the simulation, or allows the user to identify key portions of the simulation to maintain. During the high resolution simulation, a matching process ensures that the properties sampled from the low resolution simulation are maintained. This matching process keeps the different resolution simulations aligned even for complex systems, and can ensure consistency of not only the velocity field, but also advected scalar values. Because the final simulation is naturally similar to the preview simulation, only minor controlling adjustments are needed, allowing a simpler control method than that used in prior keyframing approaches. expand


Graphbased fire synthesis 

Yubo Zhang,
Carlos D. Correa,
KwanLiu Ma


Pages: 187194 

doi>10.1145/2019406.2019431 

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We present a novel graphbased datadriven technique for costeffective fire modeling. This technique allows composing long animation sequences using a small number of short simulations. While traditional techniques such as motion graphs and motion blending ...
We present a novel graphbased datadriven technique for costeffective fire modeling. This technique allows composing long animation sequences using a small number of short simulations. While traditional techniques such as motion graphs and motion blending work well for character motion synthesis, they cannot be trivially applied to fluids to produce results with physically consistent properties which are crucial to the visual appearance of fluids. Motivated by the motion graph technique used in character animations, we introduce a new type of graph which can be applied to create various fire phenomena. Each graph node consists of a group of compact spatialtemporal flow pathlines instead of a set of volumetric state fields. Consequently, achieving smooth transitions between discontinuous graph nodes for modeling turbulent fires becomes feasible and computationally efficient. The synthesized particle flow results allow direct particle controls which is much more flexible than a full volumetric representation of the simulation output. The accompanying video shows the versatility and potential power of this new technique for synthesizing realtime complex fire at the quality comparable to production animations. expand


SESSION: Facial animation 



Content retargeting using parameterparallel facial layers 

Natasha Kholgade,
Iain Matthews,
Yaser Sheikh


Pages: 195204 

doi>10.1145/2019406.2019433 

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Facial motion retargeting approaches often transfer expressions by establishing correspondences between shared units of motion, such as action units, or spatial correspondences of landmarks between the source actor and target character faces. When the ...
Facial motion retargeting approaches often transfer expressions by establishing correspondences between shared units of motion, such as action units, or spatial correspondences of landmarks between the source actor and target character faces. When the actor and character are structurally dissimilar, shared units of motion or spatial landmarks may not exist, and subtle styles of performance may differ. We present a method to deconstruct the content of an actor's facial expression into three parameterparallel layers using a composition function, transfer the content to equivalent parameterparallel layers for the character, and reconstruct the character's expression using the same composition function. Our algorithm uses the same parameterparallel layered model of facial expression for both the actor and character, separating the content of facial expressions into emotion, speech, and eyeblink layers. Facial motion in each layer is embedded in simplicial bases, each of which encodes semantically significant configurations of the face. We show the transfer of facial motion capture and videobased tracking of the eyes and mouth of an actor to a number of faces with dissimilar facial structure and expressive disposition. expand


Facial cartography: interactive scan correspondence 

Cyrus A. Wilson,
Oleg Alexander,
Borom Tunwattanapong,
Pieter Peers,
Abhijeet Ghosh,
Jay Busch,
Arno Hartholt,
Paul Debevec


Pages: 205214 

doi>10.1145/2019406.2019434 

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We present a semiautomatic technique for computing surface correspondences between 3D facial scans in different expressions, such that scan data can be mapped into a common domain for facial animation. The technique can accurately correspond highresolution ...
We present a semiautomatic technique for computing surface correspondences between 3D facial scans in different expressions, such that scan data can be mapped into a common domain for facial animation. The technique can accurately correspond highresolution scans of widely differing expressions  without requiring intermediate pose sequences  such that they can be used, together with reflectance maps, to create highquality blendshapebased facial animation. We optimize correspondences through a combination of Image, Shape, and Internal forces, as well as Directable forces to allow a user to interactively guide and refine the solution. Key to our method is a novel representation, called an Active Visage, that balances the advantages of both deformable templates and correspondence computation in a 2D canonical domain. We show that our semiautomatic technique achieves more robust results than automated correspondence alone, and is more precise than is practical with unaided manual input. expand


Realtime facial animation from live video tracking 

Taehyun Rhee,
Youngkyoo Hwang,
James Dokyoon Kim,
Changyeong Kim


Pages: 215224 

doi>10.1145/2019406.2019435 

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This paper describes a complete pipeline of a practical system for producing realtime facial expressions of a 3D virtual avatar controlled by an actor's live performances. The system handles practical challenges arising from markerless expression captures ...
This paper describes a complete pipeline of a practical system for producing realtime facial expressions of a 3D virtual avatar controlled by an actor's live performances. The system handles practical challenges arising from markerless expression captures from a single conventional video camera. For robust tracking, a localized algorithm constrained by belief propagation is applied to the upper face, and an appearance matching technique using a parameterized generic face model is exploited for lower face and head pose tracking. The captured expression features then transferred to high dimensional 3D animation controls using our facial expression space which is a structurepreserving map between two algebraic structures. The transferred animation controls drive facial animation of a 3D avatar while optimizing the smoothness of the face mesh. An examplebased face deformation technique produces nonlinear local detail deformations on the avatar that are not captured in the movement of the animation controls. expand


SESSION: Deformable objects 



Optimization for sagfree simulations 

Christopher D. Twigg,
Zoran KačićAlesić


Pages: 225236 

doi>10.1145/2019406.2019437 

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A common problem during the first few seconds of a cloth, hair, or flesh simulation is that the mesh sags under gravity, which can undo the work of hours of careful modeling. The typical response of increasing the stiffness of the mesh is unsatisfactory ...
A common problem during the first few seconds of a cloth, hair, or flesh simulation is that the mesh sags under gravity, which can undo the work of hours of careful modeling. The typical response of increasing the stiffness of the mesh is unsatisfactory as it increases the computational cost of simulation and adversely impacts the quality of the resulting motion. Modelers are accustomed to creating geometry as it is found in the real world, which already includes the effect of gravity. We propose a fast and effective approach for optimizing parameters such as spring rest lengths so that the artistically modeled shape represents the equilibrium after the mesh has settled under gravity. This eliminates sagging, preserves the quality of motion, and is intuitive for the artists. expand


Robust realtime deformation of incompressible surface meshes 

R. Diziol,
J. Bender,
D. Bayer


Pages: 237246 

doi>10.1145/2019406.2019438 

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We introduce an efficient technique for robustly simulating incompressible objects with thousands of elements in realtime. Instead of considering a tetrahedral model, commonly used to simulate volumetric bodies, we simply use their surfaces. Not requiring ...
We introduce an efficient technique for robustly simulating incompressible objects with thousands of elements in realtime. Instead of considering a tetrahedral model, commonly used to simulate volumetric bodies, we simply use their surfaces. Not requiring hundreds or even thousands of elements in the interior of the object enables us to simulate more elements on the surface, resulting in high quality deformations at low computation costs. The elasticity of the objects is robustly simulated with a geometrically motivated shape matching approach which is extended by a fast summation technique for arbitrary triangle meshes suitable for an efficient parallel computation on the GPU. Moreover, we present an oscillationfree and collisionaware volume constraint, purely based on the surface of the incompressible body. The novel heuristic we propose in our approach enables us to conserve the volume, both globally and locally. Our volume constraint is not limited to the shape matching method and can be used with any method simulating the elasticity of an object. We present several examples which demonstrate high quality volume conserving deformations and compare the runtimes of our CPU implementation, as well as our GPU implementation with similar methods. expand


Asynchronous integration with phantom meshes 

David Harmon,
Qingnan Zhou,
Denis Zorin


Pages: 247256 

doi>10.1145/2019406.2019439 

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Asynchronous variational integration of layered contact models provides a framework for robust collision handling, correct physical behavior, and guaranteed eventual resolution of even the most difficult contact problems. Yet, even for lowcontact scenarios, ...
Asynchronous variational integration of layered contact models provides a framework for robust collision handling, correct physical behavior, and guaranteed eventual resolution of even the most difficult contact problems. Yet, even for lowcontact scenarios, this approach is significantly slower compared to its less robust alternativesoften due to handling of stiff elastic forces in an explicit framework. We propose a method that retains the guarantees, but allows for variational implicit integration of some of the forces, while maintaining asynchronous integration needed for contact handling. Our method uses phantom meshes for calculations with stiff forces, which are then coupled to the original mesh through constraints. We use the augmented discrete Lagrangian of the constrained system to derive a variational integrator with the desired conservation properties. expand


Elementwise mixed implicitexplicit integration for stable dynamic simulation of deformable objects 

B. Fierz,
J. Spillmann,
M. Harders


Pages: 257266 

doi>10.1145/2019406.2019440 

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In order to evolve a deformable object in time, the underlying equations of motion have to be numerically integrated. This is commonly done by employing either an explicit or an implicit integration scheme. While explicit methods are only stable for ...
In order to evolve a deformable object in time, the underlying equations of motion have to be numerically integrated. This is commonly done by employing either an explicit or an implicit integration scheme. While explicit methods are only stable for small time steps, implicit methods are unconditionally stable. In this paper, we present a novel methodology to combine explicit and implicit linear integration approaches, based on elementwise stability considerations. First, we detect the illshaped simulation elements which hinder the stable explicit integration of the element nodes as a precomputation step. These nodes are then simulated implicitly, while the remaining parts of the mesh are explicitly integrated. As a consequence, larger integration time steps than in purely explicit methods are possible, while the computation time per step is smaller than in purely implicit integration. During modifications such as cutting or fracturing, only newly created or modified elements need to be reevaluated, thus making the technique usable in realtime simulations. In addition, our method reduces problems due to numerical dissipation. expand


SESSION: Motion editing 



Biomechanicallyinspired motion path editing 

Noah Lockwood,
Karan Singh


Pages: 267276 

doi>10.1145/2019406.2019442 

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We present a system for interactive kinematic editing of motion paths and timing that employs various biomechanical observations to augment and restrict the edited motion. Realistic path manipulations are enforced by restricting user interaction to handles ...
We present a system for interactive kinematic editing of motion paths and timing that employs various biomechanical observations to augment and restrict the edited motion. Realistic path manipulations are enforced by restricting user interaction to handles identified along a motion path using motion extrema. An asrigidaspossible deformation technique modified specifically for use on motion paths is used to deform the path to satisfy the usermanipulated handle positions. After all motion poses have been adjusted to satisfy the new path, an automatic timewarping step modifies the timing of the new motion to preserve the timing qualities of the original motion. This timewarp is based on biomechanical heuristics relating velocity to stride length and path curvature, as well as the preservation of acceleration for ballistic motion. We show that our system can be used to quickly and easily modify a variety of locomotive motions, and can accurately reproduce recorded motions that were not used during the editing process. expand


Spacetime vertex constraints for dynamicallybased adaptation of motioncaptured animation 

C. O'Brien,
J. Dingliana,
S. Collins


Pages: 277286 

doi>10.1145/2019406.2019443 

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We present a novel technique for editing motion captured animation. Our iterative solver produces physicallyplausible adaptated animations that satisfy alterations in foot and hand contact placement with the animated character's surroundings. The technique ...
We present a novel technique for editing motion captured animation. Our iterative solver produces physicallyplausible adaptated animations that satisfy alterations in foot and hand contact placement with the animated character's surroundings. The technique uses a system of particles to represent the poses and mass distribution of the character at sampled frames of the animation. Constraints between the vertices within each frame enforce the skeletal structure, including joint limits. Novel constraints extending over vertices in several frames enforce the aggregate dynamics of the character, as well as features such as joint acceleration smoothness. We demonstrate adaptation of several animations to altered foot and hand placement. expand


Perceptual evaluation of footskate cleanup 

Martin Pražák,
Ludovic Hoyet,
Carol O'Sullivan


Pages: 287294 

doi>10.1145/2019406.2019444 

Full text: PDF


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When animating virtual humans for realtime applications such as games and virtual reality, animation systems often have to edit motions in order to be responsive. In many cases, contacts between the feet and the ground are not (or cannot be) properly ...
When animating virtual humans for realtime applications such as games and virtual reality, animation systems often have to edit motions in order to be responsive. In many cases, contacts between the feet and the ground are not (or cannot be) properly enforced, resulting in a disturbing artifact know as footsliding or footskate. In this paper, we explore the perceptibility of this error and show that participants can perceive even very low levels of footsliding (<21mm in most conditions). We then explore the visual fidelity of animations where footskate has been cleaned up using two different methods. We found that corrected animations were always preferred to those with footsliding, irrespective of the extent of the correction required. We also determined that a simple approach of lengthening limbs was preferred to a more complex approach using IK fixes and trajectory smoothing. expand
