SCA: Eurographics/SIGGRAPH Symposium on Computer Animation

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Accurate Simulation of Wound Healing and Skin Deformation
(The Eurographics Association, 2016) Feess, Stefan; Kurfiss, Kathrin; Fedkiw, Ronald P.; Michels, Dominik L.; Ladislav Kavan and Chris Wojtan
We devise a method for the accurate simulation of wound healing and skin deformation. This is based on adequate formulations modeling the underlying biological processes. Cell movements and proliferation are described by a biochemical model whereas a biomechanical model covers effects like wound contraction and the influence of the healing process on the surrounding skin. The resulting simulation framework is very efficient and can be used with realistic input parameters like those measured in biochemistry and biophysics. The accurate behavior of our approach is shown by reproducing regenerative healing processes as well as specific effects such as anisotropic wound contraction, scarring and scab formation. Its efficiency and robustness is illustrated on a broad spectrum of complex examples.
Accurate Surface Embedding for Higher Order Finite Elements
(ACM SIGGRAPH / Eurographics Association, 2013) Suwelack, Stefan; Lukarski, Dimitar; Heuveline, Vincent; Dillmann, Rüdiger; Speidel, Stefanie; Theodore Kim and Robert Sumner
In this paper we present a novel approach to efficiently simulate the deformation of highly detailed meshes using higher order finite elements (FE). An efficient algorithm based on non-linear optimization is proposed in order to find the closest point in the curved computational FE mesh for each surface vertex. In order to extrapolate deformations to surface points outside the FE mesh, we introduce a mapping scheme that generates smooth surface deformations and preserves local shape even for low-resolution computational meshes. The mapping is constructed by representing each surface vertex in terms of points on the computational mesh and its distance to the FE mesh in normal direction. A numerical analysis shows that the mapping can be robustly constructed using the proposed non-linear optimization technique. Furthermore it is demonstrated that the numerical complexity of the mapping scheme is linear in the number of surface nodes and independent of the size of the coarse computational mesh.
Adaptive Tetrahedral Meshes for Brittle Fracture Simulation
(The Eurographics Association, 2014) Koschier, Dan; Lipponer, Sebastian; Bender, Jan; Vladlen Koltun and Eftychios Sifakis
We present a method for the adaptive simulation of brittle fracture of solid objects based on a novel reversible tetrahedral mesh refinement scheme. The refinement scheme preserves the quality of the input mesh to a large extent, it is solely based on topological operations, and does not alter the boundary, i.e. any geometric feature. Our fracture algorithm successively performs a stress analysis and increases the resolution of the input mesh in regions of high tensile stress. This results in an accurate location of crack origins without the need of a general high resolution mesh which would cause high computational costs throughout the whole simulation. A crack is initiated when the maximum tensile stress exceeds the material strength. The introduced algorithm then proceeds by iteratively recomputing the changed stress state and creating further cracks. Our approach can generate multiple cracks from a single impact, but effectively avoids shattering artifacts. Once the tensile stress decreases, the mesh refinement is reversed to increase the performance of the simulation. We demonstrate that our adaptive method is robust, scalable and computes highly realistic fracture results.
Anisotropic Friction for Deformable Surfaces and Solids
(ACM SIGGRAPH / Eurographics Association, 2009) Pabst, Simon; Thomaszewski, Bernhard; Straßer, Wolfgang; Eitan Grinspun and Jessica Hodgins
This paper presents a method for simulating anisotropic friction for deforming surfaces and solids. Frictional contact is a complex phenomenon that fuels research in mechanical engineering, computational contact mechanics, composite material design and rigid body dynamics, to name just a few. Many real-world materials have anisotropic surface properties. As an example, most textiles exhibit direction-dependent frictional behavior, but despite its tremendous impact on visual appearance, only simple isotropic models have been considered for cloth and solid simulation so far. In this work, we propose a simple, application-oriented but physically sound model that extends existing methods to account for anisotropic friction. The sliding properties of surfaces are encoded in friction tensors, which allows us to model frictional resistance freely along arbitrary directions. We also consider heterogeneous and asymmetric surface roughness and demonstrate the increased simulation quality on a number of two- and three-dimensional examples. Our method is computationally efficient and can easily be integrated into existing systems.
Art-Directed Muscle Simulation for High-End Facial Animation
(The Eurographics Association, 2016) Cong, Matthew; Bhat, Kiran S.; Fedkiw, Ronald P.; Ladislav Kavan and Chris Wojtan
We propose a new framework for the simulation of facial muscle and flesh that so significantly improves the technique that it allows for immediate mainstream use of anatomically and biomechanically accurate muscle models as a bread and butter technique in a high-end production quality pipeline. The key idea is to create a blendshape system for the muscles that gives the precise directability and controllability required in a high-end production environment. The blendshape muscles are used to drive the underlying anatomically and biomechanically motivated simulation in a way that is unbound by the typical restrictions of a simulation system while still retaining the desirable degree of freedom richness that leads to high quality results. We show that we are able to target production quality facial shapes, whether from scans or an animation system, and illustrate that the resulting nonlinear simulation in-betweens are of higher quality than those obtained from traditional linear blendshapes. We also demonstrate the ability to selectively improve areas on a given blendshape using the results of a simulation, as well as the ability to edit muscle shapes and paths in order to produce directability for animator control. Then, we show how these techniques can be used to transition from one blendshape to another or even track and selectively modify an entire performance. The efficacy of our system is further demonstrated by using it to retarget animation onto new creature models given only a single static rest pose as input.
Artistic Simulation of Curly Hair
(ACM SIGGRAPH / Eurographics Association, 2013) Iben, Hayley; Meyer, Mark; Petrovic, Lena; Soares, Olivier; Anderson, John; Witkin, Andrew; Theodore Kim and Robert Sumner
Artistic simulation of hair presents many challenges - ranging from incorporating artistic control to dealing with extreme motions of characters. Additionally, in a production environment, the simulation needs to be fast and results need to be usable ''out of the box'' (without extensive parameter modifications) in order to produce content efficiently. These challenges are only increased when simulating curly, stylized hair. We present a method for stably simulating stylized curly hair that addresses these artistic needs and performance demands. To satisfy the artistic requirement of maintaining the curl's helical shape during motion, we propose a hair model based upon an extensible elastic rod. We introduce a method for stably computing a frame along the hair curve, essential for stable simulation of curly hair. Our hair model uses a spring for controlling the bending of the curl and another for maintaining the helical shape during extension. We also address performance concerns often associated with handling hair-hair contact interactions by efficiently parallelizing the simulation. To do so, we present a technique for pruning both hair-hair contact pairs and hair particles. Our method has been used on two full length feature films and has proven to be robust and stable over a wide range of animated motion and on a variety of hair styles, from straight to wavy to curly. It has proven invaluable in providing controllable, stable and efficient simulation allowing our artists to achieve their desired performance even when facing strict scheduling demands.
Asynchronous Implicit Backward Euler Integration
(The Eurographics Association, 2016) Zhao, Danyong; Li, Yijing; Barbic, Jernej; Ladislav Kavan and Chris Wojtan
In standard deformable object simulation in computer animation, all the mesh elements or vertices are timestepped synchronously, i.e., under the same timestep. Previous asynchronous methods have been largely limited to explicit integration. We demonstrate how to perform spatially-varying timesteps for the widely popular implicit backward Euler integrator. Spatiallyvarying timesteps are useful when the object exhibits spatially-varying material properties such as Young's modulus or mass density. In synchronous simulation, a region with a high stiffness (or low mass density) will force a small timestep for the entire mesh, at a great computational cost, or else, the motion in the stiff (or low mass density) region will be artificially damped and inaccurate. Our method can assign smaller timesteps to stiffer (or lighter) regions, which makes it possible to properly resolve (sample) the high-frequency deformable dynamics arising from the stiff (or light) materials, resulting in greater accuracy and less artificial damping. Because soft (or heavy) regions can continue using a large timestep, our method provides a significantly higher accuracy under a fixed computational budget.
Building and Animating User-Specific Volumetric Face Rigs
(The Eurographics Association, 2016) Ichim, Alexandru-Eugen; Kavan, Ladislav; Nimier-David, Merlin; Pauly, Mark; Ladislav Kavan and Chris Wojtan
Currently, the two main approaches to realistic facial animation are 1) blendshape models and 2) physics-based simulation. Blendshapes are fast and directly controllable, but it is not easy to incorporate features such as dynamics, collision resolution, or incompressibility of the flesh. Physics-based methods can deliver these effects automatically, but modeling of muscles, bones, and other anatomical features of the face is difficult, and direct control over the resulting shape is lost. We propose a method that combines the benefits of blendshapes with the advantages of physics-based simulation. We acquire 3D scans of a given actor with various facial expressions and compute a set of volumetric blendshapes that are compatible with physics-based simulation, while accurately matching the input scans. Furthermore, our volumetric blendshapes are driven by the same weights as traditional blendshapes, which many users are familiar with. Our final facial rig is capable of delivering physics-based effects such as dynamics and secondary motion, collision response, and volume preservation without the burden of detailed anatomical modeling.
CANVAS: Computer-Assisted Narrative Animation Synthesis
(The Eurographics Association, 2016) Kapadia, Mubbasir; Frey, Seth; Shoulson, Alexander; Sumner, Robert W.; Gross, Markus; Ladislav Kavan and Chris Wojtan
Despite the maturity in solutions for animating expressive virtual characters, innovations realizing the creative intent of story writers have yet to make the same strides. The key challenge is to provide an accessible, yet expressive interface for story authoring that enables the rapid prototyping, iteration, and deployment of narrative concepts.We present CANVAS, a computerassisted visual authoring tool for synthesizing multi-character animations from sparsely-specified narrative events. In a process akin to storyboarding, authors lay out the key plot points in a story, and our system automatically fills in the missing details to synthesize a 3D animation that meets author constraints. CANVAS can be used by artists and directors to pre-visualize storyboards in an iterative fashion, and casual users may provide arbitrarily sparse specifications and harness automation to rapidly generate diverse narratives. CANVAS provides an accessible interface for rapidly authoring and pre-visualizing complex narratives. Automation reduces the authoring effort further without undermining creative control or interfering with the storytelling process.
Cloning Crowd Motions
(The Eurographics Association, 2012) Li, Yi; Christie, Marc; Siret, Orianne; Kulpa, Richard; Pettré, Julien; Jehee Lee and Paul Kry
This paper introduces a method to clone crowd motion data. Our goal is to efficiently animate large crowds from existing examples of motions of groups of characters by applying an enhanced copy and paste technique on them. Specifically, we address spatial and temporal continuity problems to enable animation of significantly larger crowds than our initial data. We animate many characters from the few examples with no limitation on duration. Moreover, our animation technique answers the needs of real-time applications through a technique of linear complexity. Therefore, it is significantly more efficient than any existing crowd simulation-based technique, and in addition, we ensure a predictable level of realism for animations. We provide virtual population designers and animators with a powerful framework which (i) enables them to clone crowd motion examples while preserving the complexity and the aspect of group motion and (ii) is able to animate large-scale crowds in real-time. Our contribution is the formulation of the cloning problem as a double search problem. Firstly, we search for almost periodic portions of crowd motion data through the available examples. Secondly, we search for almost symmetries between the conditions at the limits of these portions in order to interconnect them. The result of our searches is a set of crowd patches that contain portions of example data that can be used to compose large and endless animations. Through several examples prepared from real crowd motion data, we demonstrate the advantageous properties of our approach as well as identify its potential for future developments.
Combining Marker-based Mocap and RGB-D Camera for Acquiring High-fidelity Hand Motion Data
(The Eurographics Association, 2012) Zhao, Wenping; Chai, Jinxiang; Xu, Ying-Qing; Jehee Lee and Paul Kry
Motion capture data has been pivotal to the success of creating realistic animation for human characters. There are a number of public full-body motion databases available, but large and heterogeneous databases for hand articulations are not available. In this paper, we introduce a novel acquisition framework for acquiring a wide range of high-fidelity hand motion data. Our key idea is to leverage marker position data recorded by a twelvecamera optical motion capture system and RGB/Depth data obtained from a single Microsoft Kinect camera. We formulate the hand motion capture problem in a nonlinear optimization framework by maximizing consistency between the reconstructed motion and observed measurement. We introduce an efficient optimization technique to estimate the optimal hand pose that best matches observed data.We have demonstrated the power and effectiveness of our system by capturing a wide variety of delicate hand articulations, even in case of significant self-occlusion.
Component-based Locomotion Composition
(The Eurographics Association, 2012) Kim, Yejin; Neff, Michael; Jehee Lee and Paul Kry
When generating locomotion, it is particularly challenging to adjust the motion's style. This paper introduces a component-based system for human locomotion composition that drives off a set of example locomotion clips. The distinctive style of each example is analyzed in the form of sub-motion components decomposed from separate body parts via independent component analysis (ICA). During the synthesis process, we use these components as combinatorial ingredients to generate new locomotion sequences that are stylistically different from the example set. Our system is designed for novice users who do not have much knowledge of important locomotion properties, such as the correlations throughout the body. Thus, the proposed system analyzes the examples in a unsupervised manner and synthesizes an output locomotion from a small number of control parameters. Our experimental results show that the system can generate physically plausible locomotion in a desired style at interactive speed.
Compressing Fluid Subspaces
(The Eurographics Association, 2016) Jones, Aaron Demby; Sen, Pradeep; Kim, Theodore; Ladislav Kavan and Chris Wojtan
Subspace fluid simulations, also known as reduced-order simulations, can be extremely fast, but also require basis matrices that consume an enormous amount of memory. Motivated by the extreme sparsity of Laplacian eigenfunctions in the frequency domain, we design a frequency-space codec that is capable of compressing basis matrices by up to an order of magnitude. However, if computed naïvely, decompression can be highly inefficient and dominate the running time, effectively negating the advantage of the subspace approach. We show how to significantly accelerate the decompressor by performing the key matrix-vector product in the sparse frequency domain. Subsequently, our codec only adds a factor of three or four to the overall runtime. The compression preserves the overall quality of the simulation, which we show in a variety of examples.
Consistent Surface Model for SPH-based Fluid Transport
(ACM SIGGRAPH / Eurographics Association, 2013) Orthmann, Jens; Hochstetter, Hendrik; Bader, Julian; Bayraktar, Serkan; Kolb, Andreas; Theodore Kim and Robert Sumner
Surface effects play an essential role in fluid simulations. A vast number of dynamics including wetting of surfaces, cleansing, and foam dynamics are based on surface-surface and surface-bulk interactions, which in turn rely on a robust surface computation. In this paper we introduce a conservative Lagrangian formulation of surface effects based upon incompressible smoothed particle hydrodynamics (SPH). The key concept of our approach is to realize an implicit definition of the fluid's (free) surface by assigning each particle a value estimating its surface area. Based on this consistent surface representation, a conservative coupling of bulk and surface is achieved. We demonstrate the applicability and robustness of our approach for several types of surface-relevant effects including adsorption, diffusion and reaction kinetics.
Constrained Neighbor Lists for SPH-based Fluid Simulations
(The Eurographics Association, 2016) Winchenbach, Rene; Hochstetter, Hendrik; Kolb, Andreas; Ladislav Kavan and Chris Wojtan
In this paper we present a new approach to create neighbor lists with strict memory bounds for incompressible Smoothed Particle Hydrodynamics (SPH) simulations. Our proposed approach is based on a novel efficient predictive-corrective algorithm that locally adjusts particle support radii in order to yield neighborhoods of a user-defined maximum size. Due to the improved estimation of the initial support radius, our algorithm is able to efficiently calculate neighborhoods in a single iteration in almost any situation. We compare our neighbor list algorithm to previous approaches and show that our proposed approach can handle larger particle numbers on a single GPU due to its strict guarantees and is able to simulate more particles in real time due to its benefits in regard to performance. Additionally we demonstrate the versatility and stability of our approach in several different scenarios, for example multi-scale simulations and with different kernel functions.
Contact-Invariant Optimization for Hand Manipulation
(The Eurographics Association, 2012) Mordatch, Igor; Popovic, Zoran; Todorov, Emanuel; Jehee Lee and Paul Kry
We present a method for automatic synthesis of dexterous hand movements, given only high-level goals specifying what should happen to the object being manipulated. Results are presented on a wide range of tasks including grasping and picking up objects, twirling them between the fingers, tossing and catching, drawing. This work is an extension of the recent contact-invariant optimization (CIO) method, which introduced auxiliary decision variables directly specifying when and where contacts should occur, and optimized these variables together with the movement trajectory. Our contribution here is extending the unique contact model used in CIO which was specific to locomotion tasks, as well as applying the extended method systematically to hand manipulation.
Control of Rotational Dynamics for Ground Behaviors
(ACM SIGGRAPH / Eurographics Association, 2013) Brown, David F.; Macchietto, Adriano; Yin, KangKang; Zordan, Victor; Theodore Kim and Robert Sumner
This paper proposes a physics-based framework to generate rolling behaviors with significant rotational components. The proposed technique is a general approach for guiding coordinated action that can be layered over existing control architectures through the purposeful regulation of specific whole-body features. Namely, we apply control for rotation through the specification and execution of specific desired 'rotation indices' for whole-body orientation, angular velocity and angular momentum control. We account for the stylistic components of behaviors through reference posture control. The novelty of the described work includes control over behaviors with considerable rotational components as well as a number of characteristics useful for general control, such as flexible posture tracking and contact control planning.
Controlling Liquids Using Meshes
(The Eurographics Association, 2012) Raveendran, Karthik; Thuerey, Nils; Wojtan, Chris; Turk, Greg; Jehee Lee and Paul Kry
We present an approach for artist-directed animation of liquids using multiple levels of control over the simulation, ranging from the overall tracking of desired shapes to highly detailed secondary effects such as dripping streams, separating sheets of fluid, surface waves and ripples. The first portion of our technique is a volume preserving morph that allows the animator to produce a plausible fluid-like motion from a sparse set of control meshes. By rasterizing the resulting control meshes onto the simulation grid, the mesh velocities act as boundary conditions during the projection step of the fluid simulation. We can then blend this motion together with uncontrolled fluid velocities to achieve a more relaxed control over the fluid that captures natural inertial effects. Our method can produce highly detailed liquid surfaces with control over sub-grid details by using a mesh-based surface tracker on top of a coarse grid-based fluid simulation. We can create ripples and waves on the fluid surface attracting the surface mesh to the control mesh with spring-like forces and also by running a wave simulation over the surface mesh. Our video results demonstrate how our control scheme can be used to create animated characters and shapes that are made of water.
Creature Features: Online motion puppetry for non-human characters
(ACM SIGGRAPH / Eurographics Association, 2013) Seol, Yeongho; O'Sullivan, Carol; Lee, Jehee; Theodore Kim and Robert Sumner
We present a novel real-time motion puppetry system that drives the motion of non-human characters using human motion input. We aim to control a variety of creatures whose body structures and motion patterns can differ greatly from a human's. A combination of direct feature mapping and motion coupling enables the generation of natural creature motion, along with intuitive and expressive control for puppetry. First, in the design phase, direct feature mappings and motion classification can be efficiently and intuitively computed given crude motion mimicking as input. Later, during the puppetry phase, the user's body motions are used to control the target character in real-time, using the combination of feature mappings generated from the design phase. We demonstrate the effectiveness of our approach with several examples of natural puppetry, where a variety of non-human creatures are controlled in real-time using human motion input from a commodity motion sensing device.
Data-driven Glove Calibration for Hand Motion Capture
(ACM SIGGRAPH / Eurographics Association, 2013) Wang, Yingying; Neff, Michael; Theodore Kim and Robert Sumner
Hand motion is an important component of human motion, playing a central role in communication. However, it is difficult to capture hand motion optically, especially in conjunction with full body motion. Due to a lack of appropriate calibration methods, data gloves also do not provide sufficiently accurate hand motion. In this paper, we present a novel glove calibration approach that can map raw sensor readings to hand motion data with both accurate joint rotations and fingertip positions. Our method elegantly handles the sensor coupling problem by treating calibration as a flexible mapping from sensor readings to joint rotations. A sampling process collects data tuples according to accuracy requirements, and organizes all the tuples in a training set. From these data, a specially designed Gaussian Process Regression model is trained to infer the calibration function, and the learned model can be used to calibrate new sensor readings. For real-time hand motion capture, a sparse approximation of the model is used to enhance performance. Evaluation experiments demonstrate that our approach provides significantly better results that have more accurate hand shapes and fingertip positions, compared to other calibration methods.

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Browsing SCA: Eurographics/SIGGRAPH Symposium on Computer Animation by Subject "Animation"

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