SCA 16: Eurographics/SIGGRAPH Symposium on Computer Animation

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https://diglib.eg.org/handle/10.2312/2630803

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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.
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.
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.
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.
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.
A Data-driven Model for Lane-changing in Traffic Simulation
(The Eurographics Association, 2016) Bi, Huikun; Mao, Tianlu; Wang, Zhaoqi; Deng, Zhigang; Ladislav Kavan and Chris Wojtan
In this paper, we propose a new data-driven model to simulate the process of lane-changing in traffic simulation. Specifically, we first extract the features from surrounding vehicles that are relevant to the lane-changing of the subject vehicle. Then, we learn the lane-changing characteristics from the ground-truth vehicle trajectory data using randomized forest and back-propagation neural network algorithms. Our method can make the subject vehicle to take account of more gap options on the target lane to cut in as well as achieve more realistic lane-changing trajectories for the subject vehicle and the follower vehicle. Through many experiments and comparisons with selected state-of-the-art methods, we demonstrate that our approach can soundly outperform them in terms of the accuracy and quality of lane-changing simulation. Our model can be flexibly used together with a variety of existing car-following models to produce natural traffic animations in various virtual environments.
Enriching SPH Simulation by Approximate Capillary Waves
(The Eurographics Association, 2016) Yang, Sheng; He, Xiaowei; Wang, Huamin; Li, Sheng; Wang, Guoping; Wu, Enhua; Zhou, Kun; Ladislav Kavan and Chris Wojtan
Capillary waves are di cult to simulate due to their fast traveling speed and high frequency. In this paper, we propose to approximate capillary wave e ects by surface compression waves under the SPH framework. To achieve this goal, we present a method to convert surface tension energy changes measured from SPH simulation into high-frequency density variations. Based on the compression wave propagation model, we present an approximate technique to simulate capillary wave propagation in a high-frequency particle density field. To address noise issues in wave simulation, we develop a simple way to apply the zero pressure condition on free surfaces in projection-based incompressible SPH. Our experiment shows that the developed algorithm can produce realistic capillary wave e ects on both thin liquid features and large liquid bodies. Its computational overhead is also small.
Hierarchical hp-Adaptive Signed Distance Fields
(The Eurographics Association, 2016) Koschier, Dan; Deul, Crispin; Bender, Jan; Ladislav Kavan and Chris Wojtan
In this paper we propose a novel method to construct hierarchical hp-adaptive Signed Distance Fields (SDFs). We discretize the signed distance function of an input mesh using piecewise polynomials on an axis-aligned hexahedral grid. Besides spatial refinement based on octree subdivision to refine the cell size (h), we hierarchically increase each cell's polynomial degree (p) in order to construct a very accurate but memory-efficient representation. Presenting a novel criterion to decide whether to apply h- or p-refinement, we demonstrate that our method is able to construct more accurate SDFs at significantly lower memory consumption than previous approaches. Finally, we demonstrate the usage of our representation as collision detector for geometrically highly complex solid objects in the application area of physically-based simulation.
Large-Scale Finite State Game Engines
(The Eurographics Association, 2016) Stanton, Matt; Geddert, Sascha; Blumer, Adrian; Hormis, Paul; Nealen, Andy; Cooper, Seth; Treuille, Adrien; Ladislav Kavan and Chris Wojtan
This paper presents a new model reduction technique that exploits large-scale, parallel precomputation to create interactive, real-time games with the visual fidelity of offline rendered films.We present an algorithm to automatically discretize a continuous game into a large finite-state machine that can be pre-rendered in the film world. Despite radical differences from existing game engines, our finite-state approach is capable of preserving important characteristics of continuous games including smooth animation, responsiveness to input, triggered effects and passive animation. We demonstrate our technique with a 30-second interactive game set in an award-winning short film
Position and Orientation Based Cosserat Rods
(The Eurographics Association, 2016) Kugelstadt, Tassilo; Schömer, Elmar; Ladislav Kavan and Chris Wojtan
We present a novel method to simulate bending and torsion of elastic rods within the position-based dynamics (PBD) framework. The main challenge is that torsion effects of Cosserat rods are described in terms of material frames which are attached to the centerline of the rod. But frames or orientations do not fit into the classical position-based dynamics formulation. To solve this problem we introduce new types of constraints to couple orientations which are represented by unit quaternions. For constraint projection quaternions are treated in the exact same way as positions. Unit length is enforced with an additional constraint. This allows us to use the strain measures form Cosserat theory directly as constraints in PBD. It leads to very simple algebraic expressions for the correction displacements which only contain quaternion products and additions. Our results show that our method is very robust and accurately produces the complex bending and torsion effects of rods. Due to its simplicity our method is very efficient and more than one order of magnitude faster than existing position-based rod simulation methods. It even achieves the same performance as position-based simulations without torsion effects.
Real-Time Sound Synthesis for Paper Material Based on Geometric Analysis
(The Eurographics Association, 2016) Schreck, Camille; Rohmer, Damien; James, Doug L.; Hahmann, Stefanie; Cani, Marie-Paule; Ladislav Kavan and Chris Wojtan
In this article, we present the first method to generate plausible sounds while animating crumpling virtual paper in real time. Our method handles shape-dependent friction and crumpling sounds which typically occur when manipulating or creasing paper by hand. Based on a run-time geometric analysis of the deforming surface, we identify resonating regions characterizing the sound being produced. Coupled to a fast analysis of the surrounding elements, the sound can be efficiently spatialized to take into account nearby wall or table reflectors. Finally, the sound is synthesized in real time using a pre-recorded database of frequency- and time-domain sound sources. Our synthesized sounds are evaluated by comparing them to recordings for a specific set of paper deformations.
Topology-Aware Neighborhoods for Point-Based Simulation and Reconstruction
(The Eurographics Association, 2016) Canezin, Florian; Guennebaud, Gaël; Barthe, Loïc; Ladislav Kavan and Chris Wojtan
Particle based simulations are widely used in computer graphics. In this field, several recent results have improved the simulation itself or improved the tension of the final fluid surface. In current particle based implementations, the particle neighborhood is computed by considering the Euclidean distance between fluid particles only. Thus particles from different fluid components interact, which generates both local incorrect behavior in the simulation and blending artifacts in the reconstructed fluid surface. Our method introduces a better neighborhood computation for both the physical simulation and surface reconstruction steps. We track and store the local fluid topology around each particle using a graph structure. In this graph, only particles within the same local fluid component are neighbors and other disconnected fluid particles are inserted only if they come into contact. The graph connectivity also takes into account the asymmetric behavior of particles when they merge and split, and the fluid surface is reconstructed accordingly, thus avoiding their blending at distance before a merge. In the simulation, this neighborhood information is exploited for better controlling the fluid density and the force interactions at the vicinity of its boundaries. For instance, it prevents the introduction of collision events when two distinct fluid components are crossing without contact, and it avoids fluid interactions through thin waterproof walls. This leads to an overall more consistent fluid simulation and reconstruction.
Versatile Interactions at Interfaces for SPH-Based Simulations
(The Eurographics Association, 2016) Yang, Tao; Lin, Ming C.; Martin, Ralph R.; Chang, Jian; Hu, Shi-Min; Ladislav Kavan and Chris Wojtan
The realistic capture of various interactions at interfaces is a challenging problem for SPH-based simulation. Previous works have mainly considered a single type of interaction, while real-world phenomena typically exhibit multiple interactions at different interfaces. For instance, when cracking an egg, there are simultaneous interactions between air, egg white, egg yolk, and the shell. To conveniently handle all interactions simultaneously in a single simulation, a versatile approach is critical. In this paper, we present a new approach to the surface tension model based on pairwise interaction forces; its basis is to use a larger number of neighboring particles. Our model is stable, conserves momentum, and furthermore, prevents the particle clustering problem which commonly occurs at the free surface. It can be applied to simultaneous interactions at multiple interfaces (e.g. fluid-solid and fluid-fluid). Our method is versatile, physically plausible and easy-to-implement.We also consider the close connection between droplets and bubbles, and show how to animate bubbles in air as droplets, with the help of a new surface particle detection method. Examples are provided to demonstrate the capabilities and effectiveness of our approach.

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Browsing SCA 16: Eurographics/SIGGRAPH Symposium on Computer Animation by Subject "Animation"

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