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.
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.
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
A Macroblock Optimization for Grid-based Nonlinear Elasticity
(The Eurographics Association, 2016) Mitchell, Nathan; Doescher, Michael; Sifakis, Eftychios; Ladislav Kavan and Chris Wojtan
We introduce a new numerical approach for the solution of grid-based discretizations of nonlinear elastic models. Our method targets the linearized system of equations within each iteration of the Newton method, and combines elements of a direct factorization scheme with an iterative Conjugate Gradient method. The goal of our hybrid scheme is to inherit as many of the advantages of its constituent approaches, while curtailing several of their respective drawbacks. In particular, our algorithm converges in far fewer iterations than Conjugate Gradients, especially for systems with less-than-ideal conditioning. On the other hand, our approach largely avoids the storage footprint and memory-bound nature of direct methods, such as sparse Cholesky factorization, while offering very direct opportunities for both SIMD and thread-based parallelism. Conceptually, our method aggregates a rectangular neighborhood of grid cells (typically a 16x8x8 subgrid) into a composite element that we refer to as a ''macroblock''. Similar to conventional tetrahedral or hexahedral elements, macroblocks receive nodal inputs (e.g., displacements) and compute nodal outputs (e.g., forces). However, this input/output interface now only includes nodes on the boundary of the 16x8x8 macroblock; interior nodes are always solved exactly, by means of a direct, highly optimized solver. Models built from macroblocks are solved using Conjugate Gradients, which is accelerated due to the reduced number of degrees of freedom and improved robustness against poor conditioning thanks to the direct solver within each macroblock. We explain how we attain these benefits with just a small increase of the per-iteration cost over the simplest traditional solvers.
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.
Two-way Coupling of Fluids to Reduced Deformable Bodies
(The Eurographics Association, 2016) Lu, Wenlong; Jin, Ning; Fedkiw, Ronald P.; Ladislav Kavan and Chris Wojtan
We propose a fully monolithic two-way coupling framework that couples incompressible fluids to reduced deformable bodies. Notably, the resulting linear system matrix is both symmetric and positive-definite. Our method allows for the simulation of interesting free-surface as well as underwater phenomena, enabling the use of reduced deformable bodies as full-fledged simulation primitives alongside rigid bodies and deformable bodies. Momentum conservation is crucial to obtaining physically correct and realistic-looking motion in a fluid environment, and we achieve this by following previous work to describe reduced deformable bodies using both a rigid frame and a reduced space deformation component. Our approach partitions forces and impulses between the reduced space and the rigid frame of the reduced deformable bodies using a projection scheme that cleanly accounts for momentum losses in the reduced space via corrections in the rigid frame, resulting in a new theoretical formulation for the momentum-conserving reduced deformable body. We demonstrate that robust and stable contact, collision, articulation, and two-way coupling with fluids are all attainable in a straightforward way using this new formulation. Compared with fully deformable objects, our framework consumes less memory and scales better in large scenes, while still nicely approximating the deformation effects.
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 "I.3.7 [Computer Graphics]"

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