SCA 16: Eurographics/SIGGRAPH Symposium on Computer Animation

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

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SCA 2016: Frontmatter
(Eurographics Association, 2016) Ladislav Kavan; Chris Wojtan;
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.
Hele-Shaw Flow Simulation with Interactive Control using Complex Barycentric Coordinates
(The Eurographics Association, 2016) Segall, Aviv; Vantzos, Orestis; Ben-Chen, Mirela; Ladislav Kavan and Chris Wojtan
Hele-Shaw flow describes the slow flow of a viscous liquid between two parallel plates separated by a small gap. In some configurations such a flow generates instabilities known as Saffman-Taylor fingers, which form intricate visual patterns. While these patterns have been an inspiration for artists, as well as thoroughly analyzed by mathematicians, efficiently simulating them remains challenging. The main difficulty involves efficiently computing a harmonic function on a time-varying planar domain, a problem which has been recently addressed in the shape deformation literature using a complex-variable formulation of generalized barycentric coordinates. We propose to leverage similar machinery, and show how the model equations for the Hele-Shaw flow can be formulated in this framework. This allows us to efficiently simulate the flow, while allowing interactive user control of the behavior of the fingers. We additionally show that complex barycentric coordinates are applicable to the exterior domain, and use them to simulate two-phase flow, yielding a variety of interesting patterns.
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.
Deployable 3D Linkages with Collision Avoidance
(The Eurographics Association, 2016) Zheng, Changxi; Sun, Timothy; Chen, Xiang; Ladislav Kavan and Chris Wojtan
We present a pipeline that allows ordinary users to create deployable scissor linkages in arbitrary 3D shapes, whose mechanisms are inspired by Hoberman's Sphere. From an arbitrary 3D model and a few user inputs, our method can generate a fabricable scissor linkage resembling that shape that aims to save as much space as possible in its most contracted state. Self-collisions are the primary obstacle in this goal, and these are not addressed in prior work. One key component of our algorithm is a succinct parameterization of these types of linkages. The fast continuous collision detection that arises from this parameterization serves as the foundation for the discontinuous optimization procedure that automatically improves joint placement for avoiding collisions. While linkages are usually composed of straight bars, we consider curved bars as a means of improving the contractibility. To that end, we describe a continuous optimization algorithm for locally deforming the bars.
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.
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.
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.
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.
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.
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.
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.
Repurposing Hand Animation for Interactive Applications
(The Eurographics Association, 2016) Bailey, Stephen W.; Watt, Martin; O'Brien, James F.; Ladislav Kavan and Chris Wojtan
In this paper we describe a method for automatically animating interactive characters based on an existing corpus of key-framed hand-animation. The method learns separate low-dimensional embeddings for subsets of the hand-animation corresponding to different semantic labels. These embeddings use the Gaussian Process Latent Variable Model to map high-dimensional rig control parameters to a three-dimensional latent space. By using a particle model to move within one of these latent spaces, the method can generate novel animations corresponding to the space's semantic label. Bridges link each pose in one latent space that is similar to a pose in another space. Animations corresponding to a transitions between semantic labels are generated by creating animation paths that move though one latent space and traverse a bridge into another. We demonstrate this method by using it to interactively animate a character as it plays a simple game with the user. The character is from a previously produced animated film and the data we use for training is the data that was used to animate the character in the film. The animated motion from the film represents an enormous investment of skillful work. Our method allows this work to be repurposed and reused for interactively animating the familiar character from the film.
ADMM ⊇ Projective Dynamics: Fast Simulation of General Constitutive Models
(The Eurographics Association, 2016) Narain, Rahul; Overby, Matthew; Brown, George E.; Ladislav Kavan and Chris Wojtan
We apply the alternating direction method of multipliers (ADMM) optimization algorithm to implicit time integration of elastic bodies, and show that the resulting method closely relates to the recently proposed projective dynamics algorithm. However, as ADMM is a general-purpose optimization algorithm applicable to a broad range of objective functions, it permits the use of nonlinear constitutive models and hard constraints while retaining the speed, parallelizability, and robustness of projective dynamics. We demonstrate these benefits on several examples that include cloth, collisions, and volumetric deformable bodies with nonlinear elasticity.
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.
Dynamic Group Behaviors for Interactive Crowd Simulation
(The Eurographics Association, 2016) He, Liang; Pan, Jia; Narang, Sahil; Manocha, Dinesh; Ladislav Kavan and Chris Wojtan
We present a new algorithm to simulate dynamic group behaviors for interactive multi-agent crowd simulation. Our approach is general and makes no assumption about the environment, shape, or size of the groups.We use the least effort principle to perform coherent group navigation and present efficient inter-group and intra-group maintenance techniques. We extend the reciprocal collision avoidance scheme to perform agent-group and group-group collision avoidance that can generate collision-free and coherent trajectories. The additional overhead of dynamic group simulation is relatively small. We highlight its interactive performance in complex scenarios with hundreds of agents and highlight its benefits over prior methods.
Real-time Simulation of Large Elasto-Plastic Deformation with Shape Matching
(The Eurographics Association, 2016) Chentanez, Nuttapong; Müller, Matthias; Macklin, Miles; Ladislav Kavan and Chris Wojtan
Shape matching is a popular method for simulating deformable objects in real time as it is fast and stable at large time steps. Although shape matching can simulate large elastic deformation and ductile fracturing, until now, they are limited to scenarios with relatively small plastic deformation. In this work, we present a method for simulating deformable solids undergoing large plastic deformation and topological changes using shape matching within the position based dynamics (PBD) framework. This expands the versatility of PBD which was previously shown to be able to simulate rigid bodies, liquids, gases, cloth, and deformable solids with moderate plastic deformation. Our novel contributions include local particle re-sampling, cluster re-sampling and skinning of an explicitly tracked surface mesh.
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.
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.

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