37-Issue 8

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https://diglib.eg.org/handle/10.2312/2632544

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Robust Physics-based Motion Retargeting with Realistic Body Shapes
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Borno, Mazen Al; Righetti, Ludovic; Black, Michael J.; Delp, Scott L.; Fiume, Eugene; Romero, Javier; Thuerey, Nils and Beeler, Thabo
Motion capture is often retargeted to new, and sometimes drastically different, characters. When the characters take on realistic human shapes, however, we become more sensitive to the motion looking right. This means adapting it to be consistent with the physical constraints imposed by different body shapes. We show how to take realistic 3D human shapes, approximate them using a simplified representation, and animate them so that they move realistically using physically-based retargeting. We develop a novel spacetime optimization approach that learns and robustly adapts physical controllers to new bodies and constraints. The approach automatically adapts the motion of the mocap subject to the body shape of a target subject. This motion respects the physical properties of the new body and every body shape results in a different and appropriate movement. This makes it easy to create a varied set of motions from a single mocap sequence by simply varying the characters. In an interactive environment, successful retargeting requires adapting the motion to unexpected external forces. We achieve robustness to such forces using a novel LQR-tree formulation. We show that the simulated motions look appropriate to each character’'s anatomy and their actions are robust to perturbations.
MPM Simulation of Interacting Fluids and Solids
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Yan, Xiao; Li, Chen-Feng; Chen, Xiao-Song; Hu, Shi-Min; Thuerey, Nils and Beeler, Thabo
The material point method (MPM) has attracted increasing attention from the graphics community, as it combines the strengths of both particle- and grid-based solvers. Like the smoothed particle hydrodynamics (SPH) scheme, MPM uses particles to discretize the simulation domain and represent the fundamental unknowns. This makes it insensitive to geometric and topological changes, and readily parallelizable on a GPU. Like grid-based solvers, MPM uses a background mesh for calculating spatial derivatives, providing more accurate and more stable results than a purely particle-based scheme. MPM has been very successful in simulating both fluid flow and solid deformation, but less so in dealing with multiple fluids and solids, where the dynamic fluid-solid interaction poses a major challenge. To address this shortcoming of MPM, we propose a new set of mathematical and computational schemes which enable efficient and robust fluid-solid interaction within the MPM framework. These versatile schemes support simulation of both multiphase flow and fully-coupled solid-fluid systems. A series of examples is presented to demonstrate their capabilities and performance in the presence of various interacting fluids and solids, including multiphase flow, fluid-solid interaction, and dissolution.
Strain Rate Dissipation for Elastic Deformations
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Sánchez-Banderas, Rosa M.; Otaduy, Miguel A.; Thuerey, Nils and Beeler, Thabo
Damping determines how the energy in dynamic deformations is dissipated. The design of damping requires models where the behavior along deformation modes is easily controlled, while other motions are left unaffected. In this paper, we propose a framework for the design of damping using dissipation potentials formulated as functions of strain rate. We study simple parameterizations of the models, the application to continuum and discrete deformation models, and practical implications for implementation. We also study previous simple damping models, in particular we demonstrate limitations of Rayleigh damping. We analyze in detail the application of strain rate dissipation potentials to two highly different deformation models, StVK hyperlasticity and yarn-level cloth with sliding persistent contacts. These deformation models are representative of the range of applicability of the damping model.
Coupled Fluid Density and Motion from Single Views
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Eckert, Marie-Lena; Heidrich, Wolfgang; Thuerey, Nils; Thuerey, Nils and Beeler, Thabo
We present a novel method to reconstruct a fluid's 3D density and motion based on just a single sequence of images. This is rendered possible by using powerful physical priors for this strongly under-determined problem. More specifically, we propose a novel strategy to infer density updates strongly coupled to previous and current estimates of the flow motion. Additionally, we employ an accurate discretization and depth-based regularizers to compute stable solutions. Using only one view for the reconstruction reduces the complexity of the capturing setup drastically and could even allow for online video databases or smart-phone videos as inputs. The reconstructed 3D velocity can then be flexibly utilized, e.g., for re-simulation, domain modification or guiding purposes. We will demonstrate the capacity of our method with a series of synthetic test cases and the reconstruction of real smoke plumes captured with a Raspberry Pi camera.
Cable Joints
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Müller, Matthias; Chentanez, Nuttapong; Jeschke, Stefan; Macklin, Miles; Thuerey, Nils and Beeler, Thabo
Robustly and efficiently simulating cables and ropes that are part of a larger system such as cable driven machines, cable cars or tendons in a human or robot is a challenging task. To be able to adapt to the environment, cables are typically modeled as a large number of small segments that are connected via joints. The two main difficulties with this approach are to satisfy the inextensibility constraint and to handle the typically large mass ratio between the small segments and the larger objects they connect. In this paper we present a new approach which solves these problems in a simple and effective way. Our method is based on the idea to simulate the effect of the cables instead of the cables themselves. To this end we propose a new special type of distance constraint we call cable joint that changes both its attachment points and its rest length dynamically. A cable connecting a series of objects is then modeled as a sequence of cable joints which reduces the complexity of the simulation from the order of the number of segments to just the number of connected objects. This makes simulations both faster and more robust as we will demonstrate on a variety of examples.
Distributing and Load Balancing Sparse Fluid Simulations
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Shah, Chinmayee; Hyde, David; Qu, Hang; Levis, Philip; Thuerey, Nils and Beeler, Thabo
This paper describes a general algorithm and a system for load balancing sparse fluid simulations. Automatically distributing sparse fluid simulations efficiently is challenging because the computational load varies across the simulation domain and time. A key challenge with load balancing is that optimal decision making requires knowing the fluid distribution across partitions for future time steps, but computing this state for an arbitrary simulation requires running the simulation itself. The key insight of this paper is that it is possible to predict future load by running a speculative low resolution simulation in parallel. We mathematically formulate the problem of load balancing over multiple time steps and present a polynomial time algorithm to compute an approximate solution to it. Our experimental results show that distributing and speculatively load balancing sparse FLIP simulations over 8 nodes speeds them up by 5.3 to 7.9, and that speculative load balancing generates assignments that perform within 20% of optimal.
Time-Domain Parallelization for Accelerating Cloth Simulation
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Liang, Junbang; Lin, Ming C.; Thuerey, Nils and Beeler, Thabo
Cloth simulations, widely used in computer animation and apparel design, can be computationally expensive for real-time applications. Some parallelization techniques have been proposed for visual simulation of cloth using CPU or GPU clusters and often rely on parallelization using spatial domain decomposition techniques that have a large communication overhead. In this paper, we propose a novel time-domain parallelization technique that makes use of the two-level mesh representation to resolve the time-dependency issue and develop a practical algorithm to smooth the state transition from the corresponding coarse to fine meshes. A load estimation and a load balancing technique used in online partitioning are also proposed to maximize the performance acceleration. Our method achieves a nearly linear performance scaling on manycore clusters and outperforms spatial-domain parallelization on a diverse set of benchmarks.
HairControl: A Tracking Solution for Directable Hair Simulation
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Milliez, Antoine; Sumner, Robert W.; Gross, Markus; Thomaszewski, Bernhard; Thuerey, Nils and Beeler, Thabo
We present a method for adding artistic control to physics-based hair simulation. Taking as input an animation of a coarse set of guide hairs, we constrain a subsequent higher-resolution simulation of detail hairs to follow the input motion in a spatially-averaged sense. The resulting high-resolution motion adheres to the artistic intent, but is enhanced with detailed deformations and dynamics generated by physics-based simulation. The technical core of our approach is formed by a set of tracking constraints, requiring the center of mass of a given subset of detail hair to maintain its position relative to a reference point on the corresponding guide hair. As a crucial element of our formulation, we introduce the concept of dynamicallychanging constraint targets that allow reference points to slide along the guide hairs to provide sufficient flexibility for natural deformations. We furthermore propose to regularize the null space of the tracking constraints based on variance minimization, effectively controlling the amount of spread in the hair. We demonstrate the ability of our tracking solver to generate directable yet natural hair motion on a set of targeted experiments and show its application to production-level animations.
Learning Physically Based Humanoid Climbing Movements
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Naderi, Kourosh; Babadi, Amin; Hämäläinen, Perttu; Thuerey, Nils and Beeler, Thabo
We propose a novel learning-based solution for motion planning of physically-based humanoid climbing that allows for fast and robust planning of complex climbing strategies and movements, including extreme movements such as jumping. Similar to recent previous work, we combine a high-level graph-based path planner with low-level sampling-based optimization of climbing moves. We contribute through showing that neural network models of move success probability, effortfulness, and control policy can make both the high-level and low-level components more efficient and robust. The models can be trained through random simulation practice without any data. The models also eliminate the need for laboriously hand-tuned heuristics for graph search. As a result, we are able to efficiently synthesize climbing sequences involving dynamic leaps and one-hand swings, i.e. there are no limits to the movement complexity or the number of limbs allowed to move simultaneously. Our supplemental video also provides some comparisons between our AI climber and a real human climber.
Cosserat Rods with Projective Dynamics
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Soler, Carlota; Martin, Tobias; Sorkine-Hornung, Olga; Thuerey, Nils and Beeler, Thabo
We present a novel method to simulate Cosserat rods with Projective Dynamics (PD). The proposed method is both numerically robust and accurate with respect to the underlying physics, making it suitable for a variety of applications in computer graphics and related disciplines. Cosserat theory assigns an orientation frame to each point and is thus able to realistically simulate stretching and shearing effects, in addition to bending and twisting. Within the PD framework, it is possible to obtain accurate simulations given the implicit integration over time and its decoupling of the local-global solve. In the proposed method, we start from the continuous formulation of the Cosserat theory and derive the constraints for the PD solver.We extend the standard definition of PD and add body orientations as system variables. Thus, we include the preservation of angular momentum, so that twisting and bending can be accurately simulated. Our formulation allows the simulation of different bending behaviors with respect to a user-defined Young's modulus, the radius of the rod's cross-section, and material density. We show how different material specifications in our simulations converge within a few iterations to a reference solution, generated with a highprecision finite element method. Furthermore, we demonstrate mesh independence of our formulation: Refining the simulation mesh still results in the same characteristic motion, which is in contrast to previous position based methods.
Fast Corotated FEM using Operator Splitting
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Kugelstadt, Tassilo; Koschier, Dan; Bender, Jan; Thuerey, Nils and Beeler, Thabo
In this paper we present a novel operator splitting approach for corotated FEM simulations. The deformation energy of the corotated linear material model consists of two additive terms. The first term models stretching in the individual spatial directions and the second term describes resistance to volume changes. By formulating the backward Euler time integration scheme as an optimization problem, we show that the first term is invariant to rotations. This allows us to use an operator splitting approach and to solve both terms individually with different numerical methods. The stretching part is solved accurately with an optimization integrator, which can be done very efficiently because the system matrix is constant over time such that its Cholesky factorization can be precomputed. The volume term is solved approximately by using the compliant constraints method and Gauss-Seidel iterations. Further, we introduce the analytic polar decomposition which allows us to speed up the extraction of the rotational part of the deformation gradient and to recover inverted elements. Finally, this results in an extremely fast and robust simulation method with high visual quality that outperforms standard corotated FEMs by more than two orders of magnitude and even the fast but inaccurate PBD and shape matching methods by more than one order of magnitude without having their typical drawbacks. This enables a very efficient simulation of complex scenes containing more than a million elements.
Frontmatter: ACM SIGGRAPH / Eurographics Symposium of Computer Animation 2018
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Thuerey, Nils; Beeler, Thabo; Thuerey, Nils and Beeler, Thabo
FTP-SC: Fuzzy Topology Preserving Stroke Correspondence
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Yang, Wenwu; Seah, Hock-Soon; Chen, Quan; Liew, Hong-Ze; Sýkora, Daniel; Thuerey, Nils and Beeler, Thabo
Stroke correspondence construction is a precondition for vectorized 2D animation inbetweening and remains a challenging problem. This paper introduces the FTP-SC, a fuzzy topology preserving stroke correspondence technique, which is accurate and provides the user more effective control on the correspondence result than previous matching approaches. The method employs a two-stage scheme to progressively establish the stroke correspondence construction between the keyframes. In the first stage, the stroke correspondences with high confidence are constructed by enforcing the preservation of the so-called “fuzzy topology” which encodes intrinsic connectivity among the neighboring strokes. Starting with the high-confidence correspondences, the second stage performs a greedy matching algorithm to generate a full correspondence between the strokes. Experimental results show that the FTP-SC outperforms the existing approaches and can establish the stroke correspondence with a reasonable amount of user interaction even for keyframes with large geometric and spatial variations between strokes.
User-Guided Lip Correction for Facial Performance Capture
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Dinev, Dimitar; Beeler, Thabo; Bradley, Derek; Bächer, Moritz; Xu, Hongyi; Kavan, Ladislav; Thuerey, Nils and Beeler, Thabo
Facial performance capture is the primary method for generating facial animation in video games, feature films and virtual environments, and recent advances have produced very compelling results. Still, one of the most challenging regions is the mouth, which often contains systematic errors due to the complex appearance and occlusion/dis-occlusion of the lips. We present a novel user-guided approach to correcting these common lip shape errors present in traditional capture systems. Our approach is to allow a user to manually correct a small number of problematic frames, and then our system learns the types of corrections desired and automatically corrects the entire performance. As correcting even a single frame using traditional 3D sculpting tools can be time consuming and require great skill, we also propose a simple and fast 2D sketch-based method for generating plausible lip corrections for the problematic key frames. We demonstrate our results on captured performances of three different subjects, and validate our method with an additional sequence that contains ground truth lip reconstructions.
Collision-Aware and Online Compression of Rigid Body Simulations via Integrated Error Minimization
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Jeruzalski, Timothy; Kanji, John; Jacobson, Alec; Levin, David I. W.; Thuerey, Nils and Beeler, Thabo
Methods to compress simulation data are invaluable as they facilitate efficient transmission along the visual effects pipeline, fast and efficient replay of simulations for visualization and enable storage of scientific data. However, all current approaches to compressing simulation data require access to the entire dynamic simulation, leading to large memory requirements and additional computational burden. In this paper we perform compression of contact-dominated, rigid body simulations in an online, error-bounded fashion. This has the advantage of requiring access to only a narrow window of simulation data at a time while still achieving good agreement with the original simulation. Our approach is simulator agnostic allowing us to compress data from a variety of sources. We demonstrate the efficacy of our algorithm by compressing contact-dominated rigid body simulations from a number of sources, achieving compression rates of up to 360 times over raw data size.
Computational Design of Transformables
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Yuan, Ye; Zheng, Changxi; Coros, Stelian; Thuerey, Nils and Beeler, Thabo
We present a computational approach to designing transformables, physical characters that can shape-shift to take on vastly different forms. The design process begins with a morphological description of an input character and a target object that it should transform into. Guided by a set of objectives that model the core attributes of desirable transformable designs, optimized embeddings are interactively generated. Intuitively, embeddings represent tightly folded character configurations that fit within the target object. From any feasible embedding, skin meshes are then generated for each body part of the character. The process for generating these 3D models is based on a segmentation of the target object, which is achieved through a growth-based model applied to a multiple level set representation of the transformable. A set of transformation-aware post-processing algorithms ensure the feasibility of the final designs. Building on this technical core, our computational design system provides many opportunities for users to inject their intuition and personal preferences into the process of creating transformables, while shielding them from tasks that are challenging and tedious. As a result, they can intuitively explore the vast space of design possibilities. We demonstrated the effectiveness of our computational approach by creating a variety of transformable designs, three of which we fabricate.
Liquid Splash Modeling with Neural Networks
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Um, Kiwon; Hu, Xiangyu; Thuerey, Nils; Thuerey, Nils and Beeler, Thabo
This paper proposes a new data-driven approach to model detailed splashes for liquid simulations with neural networks. Our model learns to generate small-scale splash detail for the fluid-implicit-particle method using training data acquired from physically parametrized, high resolution simulations. We use neural networks to model the regression of splash formation using a classifier together with a velocity modifier. For the velocity modification, we employ a heteroscedastic model. We evaluate our method for different spatial scales, simulation setups, and solvers. Our simulation results demonstrate that our model significantly improves visual fidelity with a large amount of realistic droplet formation and yields splash detail much more efficiently than finer discretizations.
An Efficient Solver for Two-way Coupling Rigid Bodies with Incompressible Flow
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Aanjaneya, Mridul; Thuerey, Nils and Beeler, Thabo
We present an efficient solver for monolithic two-way coupled simulation of rigid bodies with incompressible fluids that is robust to poor conditioning of the coupled system in the presence of large density ratios between the solid and the fluid. Our method leverages ideas from the theory of Domain Decomposition, and uses a hybrid combination of direct and iterative solvers that exploits the low-dimensional nature of the solid equations. We observe that a single Multigrid V-cycle for the fluid equations serves as a very effective preconditioner for solving the Schur-complement system using Conjugate Gradients, which is the main computational bottleneck in our pipeline. We use spectral analysis to give some theoretical insights behind this observation. Our method is simple to implement, is entirely assembly-free besides the solid equations, allows for the use of large time steps because of the monolithic formulation, and remains stable even when the iterative solver is terminated early. We demonstrate the efficacy of our method on several challenging examples of two-way coupled simulation of smoke and water with rigid bodies. To illustrate that our method is applicable to other problems, we also show an example of underwater bubble simulation.
A Temporally Adaptive Material Point Method with Regional Time Stepping
(The Eurographics Association and John Wiley & Sons Ltd., 2018) Fang, Yu; Hu, Yuanming; Hu, Shi-Min; Jiang, Chenfanfu; Thuerey, Nils and Beeler, Thabo
Spatially and temporally adaptive algorithms can substantially improve the computational efficiency of many numerical schemes in computational mechanics and physics-based animation. Recently, a crucial need for temporal adaptivity in the Material Point Method (MPM) is emerging due to the potentially substantial variation of material stiffness and velocities in multimaterial scenes. In this work, we propose a novel temporally adaptive symplectic Euler scheme for MPM with regional time stepping (RTS), where different time steps are used in different regions. We design a time stepping scheduler operating at the granularity of small blocks to maintain a natural consistency with the hybrid particle/grid nature of MPM. Our method utilizes the Sparse Paged Grid (SPGrid) data structure and simultaneously offers high efficiency and notable ease of implementation with a practical multi-threaded particle-grid transfer strategy. We demonstrate the efficacy of our asynchronous MPM method on various examples including elastic objects, granular media, and fluids.

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