Volume 34 (2015)

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Adaptable Anatomical Models for Realistic Bone Motion Reconstruction
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Zhu, Lifeng; Hu, Xiaoyan; Kavan, Ladislav; Olga Sorkine-Hornung and Michael Wimmer
We present a system to reconstruct subject-specific anatomy models while relying only on exterior measurements represented by point clouds. Our model combines geometry, kinematics, and skin deformations (skinning). This joint model can be adapted to different individuals without breaking its functionality, i.e., the bones and the skin remain well-articulated after the adaptation.We propose an optimization algorithm which learns the subject-specific (anthropometric) parameters from input point clouds captured using commodity depth cameras. The resulting personalized models can be used to reconstruct motion of human subjects. We validate our approach for upper and lower limbs, using both synthetic data and recordings of three different human subjects. Our reconstructed bone motion is comparable to results obtained by optical motion capture (Vicon) combined with anatomically-based inverse kinematics (OpenSIM). We demonstrate that our adapted models better preserve the joint structure than previous methods such as OpenSIM or Anatomy Transfer.
Biologically-Inspired Visual Simulation of Insect Swarms
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Li, Weizi; Wolinski, David; Pettré, Julien; Lin, Ming C.; Olga Sorkine-Hornung and Michael Wimmer
Representing the majority of living animals, insects are the most ubiquitous biological organisms on Earth. Being able to simulate insect swarms could enhance visual realism of various graphical applications. However, the very complex nature of insect behaviors makes its simulation a challenging computational problem. To address this, we present a general biologically-inspired framework for visual simulation of insect swarms. Our approach is inspired by the observation that insects exhibit emergent behaviors at various scales in nature. At the low level, our framework automatically selects and configures the most suitable steering algorithm for the local collision avoidance task. At the intermediate level, it processes insect trajectories into piecewise-linear segments and constructs probability distribution functions for sampling waypoints. These waypoints are then evaluated by the Metropolis- Hastings algorithm to preserve global structures of insect swarms at the high level. With this biologically inspired, data-driven approach, we are able to simulate insect behaviors at different scales and we evaluate our simulation using both qualitative and quantitative metrics. Furthermore, as insect data could be difficult to acquire, our framework can be adopted as a computer-assisted animation tool to interpret sketch-like input as user control and generate simulations of complex insect swarming phenomena.
Composition-Aware Scene Optimization for Product Images
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Liu, Tianqiang; McCann, Jim; Li, Wilmot; Funkhouser, Thomas; Olga Sorkine-Hornung and Michael Wimmer
Increasingly, companies are creating product advertisements and catalog images using computer renderings of 3D scenes. A common goal for these companies is to create aesthetically appealing compositions that highlight objects of interest within the context of a scene. Unfortunately, this goal is challenging, not only due to the need to balance the trade-off among aesthetic principles and design constraints, but also because of the huge search space induced by possible camera parameters, object placement, material choices, etc. Previous methods have investigated only optimization of camera parameters. In this paper, we develop a tool that starts from an initial scene description and a set of high-level constraints provided by a stylist and then automatically generates an optimized scene whose 2D composition is improved. It does so by locally adjusting the 3D object transformations, surface materials, and camera parameters. The value of this tool is demonstrated in a variety of applications motivated by product catalogs, including rough layout refinement, detail image creation, home planning, cultural customization, and text inlay placement. Results of a perceptual study indicate that our system produces images preferable for product advertisement compared to a more traditional camera-only optimization.
Deformable Objects Collision Handling with Fast Convergence
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Li, Siwang; Pan, Zherong; Huang, Jin; Bao, Hujun; Jin, Xiaogang; Stam, Jos and Mitra, Niloy J. and Xu, Kun
We present a stable and efficient simulator for deformable objects with collisions and contacts. For stability, an optimization derived from the implicit time integrator is solved in each timestep under the inequality constraints coming from collisions. To achieve fast convergence, we extend the MPRGP based solver from handling box constraints only to handling general linear constraints and prove its convergence. This generalization introduces a cost of solving dense linear systems in each step, but these systems can be reduced into diagonal ones for efficiency without affecting the general stability via pruning redundant collisions. Our solver is an order of magnitude faster, especially for elastic objects under large deformation compared with iterative constraint anticipation method (ICA), a typical method for stability. The efficiency, robustness and stability are further verified by our results.
A Dimension-reduced Pressure Solver for Liquid Simulations
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Ando, Ryoichi; Thürey, Nils; Wojtan, Chris; Olga Sorkine-Hornung and Michael Wimmer
This work presents a method for efficiently simplifying the pressure projection step in a liquid simulation. We first devise a straightforward dimension reduction technique that dramatically reduces the cost of solving the pressure projection. Next, we introduce a novel change of basis that satisfies free-surface boundary conditions exactly, regardless of the accuracy of the pressure solve. When combined, these ideas greatly reduce the computational complexity of the pressure solve without compromising free surface boundary conditions at the highest level of detail. Our techniques are easy to parallelize, and they effectively eliminate the computational bottleneck for large liquid simulations.
An Efficient Boundary Handling with a Modified Density Calculation for SPH
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Fujisawa, Makoto; Miura, Kenjiro T.; Stam, Jos and Mitra, Niloy J. and Xu, Kun
We propose a new boundary handling method for smoothed particle hydrodynamics (SPH). Previous approaches required the use of boundary particles to prevent particles from sticking to the boundary. We address this issue by correcting the fundamental equations of SPH with the integration of a kernel function. Our approach is able to directly handle triangle mesh boundaries without the need for boundary particles.We also show how our approach can be integrated into a position-based fluid framework.
Geometrically Exact Simulation of Inextensible Ribbon
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Shen, Zhongwei; Huang, Jin; Chen, Wei; Bao, Hujun; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Narrow, inextensible, and naturally flat ribbons have some special and interesting phenomena under isometric deformations. Although a ribbon has a shape between rod and shell, directly applying the geometric representation designed for them imposes a challenge to faithfully reproduce interesting behaviors. We thus parameterize the ribbon surface as a developable ruled surface along its centerline and represent it using a framed centerline curve. Then the elastic and kinetic energy of the ribbon surface can be equivalently yet compactly described by the framed centerline curve only. To avoid numerical singularity when developability is violated, a finite Taylor series approximation to the potential energy is adopted. Under the observation that the off-centerline part of ribbon contributes little dynamic effect, the kinetic energy is simplified with respect to the centerline velocity only. For efficiency, each time step is separated into two stages: dynamically evolving the centerline, and then quasi-statically updating the ruling. We validate the method with qualitative analysis and ribbon specific phenomena comparisons with real-world scenarios. A set of comparisons to rod and shell model is also provided to demonstrate the advantages of our method.
Implicit Formulation for SPH-based Viscous Fluids
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Takahashi, Tetsuya; Dobashi, Yoshinori; Fujishiro, Issei; Nishita, Tomoyuki; Lin, Ming C.; Olga Sorkine-Hornung and Michael Wimmer
We propose a stable and efficient particle-based method for simulating highly viscous fluids that can generate coiling and buckling phenomena and handle variable viscosity. In contrast to previous methods that use explicit integration, our method uses an implicit formulation to improve the robustness of viscosity integration, therefore enabling use of larger time steps and higher viscosities. We use Smoothed Particle Hydrodynamics to solve the full form of viscosity, constructing a sparse linear system with a symmetric positive definite matrix, while exploiting the variational principle that automatically enforces the boundary condition on free surfaces. We also propose a new method for extracting coefficients of the matrix contributed by second-ring neighbor particles to efficiently solve the linear system using a conjugate gradient solver. Several examples demonstrate the robustness and efficiency of our implicit formulation over previous methods and illustrate the versatility of our method.
Improving Sampling-based Motion Control
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Liu, Libin; Yin, KangKang; Guo, Baining; Olga Sorkine-Hornung and Michael Wimmer
We address several limitations of the sampling-based motion control method of Liu et at. [LYvdP 10]. The key insight is to learn from the past control reconstruction trials through sample distribution adaptation. Coupled with a sliding window scheme for better performance and an averaging method for noise reduction, the improved algorithm can efficiently construct open-loop controls for long and challenging reference motions in good quality. Our ideas are intuitive and the implementations are simple. We compare the improved algorithm with the original algorithm both qualitatively and quantitatively, and demonstrate the effectiveness of the improved algorithm with a variety of motions ranging from stylized walking and dancing to gymnastic and Martial Arts routines.
Interactive Rigging with Intuitive Tools
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Bang, Seungbae; Choi, Byungkuk; Ribera, Roger Blanco i; Kim, Meekyoung; Lee, Sung-Hee; Noh, Junyong; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Rigging is a core element in the process of bringing a 3D character to life. The rig defines and delimits the motions of the character and provides an interface for an animator with which to interact with the 3D character. The quality of the rig has a key impact on the expressiveness of the character. Creating a usable, rich, production ready rig is a laborious task requiring direct intervention by a trained professional because the goal is difficult to achieve with fully automatic methods. We propose a semi-automatic rigging editing framework which eases the need for manual intervention while maintaining an important degree of control over the final rig. Starting by automatically generated base rig, we provide interactive operations which efficiently configure the skeleton structure and mesh skinning.
Multi-layer Lattice Model for Real-Time Dynamic Character Deformation
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Iwamoto, Naoya; Shum, Hubert P. H.; Yang, Longzhi; Morishima, Shigeo; Stam, Jos and Mitra, Niloy J. and Xu, Kun
Due to the recent advancement of computer graphics hardware and software algorithms, deformable characters have become more and more popular in real-time applications such as computer games. While there are mature techniques to generate primary deformation from skeletal movement, simulating realistic and stable secondary deformation such as jiggling of fats remains challenging. On one hand, traditional volumetric approaches such as the finite element method require higher computational cost and are infeasible for limited hardware such as game consoles. On the other hand, while shape matching based simulations can produce plausible deformation in real-time, they suffer from a stiffness problem in which particles either show unrealistic deformation due to high gains, or cannot catch up with the body movement. In this paper, we propose a unified multi-layer lattice model to simulate the primary and secondary deformation of skeleton-driven characters. The core idea is to voxelize the input character mesh into multiple anatomical layers including the bone, muscle, fat and skin. Primary deformation is applied on the bone voxels with lattice-based skinning. The movement of these voxels is propagated to other voxel layers using lattice shape matching simulation, creating a natural secondary deformation. Our multi-layer lattice framework can produce simulation quality comparable to those from other volumetric approaches with a significantly smaller computational cost. It is best to be applied in real-time applications such as console games or interactive animation creation.
Real-Time Subspace Integration for Example-Based Elastic Material
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Zhang, Wenjing; Zheng, Jianmin; Thalmann, Nadia Magnenat; Olga Sorkine-Hornung and Michael Wimmer
Example-based material allows simulating complex material behaviors in an art-directed way. This paper presents a method for fast subspace integration for example-based elastic material, which is suitable for real-time simulation in computer graphics. At the core of the method is the formulation of a new potential using example-based Green strain tensors. By using this potential, the deformation can be attracted towards the example-based deformation feature space, the example weights can be explicitly obtained and the internal force can be decomposed into the conventional one and an additional one induced by the examples. The real-time subspace integration is then developed with subspace integration costs independent of geometric complexity, and both the reduced conventional internal force and additional one being cubic polynomials in reduced coordinates. Experiments demonstrate that our method can achieve real-time simulation while providing comparable quality with the prior art.
Scalable Partitioning for Parallel Position Based Dynamics
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Fratarcangeli, Marco; Pellacini, Fabio; Olga Sorkine-Hornung and Michael Wimmer
We introduce a practical partitioning technique designed for parallelizing Position Based Dynamics, and exploiting the ubiquitous multi-core processors present in current commodity GPUs. The input is a set of particles whose dynamics is influenced by spatial constraints. In the initialization phase, we build a graph in which each node corresponds to a constraint and two constraints are connected by an edge if they influence at least one common particle. We introduce a novel greedy algorithm for inserting additional constraints (phantoms) in the graph such that the resulting topology is ˆ q-colourable, where ˆ q 2 is an arbitrary number. We color the graph, and the constraints with the same color are assigned to the same partition. Then, the set of constraints belonging to each partition is solved in parallel during the animation phase. We demonstrate this by using our partitioning technique; the performance hit caused by the GPU kernel calls is significantly decreased, leaving unaffected the visual quality, robustness and speed of serial position based dynamics.
TightCCD: Efficient and Robust Continuous Collision Detection using Tight Error Bounds
(The Eurographics Association and John Wiley & Sons Ltd., 2015) Wang, Zhendong; Tang, Min; Tong, Ruofeng; Manocha, Dinesh; Stam, Jos and Mitra, Niloy J. and Xu, Kun
We present a realtime and reliable continuous collision detection (CCD) algorithm between triangulated models that exploits the floating point hardware capability of current CPUs and GPUs. Our formulation is based on Bernstein Sign Classification that takes advantage of the geometry properties of Bernstein basis and Bézier curves to perform Boolean collision queries. We derive tight numerical error bounds on the computations and employ those bounds to design an accurate algorithm using finite-precision arithmetic. Compared with prior floatingpoint CCD algorithms, our approach eliminates all the false negatives and 90-95% of the false positives. We integrated our algorithm (TightCCD) with physically-based simulation system and observe speedups in collision queries of 5-15X compared with prior reliable CCD algorithms. Furthermore, we demonstrate its benefits in terms of improving the performance or robustness of cloth simulation systems.

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Browsing Volume 34 (2015) by Subject "Animation"

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