33-Issue 7

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https://diglib.eg.org/handle/10.2312/11789

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Advanced Hybrid Particle-Grid Method with Sub-Grid Particle Correction
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Um, Kiwon; Baek, Seungho; Han, JungHyun; J. Keyser, Y. J. Kim, and P. Wonka
This paper proposes a novel hybrid particle-grid approach to liquid simulation, which uses the fluid-implicitparticle (FLIP) method to resolve the liquid motion and a grid-based particle correction method to complement FLIP. The correction process addresses the high-frequency errors in FLIP ensuring that the particles are properly distributed. The proposed approach enables the corrective procedure to avoid directly processing the particle relationships and supports flexible corrective forces. The proposed technique effectively and efficiently improves the distribution of the particles and therefore enhances the overall simulation quality. The experimental results confirm that the technique is able to conserve the liquid volume and to produce dynamic surface motions, thin liquid sheets, and smooth surfaces without disturbing artifacts such as bumpy noise.
A Data-Driven Framework for Visual Crowd Analysis
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Charalambous, Panayiotis; Karamouzas, Ioannis; Guy, Stephen J.; Chrysanthou, Yiorgos; J. Keyser, Y. J. Kim, and P. Wonka
We present a novel approach for analyzing the quality of multi-agent crowd simulation algorithms. Our approach is data-driven, taking as input a set of user-defined metrics and reference training data, either synthetic or from video footage of real crowds. Given a simulation, we formulate the crowd analysis problem as an anomaly detection problem and exploit state-of-the-art outlier detection algorithms to address it. To that end, we introduce a new framework for the visual analysis of crowd simulations. Our framework allows us to capture potentially erroneous behaviors on a per-agent basis either by automatically detecting outliers based on individual evaluation metrics or by accounting for multiple evaluation criteria in a principled fashion using Principle Component Analysis and the notion of Pareto Optimality. We discuss optimizations necessary to allow real-time performance on large datasets and demonstrate the applicability of our framework through the analysis of simulations created by several widely-used methods, including a simulation from a commercial game.
Anisotropic Geodesics for Live-wire Mesh Segmentation
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Zhuang, Yixin; Zou, Ming; Carr, Nathan; Ju, Tao; J. Keyser, Y. J. Kim, and P. Wonka
We present an interactive method for mesh segmentation that is inspired by the classical live-wire interaction for image segmentation. The core contribution of the work is the definition and computation of wires on surfaces that are likely to lie at segment boundaries. We define wires as geodesics in a new tensor-based anisotropic metric, which improves upon previous metrics in stability and feature-awareness. We further introduce a simple but effective mesh embedding approach that allows geodesic paths in an anisotropic path to be computed efficiently using existing algorithms designed for Euclidean geodesics. Our tool is particularly suited for delineating segmentation boundaries that are aligned with features or curvature directions, and we demonstrate its use in creating artist-guided segmentations.
Locomotion Skills for Insects with Sample-based Controller
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Guo, Shihui; Chang, Jian; Yang, Xiaosong; Wang, Wencheng; Zhang, Jianjun; J. Keyser, Y. J. Kim, and P. Wonka
Natural-looking insect animation is very difficult to simulate. The fast movement and small scale of insects often challenge the standard motion capture techniques. As for the manual key-framing or physics-driven methods, significant amounts of time and efforts are necessary due to the delicate structure of the insect, which prevents practical applications. In this paper, we address this challenge by presenting a two-level control framework to efficiently automate the modeling and authoring of insects' locomotion. On the top level, we design a Triangle Placement Engine to automatically determine the location and orientation of insects' foot contacts, given the userdefined trajectory and settings, including speed, load, path and terrain etc. On the low-level, we relate the Central Pattern Generator to the triangle profiles with the assistance of a Controller Look-Up Table to fast simulate the physically-based movement of insects. With our approach, animators can directly author insects' behavior among a wide range of locomotion repertoire, including walking along a specified path or on an uneven terrain, dynamically adjusting to external perturbations and collectively transporting prey back to the nest.
Sparse Localized Decomposition of Deformation Gradients
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Huang, Zhichao; Yao, Junfeng; Zhong, Zichun; Liu, Yang; Guo, Xiaohu; J. Keyser, Y. J. Kim, and P. Wonka
Sparse localized decomposition is a useful technique to extract meaningful deformation components out of a training set of mesh data. However, existing methods cannot capture large rotational motion in the given mesh dataset. In this paper we present a new decomposition technique based on deformation gradients. Given a mesh dataset, the deformation gradient field is extracted, and decomposed into two groups: rotation field and stretching field, through polar decomposition. These two groups of deformation information are further processed through the sparse localized decomposition into the desired components. These sparse localized components can be linearly combined to form a meaningful deformation gradient field, and can be used to reconstruct the mesh through a least squares optimization step. Our experiments show that the proposed method addresses the rotation problem associated with traditional deformation decomposition techniques, making it suitable to handle not only stretched deformations, but also articulated motions that involve large rotations.
2D-3D Lifting for Shape Reconstruction
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Nan, Liangliang; Sharf, Andrei; Chen, Baoquan; J. Keyser, Y. J. Kim, and P. Wonka
We present an algorithm for shape reconstruction from incomplete 3D scans by fusing together two acquisition modes: 2D photographs and 3D scans. The two modes exhibit complementary characteristics: scans have depth information, but are often sparse and incomplete; photographs, on the other hand, are dense and have high resolution, but lack important depth information. In this work we fuse the two modes, taking advantage of their complementary information, to enhance 3D shape reconstruction from an incomplete scan with a 2D photograph. We compute geometrical and topological shape properties in 2D photographs and use them to reconstruct a shape from an incomplete 3D scan in a principled manner. Our key observation is that shape properties such as boundaries, smooth patches and local connectivity, can be inferred with high confidence from 2D photographs. Thus, we register the 3D scan with the 2D photograph and use scanned points as 3D depth cues for lifting 2D shape structures into 3D. Our contribution is an algorithm which significantly regularizes and enhances the problem of 3D reconstruction from partial scans by lifting 2D shape structures into 3D. We evaluate our algorithm on various shapes which are loosely scanned and photographed from different views, and compare them with state-of-the-art reconstruction methods.
Interactive Image-Guided Modeling of Extruded Shapes
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Cao, Yan-Pei; Ju, Tao; Fu, Zhao; Hu, Shi-Min; J. Keyser, Y. J. Kim, and P. Wonka
A recent trend in interactive modeling of 3D shapes from a single image is designing minimal interfaces, and accompanying algorithms, for modeling a specific class of objects. Expanding upon the range of shapes that existing minimal interfaces can model, we present an interactive image-guided tool for modeling shapes made up of extruded parts. An extruded part is represented by extruding a closed planar curve, called base, in the direction orthogonal to the base. To model each extruded part, the user only needs to sketch the projected base shape in the image. The main technical contribution is a novel optimization-based approach for recovering the 3D normal of the base of an extruded object by exploring both geometric regularity of the sketched curve and image contents. We developed a convenient interface for modeling multi-part shapes and a method for optimizing the relative placement of the parts. Our tool is validated using synthetic data and tested on real-world images.
Perceptually-motivated Stereoscopic Film Grain
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Templin, Krzysztof; Didyk, Piotr; Myszkowski, Karol; Seidel, Hans-Peter; J. Keyser, Y. J. Kim, and P. Wonka
Independent management of film grain in each view of a stereoscopic video can lead to visual discomfort. The existing alternative is to project the grain onto the scene geometry. Such grain, however, looks unnatural, changes object perception, and emphasizes inaccuracies in depth arising during 2D-to-3D conversion. We propose an advanced method of grain positioning that scatters the grain in the scene space. In a series of perceptual experiments, we estimate the optimal parameter values for the proposed method, analyze the user preference distribution among the proposed and the two existing methods, and show influence of the method on the object perception.
Automatic 3D Indoor Scene Updating with RGBD Cameras
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Liu, Zhenbao; Tang, Sicong; Xu, Weiwei; Bu, Shuhui; Han, Junwei; Zhou, Kun; J. Keyser, Y. J. Kim, and P. Wonka
Since indoor scenes are frequently changed in daily life, such as re-layout of furniture, the 3D reconstructions for them should be flexible and easy to update. We present an automatic 3D scene update algorithm to indoor scenes by capturing scene variation with RGBD cameras. We assume an initial scene has been reconstructed in advance in manual or other semi-automatic way before the change, and automatically update the reconstruction according to the newly captured RGBD images of the real scene update. It starts with an automatic segmentation process without manual interaction, which benefits from accurate labeling training from the initial 3D scene. After the segmentation, objects captured by RGBD camera are extracted to form a local updated scene. We formulate an optimization problem to compare to the initial scene to locate moved objects. The moved objects are then integrated with static objects in the initial scene to generate a new 3D scene. We demonstrate the efficiency and robustness of our approach by updating the 3D scene of several real-world scenes.
Template-Based Sampling of Anisotropic BRDFs
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Filip, Jiri; Vávra, Radomir; J. Keyser, Y. J. Kim, and P. Wonka
BRDFs are commonly used to represent given materials' appearance in computer graphics and related fields. Although, in the recent past, BRDFs have been extensively measured, compressed, and fitted by a variety of ana- lytical models, most research has been primarily focused on simplified isotropic BRDFs. In this paper, we present a unique database of 150 BRDFs representing a wide range of materials; the majority exhibiting anisotropic be- havior. Since time-consuming BRDF measurement represents a major obstacle in the digital material appearance reproduction pipeline, we tested several approaches estimating a very limited set of samples capable of high quality appearance reconstruction. Initially, we aligned all measured BRDFs according to the location of the anisotropic highlights. Then we propose an adaptive sampling method based on analysis of the measured BRDFs. For each BRDF, a unique sampling pattern was computed, given a predefined count of samples. Further, template-based methods are introduced based on reusing of the precomputed sampling patterns. This approach enables a more efficient measurement of unknown BRDFs while preserving the visual fidelity for the majority of tested materials. Our method exhibits better performance and stability than competing sparse sampling approaches; especially for higher numbers of samples.
Sky Based Light Metering for High Dynamic Range Images
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Gryaditskya, Yulia; Pouli, Tania; Reinhard, Erik; Seidel, Hans-Peter; J. Keyser, Y. J. Kim, and P. Wonka
Image calibration requires both linearization of pixel values and scaling so that values in the image correspond to real-world luminances. In this paper we focus on the latter and rather than rely on camera characterization, we calibrate images by analysing their content and metadata, obviating the need for expensive measuring devices or modeling of lens and camera combinations. Our analysis correlates sky pixel values to luminances that would be expected based on geographical metadata. Combined with high dynamic range (HDR) imaging, which gives us linear pixel data, our algorithm allows us to find absolute luminance values for each pixel-effectively turning digital cameras into absolute light meters. To validate our algorithm we have collected and annotated a calibrated set of HDR images and compared our estimation with several other approaches, showing that our approach is able to more accurately recover absolute luminance. We discuss various applications and demonstrate the utility of our method in the context of calibrated color appearance reproduction and lighting design.
Fast Feature-Oriented Visual Connection for Large Image Collections
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Yan, Qingan; Xu, Zhan; Xiao, Chunxia; J. Keyser, Y. J. Kim, and P. Wonka
Deriving the visual connectivity across large image collections is a computationally expensive task. Different from current image-oriented match graph construction methods which build on pairwise image matching, we present a novel and scalable feature-oriented image matching algorithm for large collections. Our method improves the match graph construction procedure in three ways. First, instead of building trees repeatedly, we put the feature points of the input image collection into a single kd-tree and select the leaves as our anchor points. Then we construct an anchor graph from which each feature can intelligently find a small portion of related candidates to match. Finally, we design a new form of adjacency matrix for fast feature similarity measuring, and return all the matches in different photos across the whole dataset directly. Experiments show that our feature-oriented correspondence algorithm can explore visual connectivity between images with significant improvement in speed.
Hybrid Particle-grid Modeling for Multi-scale Droplet/Spray Simulation
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Yang, Lipeng; Li, Shuai; Hao, Aimin; Qin, Hong; J. Keyser, Y. J. Kim, and P. Wonka
This paper presents a novel hybrid particle-grid method that tightly couples Lagrangian particle approach with Eulerian grid approach to simulate multi-scale diffuse materials varying from disperse droplets to dissipating spray and their natural mixture and transition, originated from a violent (high-speed) liquid stream. Despite the fact that Lagrangian particles are widely employed for representing individual droplets and Eulerian grid-based method is ideal for volumetric spray modeling, using either one alone has encountered tremendous difficulties when effectively simulating droplet/spray mixture phenomena with high fidelity. To ameliorate, we propose a new hybrid model to tackle such challenges with many novel technical elements. At the geometric level, we employ the particle and density field to represent droplet and spray respectively, modeling their creation from liquid as well as their seamless transition. At the physical level, we introduce a drag force model to couple droplets and spray, and specifically, we employ Eulerian method to model the interaction among droplets and marry it with the widelyused Lagrangian model. Moreover, we implement our entire hybrid model on CUDA to guarantee the interactive performance for high-effective physics-based graphics applications. The comprehensive experiments have shown that our hybrid approach takes advantages of both particle and grid methods, with convincing graphics effects for disperse droplets and spray simulation.
Time-Lapse Photometric Stereo and Applications
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Shen, Fangyang; Sunkavalli, Kalyan; Bonneel, Nicolas; Rusinkiewicz, Szymon; Pfister, Hanspeter; Tong, Xin; J. Keyser, Y. J. Kim, and P. Wonka
This paper presents a technique to recover geometry from time-lapse sequences of outdoor scenes. We build upon photometric stereo techniques to recover approximate shadowing, shading and normal components allowing us to alter the material and normals of the scene. Previous work in analyzing such images has faced two fundamental difficulties: 1. the illumination in outdoor images consists of time-varying sunlight and skylight, and 2. the motion of the sun is restricted to a near-planar arc through the sky, making surface normal recovery unstable. We develop methods to estimate the reflection component due to skylight illumination. We also show that sunlight directions are usually non-planar, thus making surface normal recovery possible. This allows us to estimate approximate surface normals for outdoor scenes using a single day of data. We demonstrate the use of these surface normals for a number of image editing applications including reflectance, lighting, and normal editing.
Fast and Scalable Mesh Superfacets
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Simari, Patricio; Picciau, Giulia; Floriani, Leila De; J. Keyser, Y. J. Kim, and P. Wonka
In the field of computer vision, the introduction of a low-level preprocessing step to oversegment images into superpixels - relatively small regions whose boundaries agree with those of the semantic entities in the scene - has enabled advances in segmentation by reducing the number of elements to be labeled from hundreds of thousands, or millions, to a just few hundred. While some recent works in mesh processing have used an analogous oversegmentation, they were not intended to be general and have relied on graph cut techniques that do not scale to current mesh sizes. Here, we present an iterative superfacet algorithm and introduce adaptations of undersegmentation error and compactness, which are well-motivated and principled metrics from the vision community. We demonstrate that our approach produces results comparable to those of the normalized cuts algorithm when evaluated on the Princeton Segmentation Benchmark, while requiring orders of magnitude less time and memory and easily scaling to, and enabling the processing of, much larger meshes.
Efficient Depth Propagation for Constructing a Layered Depth Image from a Single Image
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Iizuka, Satoshi; Endo, Yuki; Kanamori, Yoshihiro; Mitani, Jun; Fukui, Yukio; J. Keyser, Y. J. Kim, and P. Wonka
In this paper, we propose an interactive technique for constructing a 3D scene via sparse user inputs. We represent a 3D scene in the form of a Layered Depth Image (LDI) which is composed of a foreground layer and a background layer, and each layer has a corresponding texture and depth map. Given user-specified sparse depth inputs, depth maps are computed based on superpixels using interpolation with geodesic-distance weighting and an optimization framework. This computation is done immediately, which allows the user to edit the LDI interactively. Additionally, our technique automatically estimates depth and texture in occluded regions using the depth discontinuity. In our interface, the user paints strokes on the 3D model directly. The drawn strokes serve as 3D handles with which the user can pull out or push the 3D surface easily and intuitively with real-time feedback. We show our technique enables efficient modeling of LDI that produce sufficient 3D effects.
Environment-Adaptive Contact Poses for Virtual Characters
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Kang, Changgu; Lee, Sung-Hee; J. Keyser, Y. J. Kim, and P. Wonka
We present a novel method to generate a virtual character's multi-contact poses adaptive to the various shapes of the environment. Given the user-specified center of mass (CoM) position and direction as inputs, our method finds the potential contacts for the character in the surrounding geometry of the environment and generates a set of stable poses that are contact-rich. Major contributions of the work are in efficiently finding admissible support points for the target environment by precomputing candidate support points from a human pose database, and in automatically generating interactive poses that can maintain stable equilibrium. We develop the concept of support complexity to scale the set of precomputed support points by the geometric complexity of the environment. We demonstrate the effectiveness of our method by creating contact poses for various test cases of environments.
Approximate Symmetry Detection in Partial 3D Meshes
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Sipiran, Ivan; Gregor, Robert; Schreck, Tobias; J. Keyser, Y. J. Kim, and P. Wonka
Symmetry is a common characteristic in natural and man-made objects. Its ubiquitous nature can be exploited to facilitate the analysis and processing of computational representations of real objects. In particular, in computer graphics, the detection of symmetries in 3D geometry has enabled a number of applications in modeling and reconstruction. However, the problem of symmetry detection in incomplete geometry remains a challenging task. In this paper, we propose a vote-based approach to detect symmetry in 3D shapes, with special interest in models with large missing parts. Our algorithm generates a set of candidate symmetries by matching local maxima of a surface function based on the heat diffusion in local domains, which guarantee robustness to missing data. In order to deal with local perturbations, we propose a multi-scale surface function that is useful to select a set of distinctive points over which the approximate symmetries are defined. In addition, we introduce a vote-based scheme that is aware of the partiality, and therefore reduces the number of false positive votes for the candidate symmetries. We show the effectiveness of our method in a varied set of 3D shapes and different levels of partiality. Furthermore, we show the applicability of our algorithm in the repair and completion of challenging reassembled objects in the context of cultural heritage.
Using Physically Based Rendering to Benchmark Structured Light Scanners
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Medeiros, Esdras; Doraiswamy, Harish; Berger, Matthew; Silva, Claudio T.; J. Keyser, Y. J. Kim, and P. Wonka
Structured light scanning is ubiquituous in 3D acquisition. It is capable of capturing high geometric detail at a low cost under a variety of challenging scene conditions. Recent methods have demonstrated robustness in the presence of artifacts due to global illumination, such as inter-reflections and sub-surface scattering, as well as imperfections caused by projector defocus. For comparing approaches, however, the quantitative evaluation of structured lighting schemes is hindered by the challenges in obtaining ground truth data, resulting in a poor understanding for these methods across a wide range of shapes, materials, and lighting configurations. In this paper, we present a benchmark to study the performance of structured lighting algorithms in the presence of errors caused due to the above properties of the scene. In order to do this, we construct a synthetic structured lighting scanner that uses advanced physically based rendering techniques to simulate the point cloud acquisition process. We show that, under conditions similar to that of a real scanner, our synthetic scanner replicates the same artifacts found in the output of a real scanner. Using this synthetic scanner, we perform a quantitative evaluation of four different structured lighting techniques - gray-code patterns, micro-phase shifting, ensemble codes, and unstructured light scanning. The evaluation, performed on a variety of scenes,demonstrate that no one method is capable of adequately handling all sources of error - each method is appropriate for addressing distinct sources of error.
Perceptually-based Color Assignment
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Kim, Hye-Rin; Yoo, Min-Joon; Kang, Henry; Lee, In-Kwon; J. Keyser, Y. J. Kim, and P. Wonka
Color assignment is a complex task of incorporating and balancing area configuration, color harmony, and user's intent. In this paper, we present a novel method for automatic color assignment based on theories of color perception. We define color assignment as an optimization problem with respect to the color relationships as well as the spatial configuration of input segments. We also suggest possible constraints that are suitable for taskspecific purposes and for enhancing visual appeal. Our colorization scheme is useful in many applications such as infographics, computer-aided design, and visual presentation. The user study shows that our method generates perceptually pleasing results over a variety of data sets.

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