33-Issue 7

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

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Preface and Table of Contents
(The Eurographics Association and Blackwell Publishing Ltd., 2014) J. Keyser, Y. J. Kim, and P. Wonka
Sub-Pixel Anti-Aliasing Via Triangle-Based Geometry Reconstruction
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Du, Wenjun; Feng, Jieqing; Yang, Baoguang; J. Keyser, Y. J. Kim, and P. Wonka
Anti-aliasing has recently been employed as a post-processing step to adapt to the deferred shading technique in real-time applications. Some of these existing algorithms store supersampling geometric information as geometric buffer (G-buffer) to detect and alleviate sub-pixel-level aliasing artifacts. However, the anti-aliasing filter based on sampled sub-pixel geometries only may introduce unfaithful shading information to the sub-pixel color in uniform-geometry regions, and large G-buffer will increase memory storage and fetch overheads. In this paper, we present a new Triangle-based Geometry Anti-Aliasing (TGAA) algorithm, to address these problems. The coverage triangle of each screen pixel is accessed, and then, the coverage information between the triangle and neighboring sub-pixels is stored in a screen-resolution bitmask, which allows the geometric information to be stored and accessed in an inexpensive manner. Using triangle-based geometry, TGAA can exclude irrelevant neighboring shading samples and achieve faithful anti-aliasing filtering. In addition, a morphological method of estimating the geometric edges in high-frequency geometry is incorporated into the TGAA's anti-aliasing filter to complement the algorithm. The implementation results demonstrate that the algorithm is efficient and scalable for generating high-quality anti-aliased images.
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.
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.
Realistic Road Path Reconstruction from GIS Data
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Nguyen, Hoang Ha; Desbenoit, Brett; Daniel, Marc; J. Keyser, Y. J. Kim, and P. Wonka
We introduce a new approach to construct smooth piecewise curves representing realistic road paths. Given a GIS database of road networks in which sampled points are organized in 3D polylines, our method creates horizontal, then vertical curves, and finally combines them to produce 3D road paths. We first estimate the possibility of each point of being a junction between two separate primitive curve segments. Next, we design a tree-traversal algorithm to expand sequences of local best fit primitives which are then merged together with respect to the G1 continuity constraint and civil engineering rules. We apply the Levenberg-Marquardt method to minimize the error between the resulting curve and the sampled points while preserving the G1 continuity.
Real-Time Symmetry-Preserving Deformation
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Wu, Xiaokun; Wand, Michael; Hildebrandt, Klaus; Kohli, Pushmeet; Seidel, Hans-Peter; J. Keyser, Y. J. Kim, and P. Wonka
In this paper, we address the problem of structure-aware shape deformation: We specifically consider deformations that preserve symmetries of the shape being edited. While this is an elegant approach for obtaining plausible shape variations from minimal assumptions, a straightforward optimization is numerically expensive and poorly conditioned. Our paper introduces an explicit construction of bases of linear spaces of shape deformations that exactly preserve symmetries for any user-defined level of detail. This permits the construction of low-dimensional spaces of low-frequency deformations that preserve the symmetries. We obtain substantial speed-ups over alternative approaches for symmetry-preserving shape editing due to (i) the sub-space approach, which permits low-res editing, (ii) the removal of redundant, symmetric information, and (iii) the simplification of the numerical formulation due to hard-coded symmetry preservation. We demonstrate the utility in practice by applying our framework to symmetry-preserving co-rotated iterative Laplace surface editing of models with complex symmetry structure, including partial and nested symmetry.
Editing and Synthesizing Two-Character Motions using a Coupled Inverted Pendulum Model
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Hwang, Jaepyung; Suh, Il Hong; Kwon, Taesoo; J. Keyser, Y. J. Kim, and P. Wonka
This study aims to develop a controller for use in the online simulation of two interacting characters. This controller is capable of generalizing two sets of interaction motions of the two characters based on the relationships between the characters. The controller can exhibit similar motions to a captured human motion while reacting in a natural way to the opponent character in real time. To achieve this, we propose a new type of physical model called a coupled inverted pendulum on carts that comprises two inverted pendulum on a cart models, one for each individual, which are coupled by a relationship model. The proposed framework is divided into two steps: motion analysis and motion synthesis. Motion analysis is an offline preprocessing step, which optimizes the control parameters to move the proposed model along a motion capture trajectory of two interacting humans. The optimization procedure generates a coupled pendulum trajectory which represents the relationship between two characters for each frame, and is used as a reference in the synthesis step. In the motion synthesis step, a new coupled pendulum trajectory is planned reflecting the effects of the physical interaction, and the captured reference motions are edited based on the planned trajectory produced by the coupled pendulum trajectory generator. To validate the proposed framework, we used a motion capture data set showing two people performing kickboxing. The proposed controller is able to generalize the behaviors of two humans to different situations such as different speeds and turning speeds in a realistic way in real time.
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.
Learning Natural Colors for Image Recoloring
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Huang, Hao-Zhi; Zhang, Song-Hai; Martin, Ralph R.; Hu, Shi-Min; J. Keyser, Y. J. Kim, and P. Wonka
We present a data-driven method for automatically recoloring a photo to enhance its appearance or change a viewer's emotional response to it. A compact representation called a RegionNet summarizes color and geometric features of image regions, and geometric relationships between them. Correlations between color property distributions and geometric features of regions are learned from a database of well-colored photos. A probabilistic factor graph model is used to summarize distributions of color properties and generate an overall probability distribution for color suggestions. Given a new input image, we can generate multiple recolored results which unlike previous automatic results, are both natural and artistic, and compatible with their spatial arrangements.
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.
Incompressible SPH using the Divergence-Free Condition
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Kang, Nahyup; Sagong, Donghoon; J. Keyser, Y. J. Kim, and P. Wonka
In this paper, we present a novel SPH framework to simulate incompressible fluid that satisfies both the divergencefree condition and the density-invariant condition. In our framework, the two conditions are applied separately. First, the divergence-free condition is enforced when solving the momentum equation. Later, the density-invariant condition is applied after the time integration of the particle positions. Our framework is a purely Lagrangian approach so that no auxiliary grid is required. Compared to the previous density-invariant based SPH methods, the proposed method is more accurate due to the explicit satisfaction of the divergence-free condition. We also propose a modified boundary particle method for handling the free-slip condition. In addition, two simple but effective methods are proposed to reduce the particle clumping artifact induced by the density-invariant condition.
Fractional Reyes-Style Adaptive Tessellation for Continuous Level of Detail
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Liktor, Gabor; Pan, Minghao; Dachsbacher, Carsten; J. Keyser, Y. J. Kim, and P. Wonka
In this paper we present a fractional parametric splitting scheme for Reyes-style adaptive tessellation. Our parallel algorithm generates crack-free tessellation from a parametric surface, which is also free of sudden temporal changes under animation. Continuous level of detail is not addressed by existing Reyes-style methods, since these aim to produce subpixel-sized micropolygons, where topology changes are no longer noticeable. Using our method, rendering pipelines that use larger triangles, thus sensitive to geometric popping, may also benefit from the quality of the split-dice tessellation stages of Reyes. We demonstrate results on a real-time GPU implementation, going beyond the limited quality and resolution of the hardware tessellation unit. In contrast to previous split-dice methods, our split stage is compatible with the fractional hardware tessellation scheme that has been designed for continuous level of detail.
Data-Driven Reconstruction of Human Locomotion Using a Single Smartphone
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Eom, Haegwang; Choi, Byungkuk; Noh, Junyong; J. Keyser, Y. J. Kim, and P. Wonka
Generating a visually appealing human motion sequence using low-dimensional control signals is a major line of study in the motion research area in computer graphics. We propose a novel approach that allows us to reconstruct full body human locomotion using a single inertial sensing device, a smartphone. Smartphones are among the most widely used devices and incorporate inertial sensors such as an accelerometer and a gyroscope. To find a mapping between a full body pose and smartphone sensor data, we perform low dimensional embedding of full body motion capture data, based on a Gaussian Process Latent Variable Model. Our system ensures temporal coherence between the reconstructed poses by using a state decomposition model for automatic phase segmentation. Finally, application of the proposed nonlinear regression algorithm finds a proper mapping between the latent space and the sensor data. Our framework effectively reconstructs plausible 3D locomotion sequences. We compare the generated animation to ground truth data obtained using a commercial motion capture system.
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.
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.
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.
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.
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.
shade.js: Adaptive Material Descriptions
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Sons, Kristian; Klein, Felix; Sutter, Jan; Slusallek, Philipp; J. Keyser, Y. J. Kim, and P. Wonka
In computer graphics a material is a visual concept that is parameterizable and should work for arbitrary 3D assets and rendering systems. Since provided parameters and attributes as well as the capabilities of rendering systems vary considerably, a material needs to adapt to its execution environment. In current approaches, the adaptation logic is 'baked' into the rendering application based on string manipulation, compiler directives, or metaprogramming facilities. However, in order to achieve application-independent and self-contained material descriptions, the adaptation logic needs to be part of the material description itself. In this paper we present shade.js, a novel material description using a dynamic language to achieve the necessary adaptivity. A shader can inspect its execution environment and adapt to the available parameters and renderer capabilities at run time. Additionally, shade.js exploits the polymorphism that comes with non-explicit declaration of types. These two novel features allow for writing adaptable and thus more general material descriptions. Based on the concrete execution environment at run time, the accompanied compiler generates specialized shader code that is specifically typed and optimized for the target rendering system and algorithm. We evaluate shade.js with examples targeting four different rendering approaches (forward and deferred rasterization, ray-tracing, and global illumination). We show that we can improve convenience and flexibility for specifying materials without sacrificing performance.

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