33-Issue 7

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

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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.
Adaptive Multi-scale Analysis for Point-based Surface Editing
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Nader, Georges; Guennebaud, Gael; Mellado, Nicolas; J. Keyser, Y. J. Kim, and P. Wonka
This paper presents a tool that enables the direct editing of surface features in large point-clouds or meshes. This is made possible by a novel multi-scale analysis of unstructured point-clouds that automatically extracts the number of relevant features together with their respective scale all over the surface. Then, combining this ingredient with an adequate multi-scale decomposition allows us to directly enhance or reduce each feature in an independent manner. Our feature extraction is based on the analysis of the scale-variations of locally fitted surface primitives combined with unsupervised learning techniques. Our tool may be applied either globally or locally, and millions of points are handled in real-time. The resulting system enables users to accurately edit complex geometries with minimal interaction.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
G2 Surface Interpolation Over General Topology Curve Networks
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Salvi, Péter; Várady, Tamás; J. Keyser, Y. J. Kim, and P. Wonka
The basic idea of curve network-based design is to construct smoothly connected surface patches, that interpolate boundaries and cross-derivatives extracted from the curve network. While the majority of applications demands only tangent plane (G1) continuity between the adjacent patches, curvature continuous connections (G2) may also be required. Examples include special curve network configurations with supplemented internal edges, ''masterslave'' curvature constraints, and general topology surface approximations over meshes. The first step is to assign optimal surface curvatures to the nodes of the curve network; we discuss different optimization procedures for various types of nodes. Then interpolant surfaces called parabolic ribbons are created along the patch boundaries, which carry first and second derivative constraints. Our construction guarantees that the neighboring ribbons, and thus the respective transfinite patches, will be G2 continuous. We extend Gregory's multi-sided surface scheme in order to handle parabolic ribbons, involving the blending functions, and a new sweepline parameterization. A few simple examples conclude the paper.
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.
Illuminant Aware Gamut-Based Color Transfer
(The Eurographics Association and John Wiley and Sons Ltd., 2014) Nguyen, Rang M. H.; Kim, Seon Joo; Brown, Michael S.; J. Keyser, Y. J. Kim, and P. Wonka
This paper proposes a new approach for color transfer between two images. Our method is unique in its consideration of the scene illumination and the constraint that the mapped image must be within the color gamut of the target image. Specifically, our approach first performs a white-balance step on both images to remove color casts caused by different illuminations in the source and target image. We then align each image to share the same 'white axis' and perform a gradient preserving histogram matching technique along this axis to match the tone distribution between the two images. We show that this illuminant-aware strategy gives a better result than directly working with the original source and target image's luminance channel as done by many previous methods. Afterwards, our method performs a full gamut-based mapping technique rather than processing each channel separately. This guarantees that the colors of our transferred image lie within the target gamut. Our experimental results show that this combined illuminant-aware and gamut-based strategy produces more compelling results than previous methods. We detail our approach and demonstrate its effectiveness on a number of examples.
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.
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.
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.

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