32-Issue 7

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https://diglib.eg.org/handle/10.2312/185

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Second-Order Approximation for Variance Reduction in Multiple Importance Sampling
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Lu, Heqi; Pacanowski, Romain; Granier, Xavier; B. Levy, X. Tong, and K. Yin
Monte Carlo Techniques are widely used in Computer Graphics to generate realistic images. Multiple Importance Sampling reduces the impact of choosing a dedicated strategy by balancing the number of samples between different strategies. However, an automatic choice of the optimal balancing remains a difficult problem. Without any scene characteristics knowledge, the default choice is to select the same number of samples from different strategies and to use them with heuristic techniques (e.g., balance, power or maximum). In this paper, we introduce a second-order approximation of variance for balance heuristic. Based on this approximation, we introduce an automatic distribution of samples for direct lighting without any prior knowledge of the scene characteristics. We demonstrate that for all our test scenes (with different types of materials, light sources and visibility complexity), our method actually reduces variance in average.We also propose an implementation with low overhead for offline and GPU applications. We hope that this approach will help developing new balancing strategies.
An Efficient and Scalable Image Filtering Framework Using VIPS Fusion
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Zhang, Jun; Chen, Xiuhong; Zhao, Yan; Li, H.; B. Levy, X. Tong, and K. Yin
Edge-preserving image filtering is a valuable tool for a variety of applications in image processing and computer vision. Motivated by a new simple but effective local Laplacian filter, we propose a scalable and efficient image filtering framework to extend this edge-preserving image filter and construct an uniform implementation in O(N) time. The proposed framework is built upon a practical global-to-local strategy. The input image is first remapped globally by a series of tentative remapping functions to generate a virtual candidate image sequence (Virtual Image Pyramid Sequence, VIPS). This sequence is then recombined locally to a single output image by a flexible edge-aware pixel-level fusion rule. To avoid halo artifacts, both the output image and the virtual candidate image sequence are transformed into multi-resolution pyramid representations. Four examples, single image de-hazing, multi-exposure fusion, fast edge-preserving filtering and tone-mapping, are presented as the concrete applications of the proposed framework. Experiments on filtering effect and computational efficiency indicate that the proposed framework is able to build a wide range of fast image filtering that yields visually compelling results.
Artistic QR Code Embellishment
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Lin, Yi-Shan; Luo, Sheng-Jie; Chen, Bing-Yu; B. Levy, X. Tong, and K. Yin
A QR code is a two-dimensional barcode that encodes information. A standard QR code contains only regular black and white squares, and thus is unattractive. This paper proposes a novel framework for embellishing a standard QR code, to make it both attractive and recognizable by any human while maintaining its scanability. The proposed method is inspired by artistic methods. A QR code is typically embellished by stylizing the squares and embedding images into it. In the proposed framework, the regular squares are reshaped using a binary examplar, to make their local appearances resemble the example shape. Additionally, an error-aware warping technique for deforming the embedded image is proposed; it minimizes the error in the QR code that is generated by the embedding of the image to optimize the readability of the code. The proposed algorithm yields lower data error than previous global transformation techniques because the warping can locally deform the embedded image to conform to the squares that surround it. The proposed framework was examined by using it to embellish an extensive set of QR codes and to test the readability with various commercial QR code readers.
Boundary-Aware Extinction Mapping
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Gautron, Pascal; Delalandre, Cyril; Marvie, Jean-Eudes; Lecocq, Pascal; B. Levy, X. Tong, and K. Yin
We introduce Boundary-Aware Extinction Maps for interactive rendering of massive heterogeneous volumetric datasets. Our approach is based on the projection of the extinction along light rays into a boundary-aware function space, focusing on the most relevant sections of the light paths. This technique also provides an alternative representation of the set of participating media, allowing scattering simulation methods to be applied on arbitrary volume representations. Combined with a simple out-of-core rendering framework, Boundary-Aware Extinction Maps are valuable tools for interactive applications as well as production previsualization and rendering.
A Programmable Graphics Processor based on Partial Stream Rewriting
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Middendorf, Lars; Haubelt, Christian; B. Levy, X. Tong, and K. Yin
Current graphics processing units (GPU) typically offer only a limited number of programmable pipeline stages, whose usage, data flow and topology are mostly fixed. Although a more flexible, custom rendering pipeline can be emulated using the compute functionality of existing GPUs, this approach requires to manage work queues, synchronization, and scheduling in software. In this paper, we present a hardware architecture for a novel, programmable rendering pipeline, which is based on a circulating stream of data and control tokens that are iteratively modified via pattern matching. Our architecture provides light-weight mechanisms for dynamic thread creation, lock-free synchronization, and scheduling to support recursion, dynamic shader linkage and custom primitive types. A hardware prototype, running complex examples, demonstrates the improved reconfigurability also the scalability of our graphics architecture.
A Semi-Lagrangian Closest Point Method for Deforming Surfaces
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Auer, Stefan; Westermann, Rüdiger; B. Levy, X. Tong, and K. Yin
We present an Eulerian method for the real-time simulation of intrinsic fluid dynamics effects on deforming surfaces. Our method is based on a novel semi-Lagrangian closest point method for the solution of partial differential equations on animated triangle meshes.We describe this method and demonstrate its use to com- pute and visualize flow and wave propagation along such meshes at high resolution and speed. Underlying our technique is the efficient conversion of an animated triangle mesh into a time-dependent implicit repre- sentation based on closest surface points. The proposed technique is unconditionally stable with respect to the surface deformation and, in contrast to comparable Lagrangian techniques, its precision does not depend on the level of detail of the surface triangulation.
Efficient Shadow Removal Using Subregion Matching Illumination Transfer
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Xiao, Chunxia; Xiao, Donglin; Zhang, Ling; Chen, Lin; B. Levy, X. Tong, and K. Yin
This paper proposes a new shadow removal approach for input single natural image by using subregion matching illumination transfer. We first propose an effective and automatic shadow detection algorithm incorporating global successive thresholding scheme and local boundary refinement. Then we present a novel shadow removal algorithm by performing illumination transfer on the matched subregion pairs between the shadow regions and non-shadow regions, and this method can process complex images with different kinds of shadowed texture regions and illumination conditions. In addition, we develop an efficient shadow boundary processing method by using alpha matte interpolation, which produces seamless transition between the shadow and non-shadow regions. Experimental results demonstrate the capabilities of our algorithm in both the shadow removal quality and performance.
Learning and Applying Color Styles From Feature Films
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Xue, Su; Agarwala, Aseem; Dorsey, Julie; Rushmeier, Holly; B. Levy, X. Tong, and K. Yin
Directors employ a process called ''color grading'' to add color styles to feature films. Color grading is used for a number of reasons, such as accentuating a certain emotion or expressing the signature look of a director. We collect a database of feature film clips and label them with tags such as director, emotion, and genre. We then learn a model that maps from the low-level color and tone properties of film clips to the associated labels. This model allows us to examine a number of common hypotheses on the use of color to achieve goals, such as specific emotions. We also describe a method to apply our learned color styles to new images and videos. Along with our analysis of color grading techniques, we demonstrate a number of images and videos that are automatically filtered to resemble certain film styles.
Symmetry Robust Descriptor for Non-Rigid Surface Matching
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Zhang, Zhiyuan; Yin, KangKang; Foong, Kelvin W. C.; B. Levy, X. Tong, and K. Yin
In this paper, we propose a novel shape descriptor that is robust in differentiating intrinsic symmetric points on geometric surfaces. Our motivation is that even the state-of-the-art shape descriptors and non-rigid surface matching algorithms suffer from symmetry flips. They cannot differentiate surface points that are symmetric or near symmetric. Hence a left hand of one human model may be matched to a right hand of another. Our Symmetry Robust Descriptor (SRD) is based on a signed angle field, which can be calculated from the gradient fields of the harmonic fields of two point pairs. Experiments show that the proposed shape descriptor SRD results in much less symmetry flips compared to alternative methods. We further incorporate SRD into a stand-alone algorithm to minimize symmetry flips in finding sparse shape correspondences. SRD can also be used to augment other modern non-rigid shape matching algorithms with ease to alleviate symmetry confusions.
Combustion Waves on the Point Set Surface
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Jeong, SoHyeon; Kim, Chang-Hun; B. Levy, X. Tong, and K. Yin
This paper introduces a combustion model of heat transfer and fuel consumption for the propagation of a fire front on a point cloud surface. The heat transfer includes the heat advection by the airflow as well as diffusion, chemical reaction, and heat loss to generate complex, but controllable heat flows with a designed airflow velocity. For the stable heat advection, we solve a semi-Lagrangian method on point samples using discrete exponential maps to trace the position from which the wind blows while preserving the geodesic distance. We also propose angular Voronoi weights for a discrete Laplace-Beltrami operator that shows better isotropic diffusion on the inhomogeneous distribution of point clouds than the cotangent or moving least-squares schemes. We demonstrate a diversity of burning scenarios by incorporating factors affecting the fire spreading such as buoyancy and object geometries in the airflow velocity fields, or by synthesizing patterns.
Coarse-to-Fine Normal Filtering for Feature-Preserving Mesh Denoising Based on Isotropic Subneighborhoods
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Zhu, Lei; Wie, Mingqiang; Yu, Jinze; Wang, Weiming; Qin, Jing; Heng, Pheng-Ann; B. Levy, X. Tong, and K. Yin
State-of-the-art normal filters usually denoise each face normal using its entire anisotropic neighborhood. However, enforcing these filters indiscriminately on the anisotropic neighborhood will lead to feature blurring, especially in challenging regions with shallow features. We develop a novel mesh denoising framework which can effectively preserve features with various sizes. Our idea is inspired by the observation that the underlying surface of a noisy mesh is piecewise smooth. In this regard, it is more desirable that we denoise each face normal within its piecewise smooth region (we call such a region as an isotropic subneighborhood) instead of using the anisotropic neighborhood. To achieve this, we first classify mesh faces into several types using a face normal tensor voting and then perform a normal filter to obtain a denoised coarse normal field. Based on the results of normal classification and the denoised coarse normal field, we segment the anisotropic neighborhood of every feature face into a number of isotropic subneighborhoods via local spectral clustering. Thus face normal filtering can be performed again on the isotropic subneighborhoods and produce a more accurate normal field. Extensive tests on various models demonstrate that our method can achieve better performance than state-of-the-art normal filters, especially in challenging regions with features.
Polar NURBS Surface with Curvature Continuity
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Shi, Kan-Le; Yong, Jun-Hai; Tang, Lei; Sun, Jia-Guang; Paul, Jean-Claude; B. Levy, X. Tong, and K. Yin
Polar NURBS surface is a kind of periodic NURBS surface, one boundary of which shrinks to a degenerate polar point. The specific topology of its control-point mesh offers the ability to represent a cap-like surface, which is common in geometric modeling. However, there is a critical and challenging problem that hinders its application: curvature continuity at the extraordinary singular pole. We first propose a sufficient and necessary condition of curvature continuity at the pole. Then, we present constructive methods for the two key problems respectively: how to construct a polar NURBS surface with curvature continuity and how to reform an ordinary polar NURBS surface to curvature continuous. The algorithms only depend on the symbolic representation and operations of NURBS, and they introduce no restrictions on the degree or the knot vectors. Examples and comparisons demonstrate the applications of the curvature-continuous polar NURBS surface in hole-filling and free-shape modeling.
TrayGen: Arranging Objects for Exhibition and Packaging
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Yang, Yong-Liang; Huang, Qi-Xing; B. Levy, X. Tong, and K. Yin
We present a framework, called TrayGen, to generate tray designs for the exhibition and packaging of a collection of objects. Based on principles from shape perception and visual merchandising, we abstract a number of design guidelines on how to organize the objects on the tray for the exhibition of their individual features and mutual relationships. Our framework realizes these guidelines by analyzing geometric shapes of the objects and optimizing their arrangement. We demonstrate that the resultant tray designs not only save space, but also highlight the characteristic of each object and the inter-relations between objects.
Hair Interpolation for Portrait Morphing
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Weng, Yanlin; Wang, Lvdi; Li, Xiao; Chai, Menglei; Zhou, Kun; B. Levy, X. Tong, and K. Yin
In this paper we study the problem of hair interpolation: given two 3D hair models, we want to generate a sequence of intermediate hair models that transform from one input to another both smoothly and aesthetically pleasing. We propose an automatic method that efficiently calculates a many-to-many strand correspondence between two or more given hair models, taking into account the multi-scale clustering structure of hair. Experiments demonstrate that hair interpolation can be used for producing more vivid portrait morphing effects and enabling a novel example-based hair styling methodology, where a user can interactively create new hairstyles by continuously exploring a ''style space'' spanning multiple input hair models.
Implicit Integration for Particle-based Simulation of Elasto-Plastic Solids
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Zhou, Yahan; Lun, Zhaoliang; Kalogerakis, Evangelos; Wang, Rui; B. Levy, X. Tong, and K. Yin
We present a novel particle-based method for stable simulation of elasto-plastic materials. The main contribution of our method is an implicit numerical integrator, using a physically-based model, for computing particles that undergo both elastic and plastic deformations. The main advantage of our implicit integrator is that it allows the use of large time steps while still preserving stable and physically plausible simulation results. As a key component of our algorithm, at each time step we compute the particle positions and velocities based on a sparse linear system, which we solve efficiently on the graphics hardware. Compared to existing techniques, our method allows for a much wider range of stiffness and plasticity settings. In addition, our method can significantly reduce the computation cost for certain range of material types. We demonstrate fast and stable simulations for a variety of elasto-plastic materials, ranging from highly stiff elastic materials to highly plastic ones.
The POP Buffer: Rapid Progressive Clustering by Geometry Quantization
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Limper, Max; Jung, Yvonne; Behr, Johannes; Alexa, Marc; B. Levy, X. Tong, and K. Yin
Within this paper, we present a novel, straightforward progressive encoding scheme for general triangle soups, which is particularly well-suited for mobile and Web-based environments due to its minimal requirements on the client's hardware and software. Our rapid encoding method uses a hierarchy of quantization to effectively reorder the original primitive data into several nested levels of detail. The resulting stateless buffer can progressively be transferred as-is to the GPU, where clustering is efficiently performed in parallel during rendering. We combine our approach with a crack-free mesh partitioning scheme to obtain a straightforward method for fast streaming and basic view-dependent LOD control.
As-Rigid-As-Possible Distance Field Metamorphosis
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Weng, Yanlin; Chai, Menglei; Xu, Weiwei; Tong, Yiying; Zhou, Kun; B. Levy, X. Tong, and K. Yin
Widely used for morphing between objects with arbitrary topology, distance field interpolation (DFI) handles topological transition naturally without the need for correspondence or remeshing, unlike surface-based interpolation approaches. However, lack of correspondence in DFI also leads to ineffective control over the morphing process. In particular, unless the user specifies a dense set of landmarks, it is not even possible to measure the distortion of intermediate shapes during interpolation, let alone control it. To remedy such issues, we introduce an approach for establishing correspondence between the interior of two arbitrary objects, formulated as an optimal mass transport problem with a sparse set of landmarks. This correspondence enables us to compute non-rigid warping functions that better align the source and target objects as well as to incorporate local rigidity constraints to perform as-rigid-as-possible DFI. We demonstrate how our approach helps achieve flexible morphing results with a small number of landmarks.
A Progressive Tri-level Segmentation Approach for Topology-Change-Aware Video Matting
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Ju, Jinlong; Wang, Jue; Liu, Yebin; Wang, Haoqian; Dai, Qionghai; B. Levy, X. Tong, and K. Yin
Previous video matting approaches mostly adopt the ''binary segmentation + matting'' strategy, i.e., first segment each frame into foreground and background regions, then extract the fine details of the foreground boundary using matting techniques. This framework has several limitations due to the fact that binary segmentation is employed. In this paper, we propose a new supervised video matting approach. Instead of applying binary segmentation, we explicitly model segmentation uncertainty in a novel tri-level segmentation procedure. The segmentation is done progressively, enabling us to handle difficult cases such as large topology changes, which are challenging to previous approaches. The tri-level segmentation results can be naturally fed into matting techniques to generate the final alpha mattes. Experimental results show that our system can generate high quality results with less user inputs than the state-of-the-art methods.
Modeling by Drawing with Shadow Guidance
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Fan, Lubin; Wang, Ruimin; Xu, Linlin; Deng, Jiansong; Liu, Ligang; B. Levy, X. Tong, and K. Yin
Modeling 3D objects is difficult, especially for the user who lacks the knowledge on 3D geometry or even on 2D sketching. In this paper, we present a novel sketch-based modeling system which allows novice users to create 3D custom models by assembling parts based on a database of pre-segmented 3D models. Different from previous systems, our system supports the user with visualized and meaningful shadow guidance under his strokes dynamically to guide the user to convey his design concept easily and quickly. Our system interprets the user's strokes as similarity queries into database to generate the shadow image for guiding the user's further drawing and returns the 3D candidate parts for modeling simultaneously. Moreover, our system preserves the high-level structure in generated models based on prior knowledge pre-analyzed from the database, and allows the user to create custom parts with geometric variations. We demonstrate the applicability and effectiveness of our modeling system with human subjects and present various models designed using our system.
Level-of-Detail Streaming and Rendering using Bidirectional Sparse Virtual Texture Functions
(The Eurographics Association and Blackwell Publishing Ltd., 2013) Schwartz, Christopher; Ruiters, Roland; Klein, Reinhard; B. Levy, X. Tong, and K. Yin
Bidirectional Texture Functions (BTFs) are among the highest quality material representations available today and thus well suited whenever an exact reproduction of the appearance of a material or complete object is required. In recent years, BTFs have started to find application in various industrial settings and there is also a growing interest in the cultural heritage domain. BTFs are usually measured from real-world samples and easily consist of tens or hundreds of gigabytes. By using data-driven compression schemes, such as matrix or tensor factorization, a more compact but still faithful representation can be derived. This way, BTFs can be employed for real-time rendering of photo-realistic materials on the GPU. However, scenes containing multiple BTFs or even single objects with high-resolution BTFs easily exceed available GPU memory on today's consumer graphics cards unless quality is drastically reduced by the compression. In this paper, we propose the Bidirectional Sparse Virtual Texture Function, a hierarchical level-of-detail approach for the real-time rendering of large BTFs that requires only a small amount of GPU memory. More importantly, for larger numbers or higher resolutions, the GPU and CPU memory demand grows only marginally and the GPU workload remains constant. For this, we extend the concept of sparse virtual textures by choosing an appropriate prioritization, finding a trade off between factorization components and spatial resolution. Besides GPU memory, the high demand on bandwidth poses a serious limitation for the deployment of conventional BTFs. We show that our proposed representation can be combined with an additional transmission compression and then be employed for streaming the BTF data to the GPU from from local storage media or over the Internet. In combination with the introduced prioritization this allows for the fast visualization of relevant content in the users field of view and a consecutive progressive refinement.

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