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Minimal Sampling for Effective Acquisition of Anisotropic BRDFs

computer graphics FORUM 2016年第7期35卷 299-309页

作者： Vavra, R. Filip, J. CAS Inst Informat Theory & Automat Prague Czech Republic

BRDFs are commonly used for material appearance representation in applications ranging from gaming and the movie industry, to product design and specification. Most applications rely on isotropic BRDFs due to their better availability as a result of their easier acquisition process. On the other hand, anisotropic BRDF due to their structure-dependent anisotropic highlights, are more challenging to measure and process. This paper thus leverages the measurement process of anisotropic BRDF by representing such BRDF by the collection of isotropic BRDFs. Our method relies on an anisotropic BRDF database decomposition into training isotropic slices forming a linear basis, where appropriate sparse samples are identified using numerical optimization. When an unknown anisotropic BRDF is measured, these samples are repeatably captured in a small set of azimuthal directions. All collected samples are then used for an entire measured BRDF reconstruction from a linear isotropic basis. Typically, below 100 samples are sufficient for the capturing of main visual features of complex anisotropic materials, and we provide a minimal directional samples to be regularly measured at each sample rotation. We conclude, that even simple setups relying on five bidirectional samples (maximum of five stationary sensors/lights) in combination with eight rotations (rotation stage for specimen) can yield a promising reconstruction of anisotropic behavior. Next, we outline extension of the proposed approach to adaptive sampling of anisotropic BRDF to gain even better performance. Finally, we show that our method allows using standard geometries, including industrial multi-angle reflectometers, for the fast measurement of anisotropic BRDFs.

关键词： Categories and Subject Descriptors (according to ACM CCS) I.3.7 [computer graphics]: Three-Dimensional graphics and Realism&mdash Color shading shadowing and texture I.4.1 [computer graphics]: Digitization and Image Capture&mdash Reflectance

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Anaglyph Caustics with Motion Parallax

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computer graphics FORUM 2016年第7期35卷 343-352页

作者： Lancelle, M. Martin, T. Solenthaler, B. Gross, M. Swiss Fed Inst Technol Dept Comp Sci Zurich Switzerland Disney Res Zurich Zurich Switzerland

In this paper we present a method to model and simulate a lens such that its caustic reveals a stereoscopic 3D image when viewed through anaglyph glasses. By interpreting lens dispersion as stereoscopic disparity, our method optimizes the shape and arrangement of prisms constituting the lens, such that the resulting anaglyph caustic corresponds to a given input image defined by intensities and disparities. In addition, a slight change of the lens' distance to the screen causes a 3D parallax effect that can also be perceived without glasses. Our proposed relaxation method carefully balances the resulting pixel intensity and disparity error, while taking the subsequent physical fabrication process into account. We demonstrate our method on a representative set of input images and evaluate the anaglyph caustics using multi-spectral photon tracing. We further show the fabrication of prototype lenses with a laser cutter as a proof of concept.

关键词： Categories and Subject Descriptors (according to ACM CCS) I.3.3 [computer graphics]: Three-Dimensional graphics and Realism&mdash Display Algorithms

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Planar Shape Interpolation Based On Teichmuller Mapping

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computer graphics FORUM 2016年第7期35卷 43-56页

作者： Nian, Xianshun Chen, Falai Univ Sci & Technol China Sch Math Sci Hefei 230026 Anhui Peoples R China

Shape interpolation is a classical problem in computer graphics and has been widely investigated in the past two decades. Ideal shape interpolation should be natural and smooth which have good properties such as affine and conformal reproduction, bounded distortion, no fold-overs, etc. In this paper, we present a new approach for planar shape interpolation based on Teichmuller maps - a special type of maps in the class of quasi-conformal maps. The algorithm consists of two steps. In the first step, a Teichmuller map is computed from the source shape to the target shape, and then the Beltrami coefficient is interpolated such that the conformal distortion is linear with respect to the time variable. In the second step, the intermediate shape is reconstructed by solving the Beltrami equation locally over each triangle and then stitching the mapped triangles by conformal transformations. The new approach preserves all the good properties mentioned above and produces more natural and more uniform intermediate shapes than the start-of-the-art methods. Especially, the conformal distortion changes linearly with respect to the time variable. Experiment results show that our method can produce appealing results regardless of interpolating between the same or different objects.

关键词： Categories and Subject Descriptors (according to ACM CCS) I.3.3 [computer graphics]: Picture/Image Generation&mdash Display Algorithms I.3.7 [computer graphics]: Three-Dimensional graphics and Realism&mdash Animation

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Facial Feature Exaggeration According to Social Psychology of Face Perception

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computer graphics FORUM 2016年第7期35卷 391-399页

作者： Tian, Lihui Xiao, Shuangjiu Shanghai Jiao Tong Univ Shanghai 200030 Peoples R China

We propose a personality trait exaggeration system emphasizing the impression of human face in images, based on multi-level features learning and exaggeration. These features are called Personality Trait Model(PTM). Abstract level of PTM is social psychology trait of face perception such as amiable, mean, cute and so on. Concrete level of PTM is shape feature and texture feature. A training phase is presented to learn multi-level features of faces from different images. Statistical survey is taken to label sample images with people's first impressions. From images with the same labels, we capture not only shape features but also texture features to enhance exaggeration effect. Texture feature is expressed by matrix to reflect depth of facial organs, wrinkles and so on. In application phase, original images will be exaggerated using PTM iteratively. And exaggeration rate for each iteration is constrained to keep likeness with the original face. Experimental results demonstrate that our system can emphasize chosen social psychology traits effectively.

关键词： Categories and Subject Descriptors (according to ACM CCS) I.3.3 [computer graphics]: Picture/Image Generation&mdash

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Anisotropic Superpixel Generation Based on Mahalanobis Distance

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computer graphics FORUM 2016年第7期35卷 199-207页

作者： Cai, Yiqi Guo, Xiaohu Univ Texas Dallas Dallas TX 75080 USA

Superpixels have been widely used as a preprocessing step in various computer vision tasks. Spatial compactness and color homogeneity are the two key factors determining the quality of the superpixel representation. In this paper, these two objectives are considered separately and anisotropic superpixels are generated to better adapt to local image content. We develop a unimodular Gaussian generative model to guide the color homogeneity within a superpixel by learning local pixel color variations. It turns out maximizing the log-likelihood of our generative model is equivalent to solving a Centroidal Voronoi Tessellation (CVT) problem. Moreover, we provide the theoretical guarantee that the CVT result is invariant to affine illumination change, which makes our anisotropic superpixel generation algorithm well suited for image/video analysis in varying illumination environment. The effectiveness of our method in image/video superpixel generation is demonstrated through the comparison with other state-of-the-art methods.

关键词： computer vision GAUSSIAN distribution PIXELS DIGITAL images IMAGE processing RESOLUTION (Optics) CENTROIDAL Voronoi tessellations VORONOI polygons

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Scale-aware Structure-Preserving Texture Filtering

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computer graphics FORUM 2016年第7期35卷 77-86页

作者： Jeon, Junho Lee, Hyunjoon Kang, Henry Lee, Seungyong POSTECH Dept Comp Sci & Engn Pohang South Korea Univ Missouri Dept Math & Comp Sci St Louis MO 63121 USA

This paper presents a novel method to enhance the performance of structure-preserving image and texture filtering. With conventional edge-aware filters, it is often challenging to handle images of high complexity where features of multiple scales coexist. In particular, it is not always easy to find the right balance between removing unimportant details and protecting important features when they come in multiple sizes, shapes, and contrasts. Unlike previous approaches, we address this issue from the perspective of adaptive kernel scales. Relying on patch-based statistics, our method identifies texture from structure and also finds an optimal per-pixel smoothing scale. We show that the proposed mechanism helps achieve enhanced image/ texture filtering performance in terms of protecting the prominent geometric structures in the image, such as edges and corners, and keeping them sharp even after significant smoothing of the original signal.

关键词： Categories and Subject Descriptors (according to ACM CCS) I.4.3 [Image Processing and computer Vision]: Enhancement&mdash Smoothing

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Geometrically Based Linear Iterative Clustering for Quantitative Feature Correspondence

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computer graphics FORUM 2016年第7期35卷 1-10页

作者： Yan, Qingan Yang, Long Liang, Chao Liu, Huajun Hu, Ruimin Xiao, Chunxia Wuhan Univ Sch Comp Wuhan 430072 Peoples R China Wuhan Univ State Key Lab Software Engn Wuhan 430072 Peoples R China

A major challenge in feature matching is the lack of objective criteria to determine corresponding points. Recent methods find match candidates first by exploring the proximity in descriptor space, and then rely on a ratio-test strategy to determine final correspondences. However, these measurements are heuristic and subjectively excludes massive true positive correspondences that should be matched. In this paper, we propose a novel feature matching algorithm for image collections, which is capable of providing quantitative depiction to the plausibility of feature matches. We achieve this by exploring the epipolar consistency between feature points and their potential correspondences, and reformulate feature matching as an optimization problem in which the overall geometric inconsistency across the entire image set ought to be minimized. We derive the solution of the optimization problem in a simple linear iterative manner, where a k-means-type approach is designed to automatically generate consistent feature clusters. Experiments show that our method produces precise correspondences on a variety of image sets and retrieves many matches that are subjectively rejected by recent methods. We also demonstrate the usefulness of the framework in structure from motion task for denser point cloud reconstruction.

关键词： Categories and Subject Descriptors (according to ACM CCS) I.3.m [computer graphics]: Computational photography&mdash I.4.0 [computer graphics]: General&mdash I.4.7 [computer graphics]: Feature Measurement&mdash

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Efficient Gradient-Domain Compositing Using an Approximate Curl-free Wavelet Projection

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computer graphics FORUM 2017年第7期36卷 207-215页

作者： Ren, Xiaohua Lyu, Luan He, Xiaowei Zhang, Yanci Wu, Enhua Univ Macau Taipa Macao Peoples R China ISCAS State Key Lab CS Beijing Peoples R China Univ CAS Beijing Peoples R China Sichuan Univ Chengdu Sichuan Peoples R China Zhuhai UM Sci & Technol Res Inst Zhuhai Peoples R China

Gradient-domain compositing has been widely used to create a seamless composite with gradient close to a composite gradient field generated from one or more registered images. The key to this problem is to solve a Poisson equation, whose unknown variables can reach the size of the composite if no region of interest is drawn explicitly, thus making both the time and memory cost expensive in processing multi-megapixel images. In this paper, we propose an approximate projection method based on biorthogonal Multiresolution Analyses (MRA) to solve the Poisson equation. Unlike previous Poisson equation solvers which try to converge to the accurate solution with iterative algorithms, we use biorthogonal compactly supported curl-free wavelets as the fundamental bases to approximately project the composite gradient field onto a curl-free vector space. Then, the composite can be efficiently recovered by applying a fast inverse wavelet transform. Considering an n-pixel composite, our method only requires 2n of memory for all vector fields and is more efficient than state-of-the-art methods while achieving almost identical results. Specifically, experiments show that our method gains a 5x speedup over the streaming multigrid in certain cases.

关键词： Categories and Subject Descriptors (according to ACM CCS) I.3.3 [computer graphics]: Picture/Image Generation—Display algorithms

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Harmonic Functions for Rotational Symmetry Vector Fields

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computer graphics FORUM 2016年第7期35卷 507-516页

作者： Shen, Zhongwei Fang, Xianzhong Liu, Xinguo Bao, Hujun Huang, Jin Zhejiang Univ State Key Lab CAD&CG Hangzhou Zhejiang Peoples R China

Representing rotational symmetry vector as a set of vectors is not suitable for design due to lacking of a consistent ordering for measurement. In this paper we introduce a spectral method to find rotation invariant harmonic functions for symmetry vector field design. This method is developed for 3D vector fields, but it is applicable in 2D. Given the finite symmetry group G of a symmetry vector field v(x) on a 3D domain Omega, we formulate the harmonic function h (s) as a stationary point of group G. Using the real spherical harmonic (SH) bases, we showed the coefficients of the harmonic functions are an eigenvector of the SH rotation matrices corresponding to group G. Instead of solving eigen problems to obtain the eigenvector, we developed a forward constructive method based on orthogonal group theory. The harmonic function found by our method is not only invariant under G, but also expressive and can distinguish different rotations with respect to G. At last, we demonstrate some vector field design results with tetrahedron-symmetry, cube-symmetry and dodecahedron-symmetry groups.

关键词： Categories and Subject Descriptors (according to ACM CCS) I.3.5 [computer graphics]: Computational Geometry and Object Modeling&mdash Geometric algorithms

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Piecewise smooth reconstruction of normal vector field on digital data

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computer graphics FORUM 2016年第7期35卷 157-167页

作者： Coeurjolly, David Foare, Marion Gueth, Pierre Lachaud, Jacques-Olivier Univ Lyon CNRS LIRIS UMR 5205 F-69622 Lyon France Univ Savoie Mont Blanc UMR 5127 CNRS Lab Math Chambery France

We propose a novel method to regularize a normal vector field defined on a digital surface (boundary of a set of voxels). When the digital surface is a digitization of a piecewise smooth manifold, our method localizes sharp features (edges) while regularizing the input normal vector field at the same time. It relies on the optimisation of a variant of the Ambrosio-Tortorelli functional, originally defined for denoising and contour extraction in image processing [AT90]. We reformulate this functional to digital surface processing thanks to discrete calculus operators. Experiments show that the output normal field is very robust to digitization artifacts or noise, and also fairly independent of the sampling resolution. The method allows the user to choose independently the amount of smoothing and the length of the set of discontinuities. Sharp and vanishing features are correctly delineated even on extremely damaged data. Finally, our method can be used to enhance considerably the output of state-of-the-art normal field estimators like Voronoi Covariance Measure [MOG11] or Randomized Hough Transform [BM12].

关键词： Categories and Subject Descriptors (according to ACM CCS) [computer graphics]: Modeling - Digital Geometry Processing&mdash Modeling - Shape Analysis

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