The Experts below are selected from a list of 15387 Experts worldwide ranked by ideXlab platform
Darrin M York - One of the best experts on this subject based on the ideXlab platform.
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spherical tensor Gradient Operator method for integral rotation a simple efficient and extendable alternative to slater koster tables
Journal of Chemical Physics, 2008Co-Authors: Timothy J Giese, Darrin M YorkAbstract:We present a novel alternative to the use of Slater–Koster tables for the efficient rotation and Gradient evaluation of two-center integrals used in tight-binding Hamiltonian models. The method recasts the problem into an exact, yet implicit, basis representation through which the properties of the spherical tensor Gradient Operator are exploited. These properties provide a factor of 3 to 4 speedup in the evaluation of the integral Gradients and afford a compact code structure that easily extends to high angular momentum without loss in efficiency. Thus, the present work is important in improving the performance of tight-binding models in molecular dynamics simulations and has particular use for methods that require the evaluation of two-center integrals that involve high angular momentum basis functions. These advances have a potential impact for the design of new tight-binding models that incorporate polarization or transition metal basis functions and methods based on electron density fitting of molecular fragments.
Enmin Song - One of the best experts on this subject based on the ideXlab platform.
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a quaternion Gradient Operator for color image edge detection
International Conference on Image Processing, 2013Co-Authors: Lianghai Jin, Enmin SongAbstract:Estimating Gradients of color images is important to many color image processing tasks. However, the research on color image Gradients is limited. In this paper, a novel method of estimating color image Gradients is presented. The proposed Gradient mechanism is based on measuring the squared local contrast variation of a color image function, in which the chromatic variation is evaluated by quaternion representation. As an application in color image processing, we apply the proposed Gradient Operator to the traditional grayscale image Canny Operator for color edge detection. The edge detection results indicate that the proposed color Gradient Operator is superior to other state-of-the-art color image Gradient methods.
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ICIP - A quaternion Gradient Operator for color image edge detection
2013 IEEE International Conference on Image Processing, 2013Co-Authors: Lianghai Jin, Enmin SongAbstract:Estimating Gradients of color images is important to many color image processing tasks. However, the research on color image Gradients is limited. In this paper, a novel method of estimating color image Gradients is presented. The proposed Gradient mechanism is based on measuring the squared local contrast variation of a color image function, in which the chromatic variation is evaluated by quaternion representation. As an application in color image processing, we apply the proposed Gradient Operator to the traditional grayscale image Canny Operator for color edge detection. The edge detection results indicate that the proposed color Gradient Operator is superior to other state-of-the-art color image Gradient methods.
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improved direction estimation for di zenzo s multichannel image Gradient Operator
Pattern Recognition, 2012Co-Authors: Xiangyang Xu, Enmin SongAbstract:Gradient estimation is one of the most important tasks in image/video processing. For multichannel images, a classical and widely-used Gradient method is Di Zenzo's Gradient Operator, which is based on the measure of squared local contrast variation of multichannel images. However, up to now, the indetermination of Di Zenzo's Gradient direction has not been well solved, which results in errors occurring in most of the subsequent studies in which Di Zenzo's vector Gradient is used. In this paper, this problem is solved thoroughly. Furthermore, the ranges of the values that the Gradient angle should take in various cases are also analyzed. As an application in color image processing, a color version of Canny edge detector is implemented by introducing the new Gradient estimator to the traditional grayscale image Canny Operator. The experimental results indicate that the improved Di Zenzo's Gradient Operator is currently one of the best color Gradient estimators and outperforms other state-of-the-art color image Gradient methods. The improved multichannel Gradient Operator not only provides accurate Gradient estimation but also is efficient and easy to implement.
Seong G Kong - One of the best experts on this subject based on the ideXlab platform.
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focusing in thermal imagery using morphological Gradient Operator
Pattern Recognition Letters, 2014Co-Authors: Myung Geun Chun, Seong G KongAbstract:This paper presents focusing on an object of interest in thermal infrared (IR) imagery using the morphological Gradient Operator. Most existing focus metrics measure the degree of sharpness on the edge of an object in the field of view, often based on the local Gradient Operators of pixel brightness intensity. However, such focus measures may fail to find the optimal focusing distance to the object in thermal IR images, where strong edge components of an object do not exist. In particular, when the end goal of image acquisition is object recognition, focusing on an object must retain prominent features of the object for recognition. In this paper, the performances of various focus measures are evaluated in terms of sharpness as well as recognition accuracies for face recognition in thermal IR images. Experiment results show that the morphological Gradient Operator outperforms conventional Gradient Operators in terms of autofocusing resolution metric as well as face recognition accuracy.
John Tsamopoulos - One of the best experts on this subject based on the ideXlab platform.
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On the order of accuracy of the divergence theorem (Green-Gauss) method for calculating the Gradient in finite volume methods
2017Co-Authors: Alexandros Syrakos, Stylianos Varchanis, Yannis Dimakopoulos, Apostolos Goulas, John TsamopoulosAbstract:The divergence theorem (or Green-Gauss) Gradient scheme is among the most popular methods for discretising the Gradient Operator in second-order accurate finite volume methods, with a long history of successful application on structured grids. This together with the ease of application of the scheme on unstructured grids has led to its widespread use in unstructured finite volume methods (FVMs). However, the present study shows both theoretically and through numerical tests that the common variant of this scheme is zeroth-order accurate (it does not converge to the exact Gradient) on grids of arbitrary skewness, such as typically produced by unstructured grid generation algorithms. Moreover, we use the scheme in the FVM solution of a diffusion (Poisson) equation problem, with both an in-house code and the popular open-source solver OpenFOAM, and observe that the zeroth-order accuracy of the Gradient Operator is inherited by the FVM solver as a whole. However, a simple iterative procedure that exploits the outer iterations of the FVM solver is shown to effect first-order accuracy to the Gradient and second-order accuracy to the FVM at almost no extra cost compared to the original scheme. Second-order accurate results are also obtained if a least-squares Gradient Operator is used instead.
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A critical analysis of some popular methods for the discretisation of the Gradient Operator in finite volume methods
Physics of Fluids, 2017Co-Authors: Alexandros Syrakos, Stylianos Varchanis, Yannis Dimakopoulos, Apostolos Goulas, John TsamopoulosAbstract:Finite volume methods (FVMs) constitute a popular class of methods for the numerical simulation of fluid flows. Among the various components of these methods, the discretisation of the Gradient Operator has received less attention despite its fundamental importance with regards to the accuracy of the FVM. The most popular Gradient schemes are the divergence theorem (DT) (or Green-Gauss) scheme and the least-squares (LS) scheme. Both are widely believed to be second-order accurate, but the present study shows that in fact the common variant of the DT Gradient is second-order accurate only on structured meshes whereas it is zeroth-order accurate on general unstructured meshes, and the LS Gradient is second-order and first-order accurate, respectively. This is explained through a theoretical analysis and is confirmed by numerical tests. The schemes are then used within a FVM to solve a simple diffusion equation on unstructured grids generated by several methods; the results reveal that the zeroth-order accur...
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a critical analysis of some popular methods for the discretisation of the Gradient Operator in finite volume methods
arXiv: Numerical Analysis, 2016Co-Authors: Alexandros Syrakos, Stylianos Varchanis, Yannis Dimakopoulos, Apostolos Goulas, John TsamopoulosAbstract:Finite volume methods (FVMs) constitute a popular class of methods for the numerical simulation of fluid flows. Among the various components of these methods, the discretisation of the Gradient Operator has received less attention despite its fundamental importance with regards to the accuracy of the FVM. The most popular Gradient schemes are the divergence theorem (DT) (or Green-Gauss) scheme, and the least-squares (LS) scheme. Both are widely believed to be second-order accurate, but the present study shows that in fact the common variant of the DT Gradient is second-order accurate only on structured meshes whereas it is zeroth-order accurate on general unstructured meshes, and the LS Gradient is second-order and first-order accurate, respectively. This is explained through a theoretical analysis and is confirmed by numerical tests. The schemes are then used within a FVM to solve a simple diffusion equation on unstructured grids generated by several methods; the results reveal that the zeroth-order accuracy of the DT Gradient is inherited by the FVM as a whole, and the discretisation error does not decrease with grid refinement. On the other hand, use of the LS Gradient leads to second-order accurate results, as does the use of alternative, consistent, DT Gradient schemes, including a new iterative scheme that makes the common DT Gradient consistent at almost no extra cost. The numerical tests are performed using both an in-house code and the popular public domain PDE solver OpenFOAM.
Timothy J Giese - One of the best experts on this subject based on the ideXlab platform.
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spherical tensor Gradient Operator method for integral rotation a simple efficient and extendable alternative to slater koster tables
Journal of Chemical Physics, 2008Co-Authors: Timothy J Giese, Darrin M YorkAbstract:We present a novel alternative to the use of Slater–Koster tables for the efficient rotation and Gradient evaluation of two-center integrals used in tight-binding Hamiltonian models. The method recasts the problem into an exact, yet implicit, basis representation through which the properties of the spherical tensor Gradient Operator are exploited. These properties provide a factor of 3 to 4 speedup in the evaluation of the integral Gradients and afford a compact code structure that easily extends to high angular momentum without loss in efficiency. Thus, the present work is important in improving the performance of tight-binding models in molecular dynamics simulations and has particular use for methods that require the evaluation of two-center integrals that involve high angular momentum basis functions. These advances have a potential impact for the design of new tight-binding models that incorporate polarization or transition metal basis functions and methods based on electron density fitting of molecular fragments.
