TY - CHAP A1 - Kleefeld, Andreas A1 - Burgeth, Bernhard ED - Burgeth, Bernhard ED - Vilanova, Anna ED - Westin, Carl-Fredrik T1 - Order based morphology for color images via matrix fields T2 - Visualization and Processing of Tensor Fields and Higher Order Descriptors for Multi-Valued Data N2 - Mathematical morphology is a successful branch of image processing with a history of more than four decades. Its fundamental operations are dilation and erosion, which are based on the notion of supremum and infimum with respect to an order. From dilation and erosion one can build readily other useful elementary morphological operators and filters, such as opening, closing, morphological top-hats, derivatives, and shock filters. Such operators are available for grey value images, and recently useful analogs of these processes for matrix-valued images have been introduced by taking advantage of the so-called Loewner order. There is a number of approaches to morphology for vector-valued images, that is, color images based on various orders, however, each with its merits and shortcomings. In this chapter we propose an approach to (elementary) morphology for color images that relies on the existing order based morphology for matrix fields of symmetric 2 × 2-matrices. An RGB-image is embedded into a field of those 2 × 2-matrices by exploiting the geometrical properties of the order cone associated with the Loewner order. To this end a modification of the HSL-color model and a relativistic addition of matrices is introduced. The experiments performed with various morphological elementary operators on synthetic and real images provide results promising enough to serve as a proof-of-concept. KW - Mathematical Morphology KW - Color image KW - Matrix field Y1 - 2014 UR - http://link.springer.com/chapter/10.1007%2F978-3-642-54301-2_4 SN - 978-3-642-54300-5 SN - 978-3-642-54301-2 U6 - https://doi.org/10.1007/978-3-642-54301-2_4 SP - 75 EP - 95 PB - Springer CY - Berlin ER - TY - CHAP A1 - Kleefeld, Andreas A1 - Burgeth, Bernhard ED - Hendriks, Cris L. Luengo ED - Borgefors, Gunilla ED - Strand, Robin T1 - Morphology for Color Images via Loewner Order for Matrix Fields T2 - Mathematical Morphology and Its Applications to Signal and Image Processing N2 - Mathematical morphology is a very successful branch of image processing with a history of more than four decades. Its fundamental operations are dilation and erosion, which are based on the notion of a maximum and a minimum with respect to an order. Many operators constructed from dilation and erosion are available for grey value images, and recently useful analogs of these processes for matrix-valued images have been introduced by taking advantage of the so-called Loewner order. There has been a number of approaches to morphology for vector-valued images, that is, colour images based on various orders, however, each with its merits and shortcomings. In this article we propose an approach to (elementary) morphology for colour images that relies on the existing order based morphology for matrix fields of symmetric 2×2-matrices. An RGB-image is embedded into a field of those 2×2-matrices by exploiting the geometrical properties of the order cone associated with the Loewner order. To this end a modification of the HSL-colour model and a relativistic addition of matrices is introduced. The experiments performed with various morphological elementary operators on synthetic and real images demonstrate the capabilities and restrictions of the novel approach. KW - matrix field KW - tensor field KW - symmetric matrix KW - colour images KW - dilation KW - erosion KW - colour space KW - Einstein addition Y1 - 2013 UR - http://link.springer.com/chapter/10.1007/978-3-642-38294-9_21 SN - 978-3-642-38293-2 SN - 978-3-642-38294-9 U6 - https://doi.org/10.1007/978-3-642-38294-9_21 SP - 243 EP - 254 PB - Springer CY - Berlin ER - TY - GEN A1 - Anagnostōpulos, Kōnstantinos N. A1 - Charalambopoulos, Antonios A1 - Kleefeld, Andreas T1 - The factorization method for the acoustic transmission problem T2 - Inverse Problems N2 - In this work, the shape reconstruction problem of acoustically penetrable bodies from the far-field data corresponding to time-harmonic plane wave incidence is investigated within the framework of the factorization method. Although the latter technique has received considerable attention in inverse scattering problems dealing with impenetrable scatterers and it has not been elaborated for inverse transmission problems with the only exception being a work by the first two authors and co-workers. We aim to bridge this gap in the field of acoustic scattering; the paper on one hand focuses on establishing rigorously the necessary theoretical framework for the application of the factorization method to the inverse acoustic transmission problem. The main outcome of the investigation undertaken is the derivation of an explicit formula for the scatterer's characteristic function, which depends solely on the far-field data feeding the inverse scattering scheme. Extended numerical examples in three dimensions are also presented, where a variety of different surfaces are successfully reconstructed by the factorization method, thus, complementing the method's validation from the computational point of view. KW - integral equations KW - Factorization method KW - scattering KW - inverse problem Y1 - 2013 UR - http://iopscience.iop.org/0266-5611/29/11/115015/ U6 - https://doi.org/10.1088/0266-5611/29/11/115015 SN - 0266-5611 SN - 1361-6420 VL - 29 IS - 11 SP - 115015 ER - TY - GEN A1 - Kleefeld, Andreas T1 - A numerical method to compute interior transmission eigenvalues T2 - Inverse Problems N2 - In this paper the numerical calculation of eigenvalues of the interior transmission problem arising in acoustic scattering for constant contrast in three dimensions is considered. From the computational point of view existing methods are very expensive, and are only able to show the existence of such transmission eigenvalues. Furthermore, they have trouble finding them if two or more eigenvalues are situated closely together. We present a new method based on complex-valued contour integrals and the boundary integral equation method which is able to calculate highly accurate transmission eigenvalues. So far, this is the first paper providing such accurate values for various surfaces different from a sphere in three dimensions. Additionally, the computational cost is even lower than those of existing methods. Furthermore, the algorithm is capable of finding complex-valued eigenvalues for which no numerical results have been reported yet. Until now, the proof of existence of such eigenvalues is still open. Finally, highly accurate eigenvalues of the interior Dirichlet problem are provided and might serve as test cases to check newly derived Faber–Krahn type inequalities for larger transmission eigenvalues that are not yet available. KW - interior transmission eigenvalues Y1 - 2013 UR - http://iopscience.iop.org/0266-5611/29/10/104012/article U6 - https://doi.org/10.1088/0266-5611/29/10/104012 SN - 0266-5611 SN - 1361-6420 IS - 29 SP - 104012 ER - TY - GEN A1 - Kleefeld, Andreas A1 - Lin, Tzu-Chu T1 - A Global Galerkin Method for Solving the Exterior Neumann Problem for the Helmholtz Equation Using Panich's Integral Equation Approach T2 - SIAM Journal on Scientific Computing N2 - We describe a boundary integral equation that solves the exterior Neumann problem for the Helmholtz equation in three dimensions for smooth obstacles that can be described globally in spherical coordinates. The unique solution is found by approximating the Fredholm integral equation of the second kind with the Galerkin method, where the basis functions are spherical harmonics. This leads to a fast method for small and medium wave numbers, since the system of equations is of size smaller than 64. To smooth the integrand a new map is introduced which significantly improves the accuracy. Numerical results for several smooth surfaces are presented. KW - Fredholm integral equation of the second kind KW - Helmholtz equation KW - exterior Neumann problem KW - Galerkin method Y1 - 2013 U6 - https://doi.org/10.1137/120873066 SN - 1064-8275 SN - 1095-7197 VL - 35 IS - 3 SP - A1709 EP - A1735 ER - TY - GEN A1 - Kleefeld, Andreas T1 - The transmission problem for the Helmholtz equation in R³ T2 - Computational Methods in Applied Mathematics N2 - The problem under consideration is the three-dimensional transmission problem for time harmonic acoustic waves for two homogeneous media. A simply-connected and bounded region with sufficiently smooth boundary is immersed in an infinite medium. Each medium is c haracterized by the space independent wave number κ and the density μ. The system of boundary integral equations is reviewed as well as an existence and uniqueness result. The system is approximated by the boundary element collocation method and consistency, stability, and convergence is proved. In addition, superconvergence is proved and numerical results illustrate the agreement with these theoretical results. No numerical results seem to be reported for this method yet. KW - Helmholtz's equation; integral equation; transmission problem; boundary element method; superconvergence Y1 - 2012 UR - http://www.degruyter.com/view/j/cmam.2012.12.issue-3/cmam-2012-0008/cmam-2012-0008.xml?format=INT SN - 1609-4840 VL - 12 IS - 3 SP - 330 EP - 350 ER - TY - CHAP A1 - Kleefeld, Andreas A1 - Ju, Yong Chul A1 - Tozza, Silvia A1 - Breuß, Michael A1 - Bruhn, Andrés ED - Burghardt, Tilo ED - Damen, Dima ED - Mayol-Cuevas, Walterio ED - Mirmehdi, Majid T1 - Generalised Perspective Shape from Shading with Oren-Nayar Reflectance T2 - Proceedings 24th British Machine Vision Conference, BMVC 2013, Bristol, UK, September 2013 N2 - In spite of significant advances in Shape from Shading (SfS) over the last years, it is still a challenging task to design SfS approaches that are flexible enough to handle a wide range of input scenes. In this paper, we address this lack of flexibility by proposing a novel model that extends the range of possible applications. To this end, we consider the class of modern perspective SfS models formulated via partial differential equations (PDEs). By combining a recent spherical surface parametrisation with the advanced non-Lambertian Oren-Nayar reflectance model, we obtain a robust approach that allows to deal with an arbitrary position of the light source while being able to handle rough surfaces and thus more realistic objects at the same time. To our knowledge, the resulting model is currently the most advanced and most flexible approach in the literature on PDE-based perspective SfS. Apart from deriving our model, we also show how the corresponding set of sophisticated Hamilton-Jacobi equations can be efficiently solved by a specifically tailored fast marching scheme. Experiments with medical real-world data demonstrate that our model works in practice and that is offers the desired flexibility. Y1 - 2013 UR - http://www.bmva.org/bmvc/2013/Papers/paper0042/index.html U6 - https://doi.org/10.5244/C.27.42 SP - 42.1 EP - 42.11 PB - BMVA Press ER - TY - GEN A1 - Kleefeld, Andreas A1 - Brazauskas, Vytaras T1 - A statistical application of the quantile mechanics approach: MTM estimators for the parameters of t and gamma distribution T2 - European Journal of Applied Mathematics N2 - In this paper, we revisit the quantile mechanics approach, which was introduced by Steinbrecher and Shaw (Steinbrecher, G. & Shaw, W. T. (2008) Quantile mechanics. European. J. Appl. Math. 19, 87–112). Our objectives are (i) to derive the method of trimmed moments (MTM) estimators for the parameters of gamma and Student's t distributions, and (ii) to examine their large- and small-sample statistical properties. Since trimmed moments are defined through the quantile function of the distribution, quantile mechanics seems like a natural approach for achieving objective (i). To accomplish the second goal, we rely on the general large sample results for MTMs, which were established by Brazauskas et al. (Brazauskas, V., Jones, B. L. & Zitikis, R. (2009) Robust fitting of claim severity distributions and the method of trimmed moments. J. Stat. Plan. Inference 139, 2028–2043), and then use Monte Carlo simulations to investigate small-sample behaviour of the newly derived estimators. We find that, unlike the maximum likelihood method, which usually yields fully efficient but non-robust estimators, the MTM estimators are robust and offer competitive trade-offs between robustness and efficiency. These properties are essential when one employs gamma or Student's t distributions in such outlier-prone areas as insurance and finance. KW - Point estimation (62F10); Asymptotic properties of estimators (62F12); Robustness and adaptive procedures (62F35); Method of trimmed moments (62F99); Computational problems in statistics (65C60) Y1 - 2012 UR - http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8678315 U6 - https://doi.org/10.1017/S0956792512000137 SN - 0956-7925 VL - 23 IS - 5 SP - 593 EP - 610 ER - TY - GEN A1 - Kleefeld, Andreas T1 - A modified boundary integral equation for solving the exterior Robin problem for the Helmholtz equation in three dimensions T2 - Applied Mathematics and Computation N2 - A boundary integral equation with modified fundamental solution to solve the exterior Robin problem for Helmholtz’s equation is considered. A specific choice of the coefficients of these added terms ensure the unique solvability and, in addition, they can be chosen to ensure the minimization of the least-squares difference of the modified and the exact Green’s function or the minimization of the condition number. Numerical results are reported showing the robustness and the superconvergence of the new method. KW - Boundary integral equation; Modified fundamental solution; Helmholtz equation; Exterior Robin problem; Superconvergence Y1 - 2012 UR - http://www.sciencedirect.com/science/article/pii/S009630031200848X# U6 - https://doi.org/10.1016/j.amc.2012.08.055 SN - 0096-3003 VL - 2012 IS - 219 SP - 2114 EP - 2123 ER - TY - GEN A1 - Welk, Martin A1 - Kleefeld, Andreas A1 - Breuß, Michael T1 - Quantile Filtering of Colour Images via Symmetric Matrices T2 - Mathematical Morphology - Theory and Applications N2 - Quantile filters, or rank-order filters, are local image filters which assign quantiles of intensities of the input image within neighbourhoods as output image values. Combining a multivariate quantile definition developed in matrix-valued morphology with a recently introduced mapping between the RGB colour space and the space of symmetric 2 × 2 matrices, we state a class of colour image quantile filters, along with a class of morphological gradient filters derived from these.We consider variants of these filters based on three matrix norms – the nuclear, Frobenius, and spectral norm – and study their differences. We investigate the properties of the quantile and gradient filters and their links to dilation and erosion operators. Using amoeba structuring elements,we devise image-adaptive versions of our quantile and gradient filters. Experiments are presented to demonstrate the favourable properties of the filters, and compare them to existing approaches in colour morphology. KW - quantile KW - rank-order filter KW - colour image KW - matrix field KW - amoebas Y1 - 2016 U6 - https://doi.org/10.1515/mathm-2016-0008 SN - 2353-3390 VL - 1 IS - 1 SP - 136 EP - 174 ER -