64 research outputs found
A flexible space-variant anisotropic regularisation for image restoration with automated parameter selection
We propose a new space-variant anisotropic regularisation term for
variational image restoration, based on the statistical assumption that the
gradients of the target image distribute locally according to a bivariate
generalised Gaussian distribution. The highly flexible variational structure of
the corresponding regulariser encodes several free parameters which hold the
potential for faithfully modelling the local geometry in the image and
describing local orientation preferences. For an automatic estimation of such
parameters, we design a robust maximum likelihood approach and report results
on its reliability on synthetic data and natural images. For the numerical
solution of the corresponding image restoration model, we use an iterative
algorithm based on the Alternating Direction Method of Multipliers (ADMM). A
suitable preliminary variable splitting together with a novel result in
multivariate non-convex proximal calculus yield a very efficient minimisation
algorithm. Several numerical results showing significant quality-improvement of
the proposed model with respect to some related state-of-the-art competitors
are reported, in particular in terms of texture and detail preservation
Joint Image Reconstruction and Segmentation Using the Potts Model
We propose a new algorithmic approach to the non-smooth and non-convex Potts
problem (also called piecewise-constant Mumford-Shah problem) for inverse
imaging problems. We derive a suitable splitting into specific subproblems that
can all be solved efficiently. Our method does not require a priori knowledge
on the gray levels nor on the number of segments of the reconstruction.
Further, it avoids anisotropic artifacts such as geometric staircasing. We
demonstrate the suitability of our method for joint image reconstruction and
segmentation. We focus on Radon data, where we in particular consider limited
data situations. For instance, our method is able to recover all segments of
the Shepp-Logan phantom from angular views only. We illustrate the
practical applicability on a real PET dataset. As further applications, we
consider spherical Radon data as well as blurred data
Efficient Reconstruction of Piecewise Constant Images Using Nonsmooth Nonconvex Minimization
We consider the restoration of piecewise constant images where the number of the regions and their
values are not fixed in advance, with a good difference of piecewise constant values between neighboring regions, from noisy data obtained at the output of a linear operator (e.g., a blurring kernel or
a Radon transform). Thus we also address the generic problem of unsupervised segmentation in the
context of linear inverse problems. The segmentation and the restoration tasks are solved jointly
by minimizing an objective function (an energy) composed of a quadratic data-fidelity term and a
nonsmooth nonconvex regularization term. The pertinence of such an energy is ensured by the analytical properties of its minimizers. However, its practical interest used to be limited by the difficulty
of the computational stage which requires a nonsmooth nonconvex minimization. Indeed, the existing
methods are unsatisfactory since they (implicitly or explicitly) involve a smooth approximation
of the regularization term and often get stuck in shallow local minima. The goal of this paper is to
design a method that efficiently handles the nonsmooth nonconvex minimization. More precisely,
we propose a continuation method where one tracks the minimizers along a sequence of approximate
nonsmooth energies {Jฮต}, the first of which being strictly convex and the last one the original energy
to minimize. Knowing the importance of the nonsmoothness of the regularization term for the segmentation task, each Jฮต is nonsmooth and is expressed as the sum of an l1 regularization term and
a smooth nonconvex function. Furthermore, the local minimization of each Jฮต is reformulated as the
minimization of a smooth function subject to a set of linear constraints. The latter problem is solved
by the modified primal-dual interior point method, which guarantees the descent direction at each
step. Experimental results are presented and show the effectiveness and the efficiency of the proposed
method. Comparison with simulated annealing methods further shows the advantage of our method.published_or_final_versio
On the convergence of a linesearch based proximal-gradient method for nonconvex optimization
We consider a variable metric linesearch based proximal gradient method for
the minimization of the sum of a smooth, possibly nonconvex function plus a
convex, possibly nonsmooth term. We prove convergence of this iterative
algorithm to a critical point if the objective function satisfies the
Kurdyka-Lojasiewicz property at each point of its domain, under the assumption
that a limit point exists. The proposed method is applied to a wide collection
of image processing problems and our numerical tests show that our algorithm
results to be flexible, robust and competitive when compared to recently
proposed approaches able to address the optimization problems arising in the
considered applications
์ฝ์์ก์ ์ ๊ฑฐ๋ฅผ ์ํ ๋ณ๋ถ๋ฒ์ ์ ๊ทผ
ํ์๋
ผ๋ฌธ(๋ฐ์ฌ)--์์ธ๋ํ๊ต ๋ํ์ :์์ฐ๊ณผํ๋ํ ์๋ฆฌ๊ณผํ๋ถ,2020. 2. ๊ฐ๋ช
์ฃผ.In image processing, image noise removal is one of the most important problems. In this thesis, we study Cauchy noise removal by variational approaches. Cauchy noise occurs often in engineering applications. However, because of the non-convexity of the variational model of Cauchy noise, it is difficult to solve and were not studied much. To denoise Cauchy noise, we use the non-convex alternating direction method of multipliers and present two variational models.
The first thing is fractional total variation(FTV) model. FTV is derived by fractional derivative which is an extended version of integer order derivative to real order derivative.
The second thing is the weighted nuclear norm model. Weighted nuclear norm has an excellent performance in low-level vision. We have combined our novel ideas with weighted nuclear norm minimization to achieve better results than existing models in Cauchy noise removal. Finally, we show the superiority of the proposed model from numerical experiments.์ด๋ฏธ์ง ์ฒ๋ฆฌ์์ ์ด๋ฏธ์ง ์ก์ ์ ๊ฑฐ๋ ๊ฐ์ฅ ์ค์ํ ๋ฌธ์ ์ค ํ๋๋ค. ์ด ๋
ผ๋ฌธ์์ ์ฐ๋ฆฌ๋ ๋ค์ํ ์ ๊ทผ ๋ฐฉ์์ ์ํ ์ฝ์ ์ก์ ์ ๊ฑฐ๋ฅผ ์ฐ๊ตฌํ๋ค. ์ฝ์ ์ก์์ ์์ง๋์ด๋ง ์ ํ๋ฆฌ์ผ์ด์
์์ ์์ฃผ ๋ฐ์ํ๋ ์ฝ์ ์ก์์ ๋ณ๋ถ๋ฒ์ ๋ชจ๋ธ์ ๋น ๋ณผ๋ก์ฑ์ผ๋ก ์ธํด ํด๊ฒฐํ๊ธฐ๊ฐ ์ด๋ ต๊ณ ๋ง์ด ์ฐ๊ตฌ๋์ง ์์๋ค. ์ฝ์ ๋
ธ์ด์ฆ๋ฅผ ์ ๊ฑฐํ๊ธฐ ์ํด ์ฐ๋ฆฌ๋ ๊ณฑ์
๊ธฐ์ ๋ณผ๋กํ์ง ์์ ๊ต๋ฅ ๋ฐฉํฅ ๋ฐฉ๋ฒ(nonconvex ADMM)์ ์ฌ์ฉํ์์ผ๋ฉฐ ๋ ๊ฐ์ง ๋ณ๋ถ๋ฒ์ ๋ชจ๋ธ์ ์ ์ํ๋ค.
์ฒซ ๋ฒ์งธ๋ ๋ถ์ ์ด ๋ณ์ด(FTV)๋ฅผ ์ด์ฉํ ๋ชจ๋ธ์ด๋ค. ๋ถ์ ์ด ๋ณ์ด๋ ์ผ๋ฐ์ ์ธ ์ ์ ๋ํจ์๋ฅผ ์ค์ ๋ํจ์๋ก ํ์ฅ ํ ๋ถ์ ๋ํจ์์ ์ํด ์ ์๋๋ค.
๋ ๋ฒ์งธ๋ ๊ฐ์ค ํต ๋
ธ๋ฆ์ ์ด์ฉํ ๋ชจ๋ธ์ด๋ค. ๊ฐ์ค ํต ๋
ธ๋ฆ์ ์ ์์ค ์์์ฒ๋ฆฌ์์ ํ์ํ ์ฑ๋ฅ์ ๋ฐํํ๋ค. ์ฐ๋ฆฌ๋ ๊ฐ์ค ํต ๋
ธ๋ฆ์ด ์ฝ์ ์ก์ ์ ๊ฑฐ์์๋ ๋ฐ์ด๋ ์ฑ๋ฅ์ ๋ฐํํ ๊ฒ์ผ๋ก ์์ํ์๊ณ , ์ฐ๋ฆฌ์ ์๋ก์ด ์์ด๋์ด๋ฅผ ๊ฐ์ค ํต ๋
ธ๋ฆ ์ต์ํ์ ๊ฒฐํฉํ์ฌ ํ์กดํ๋ ์ฝ์ ์ก์ ์ ๊ฑฐ ์ต์ ๋ชจ๋ธ๋ค๋ณด๋ค ๋ ๋์ ๊ฒฐ๊ณผ๋ฅผ ์ป์ ์ ์์๋ค. ๋ง์ง๋ง ์ฅ์์ ์ค์ ์ฝ์ ์ก์ ์ ๊ฑฐ ํ
์คํธ๋ฅผ ํตํด ์ฐ๋ฆฌ ๋ชจ๋ธ์ด ์ผ๋ง๋ ๋ฐ์ด๋์ง ํ์ธํ๋ฉฐ ๋
ผ๋ฌธ์ ๋ง์น๋ค.1 Introduction 1
2 The Cauchy distribution and the Cauchy noise 5
2.1 The Cauchy distribution 5
2.1.1 The alpha-stable distribution 5
2.1.2 The Cauchy distribution 8
2.2 The Cauchy noise 13
2.2.1 Analysis of the Cauchy noise 13
2.2.2 Variational model of Cauchy noise 14
2.3 Previous work 16
3 Fractional order derivatives and total fractional order variational model 19
3.1 Some fractional derivatives and integrals 19
3.1.1 Grunwald-Letnikov Fractional Derivatives 20
3.1.2 Riemann-Liouville Fractional Derivatives 28
3.2 Proposed model: Cauchy noise removal model by fractional total variation 33
3.2.1 Fractional total variation and Cauchy noise removal model 34
3.2.2 nonconvex ADMM algorithm 37
3.2.3 The algorithm for solving fractional total variational model of Cauchy noise 39
3.3 Numerical results of fractional total variational model 51
3.3.1 Parameter and termination condition 51
3.3.2 Experimental results 54
4 Nuclear norm minimization and Cauchy noise denoising model 67
4.1 Weighted Nuclear Norm 67
4.1.1 Weighted Nuclear Norm and Its Applications 68
4.1.2 Iteratively Reweighted l1 Minimization 74
4.2 Proposed Model: Weighted Nuclear Norm For Cauchy Noise Denoising 77
4.2.1 Model and algorithm description 77
4.2.2 Convergence of algorithm7 79
4.2.3 Block matching method 81
4.3 Numerical Results OfWeighted Nuclear Norm Denoising Model For Cauchy Noise 83
4.3.1 Parameter setting and truncated weighted nuclear norm 84
4.3.2 Termination condition 85
4.3.3 Experimental results 86
5 Conclusion 95
Abstract (in Korean) 105Docto
A Unified Bregman Alternating Minimization Algorithm for Generalized DC Programming with Application to Imaging Data
In this paper, we consider a class of nonconvex (not necessarily
differentiable) optimization problems called generalized DC
(Difference-of-Convex functions) programming, which is minimizing the sum of
two separable DC parts and one two-block-variable coupling function. To
circumvent the nonconvexity and nonseparability of the problem under
consideration, we accordingly introduce a Unified Bregman Alternating
Minimization Algorithm (UBAMA) by maximally exploiting the favorable DC
structure of the objective. Specifically, we first follow the spirit of
alternating minimization to update each block variable in a sequential order,
which can efficiently tackle the nonseparablitity caused by the coupling
function. Then, we employ the Fenchel-Young inequality to approximate the
second DC components (i.e., concave parts) so that each subproblem reduces to a
convex optimization problem, thereby alleviating the computational burden of
the nonconvex DC parts. Moreover, each subproblem absorbs a Bregman proximal
regularization term, which is usually beneficial for inducing closed-form
solutions of subproblems for many cases via choosing appropriate Bregman kernel
functions. It is remarkable that our algorithm not only provides an algorithmic
framework to understand the iterative schemes of some novel existing
algorithms, but also enjoys implementable schemes with easier subproblems than
some state-of-the-art first-order algorithms developed for generic nonconvex
and nonsmooth optimization problems. Theoretically, we prove that the sequence
generated by our algorithm globally converges to a critical point under the
Kurdyka-{\L}ojasiewicz (K{\L}) condition. Besides, we estimate the local
convergence rates of our algorithm when we further know the prior information
of the K{\L} exponent.Comment: 44 pages, 7figures, 5 tables. Any comments are welcom
Combining Weighted Total Variation and Deep Image Prior for natural and medical image restoration via ADMM
In the last decades, unsupervised deep learning based methods have caught
researchers attention, since in many real applications, such as medical
imaging, collecting a great amount of training examples is not always feasible.
Moreover, the construction of a good training set is time consuming and hard
because the selected data have to be enough representative for the task. In
this paper, we focus on the Deep Image Prior (DIP) framework and we propose to
combine it with a space-variant Total Variation regularizer with an automatic
estimation of the local regularization parameters. Differently from other
existing approaches, we solve the arising minimization problem via the flexible
Alternating Direction Method of Multipliers (ADMM). Furthermore, we provide a
specific implementation also for the standard isotropic Total Variation. The
promising performances of the proposed approach, in terms of PSNR and SSIM
values, are addressed through several experiments on simulated as well as real
natural and medical corrupted images.Comment: conference pape
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