6 research outputs found
Minimizing L1 over L2 norms on the gradient
In this paper, we study the L1/L2 minimization on the gradient for imaging
applications. Several recent works have demonstrated that L1/L2 is better than
the L1 norm when approximating the L0 norm to promote sparsity. Consequently,
we postulate that applying L1/L2 on the gradient is better than the classic
total variation (the L1 norm on the gradient) to enforce the sparsity of the
image gradient. To verify our hypothesis, we consider a constrained formulation
to reveal empirical evidence on the superiority of L1/L2 over L1 when
recovering piecewise constant signals from low-frequency measurements.
Numerically, we design a specific splitting scheme, under which we can prove
subsequential and global convergence for the alternating direction method of
multipliers (ADMM) under certain conditions. Experimentally, we demonstrate
visible improvements of L1/L2 over L1 and other nonconvex regularizations for
image recovery from low-frequency measurements and two medical applications of
MRI and CT reconstruction. All the numerical results show the efficiency of our
proposed approach.Comment: 26 page
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μ£Ό.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.μ΄λ―Έμ§ μ²λ¦¬μμ μ΄λ―Έμ§ μ‘μ μ κ±°λ κ°μ₯ μ€μν λ¬Έμ μ€ νλλ€. μ΄ λ
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Όλ¬Έμ λ§μΉλ€.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
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μ£Ό.In this thesis, we discuss regularization methods for denoising images corrupted by Gaussian or Cauchy noise and image dehazing in underwater. In image denoising, we introduce the second-order extension of structure tensor total variation and propose a hybrid method for additive Gaussian noise. Furthermore, we apply the weighted nuclear norm under nonlocal framework to remove additive Cauchy noise in images. We adopt the nonconvex alternating direction method of multiplier to solve the problem iteratively. Subsequently, based on the color ellipsoid prior which is effective for restoring hazy image in the atmosphere, we suggest novel dehazing method adapted for underwater condition. Because attenuation rate of light varies depending on wavelength of light in water, we apply the color ellipsoid prior only for green and blue channels and combine it with intensity map of red channel to refine the obtained depth map further. Numerical experiments show that our proposed methods show superior results compared with other methods both in quantitative and qualitative aspects.λ³Έ λ
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Ήμκ³Ό μ²μ μ±λμ μ μ©νκ³ κ·Έλ‘λΆν° μ»μ κΉμ΄ μ§λλ₯Ό μ μ μ±λμ κ°λ μ§λμ νΌν©νμ¬ κ°μ λ κΉμ΄ μ§λλ₯Ό μ»λλ€. μμΉμ μ€νμ ν΅ν΄μ μ°λ¦¬κ° μ μν λ°©λ²λ€μ λ€λ₯Έ λ°©λ²κ³Ό λΉκ΅νκ³ μ§μ μΈ μΈ‘λ©΄κ³Ό νκ° μ§νμ λ°λ₯Έ μμ μΈ μΈ‘λ©΄ λͺ¨λμμ μ°μν¨μ νμΈνλ€.1 Introduction 1
1.1 Image denoising for Gaussian and Cauchy noise 2
1.2 Underwater image dehazing 5
2 Preliminaries 9
2.1 Variational models for image denoising 9
2.1.1 Data-fidelity 9
2.1.2 Regularization 11
2.1.3 Optimization algorithm 14
2.2 Methods for image dehazing in the air 15
2.2.1 Dark channel prior 16
2.2.2 Color ellipsoid prior 19
3 Image denoising for Gaussian and Cauchy noise 23
3.1 Second-order structure tensor and hybrid STV 23
3.1.1 Structure tensor total variation 24
3.1.2 Proposed model 28
3.1.3 Discretization of the model 31
3.1.4 Numerical algorithm 35
3.1.5 Experimental results 37
3.2 Weighted nuclear norm minimization for Cauchy noise 46
3.2.1 Variational models for Cauchy noise 46
3.2.2 Low rank minimization by weighted nuclear norm 52
3.2.3 Proposed method 55
3.2.4 ADMM algorithm 56
3.2.5 Numerical method and experimental results 58
4 Image restoration in underwater 71
4.1 Scientific background 72
4.2 Proposed method 73
4.2.1 Color ellipsoid prior on underwater 74
4.2.2 Background light estimation 78
4.3 Experimental results 80
5 Conclusion 87
Appendices 89Docto