Abstract
The recently introduced preconditioned Douglas–Rachford iteration (PDR) for convex–concave saddle-point problems is studied with respect to convergence rates and applied to variational imaging problems with total variation (TV) and total generalized variation (TGV) penalty. A rate of \({\mathcal {O}}(1/k)\) for restricted primal–dual gaps evaluated for ergodic sequences generated by the PDR iteration is established. Based on PDR, new fast iterative algorithms for TV-denoising, TV-deblurring, and TGV-denoising of second order with \(L^2\) and \(L^1\) discrepancy are proposed. While for denoising, symmetric (block) Red–Black Gauss–Seidel preconditioners are effective, fast Fourier transform-based preconditioners are employed for the deblurring problems. Finally, for the \(L^2\)-TGV-denoising problem, an effective modified primal–dual gap is developed which may serve as a stopping criterion. All algorithms are tested and compared in numerical experiments. In particular, for problems where strong convexity does not hold, it turns out that the proposed preconditioning techniques are beneficial and lead to competitive results.
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The work of Kristian Bredies and Hongpeng Sun is supported by the Austrian Science Fund (FWF) under grant SFB32 (SFB “Mathematical Optimization and Applications in the Biomedical Sciences”). The Institute for Mathematics and Scientific Computing at the University of Graz is a member of NAWI Graz (http://www.nawigraz.at/).
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Bredies, K., Sun, H.P. Preconditioned Douglas–Rachford Algorithms for TV- and TGV-Regularized Variational Imaging Problems. J Math Imaging Vis 52, 317–344 (2015). https://doi.org/10.1007/s10851-015-0564-1
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DOI: https://doi.org/10.1007/s10851-015-0564-1