On the Proof of Fixed-Point Convergence for Plug-and-Play ADMM

Gavaskar, RG and Chaudhury, KN (2019) On the Proof of Fixed-Point Convergence for Plug-and-Play ADMM. In: IEEE Signal Processing Letters, 26 (12). pp. 1817-1821.

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Abstract

In most state-of-the-art image restoration methods, the sum of a data-fidelity and a regularization term is optimized using an iterative algorithm such as ADMM (alternating direction method of multipliers). In recent years, the possibility of using denoisers for regularization has been explored in several works. A popular approach is to formally replace the proximal operator within the ADMM framework with some powerful denoiser. However, since most state-of-the-art denoisers cannot be posed as a proximal operator, one cannot guarantee the convergence of these so-called plug-and-play (PnP) algorithms. In fact, the theoretical convergence of PnP algorithms is an active research topic. In this letter, we consider the result of Chan et al. (IEEE TCI, 2017), where fixed-point convergence of an ADMM-based PnP algorithm was established for a class of denoisers. We argue that the original proof is incomplete, since convergence is not analyzed for one of the three possible cases outlined in the letter. Moreover, we explain why the argument for the other cases does not apply in this case. We give a different analysis to fill this gap, which firmly establishes the original convergence theorem.

Item Type:	Journal Article
Publication:	IEEE Signal Processing Letters
Publisher:	Institute of Electrical and Electronics Engineers Inc.
Additional Information:	Copyright of this article belongs to Institute of Electrical and Electronics Engineers Inc.
Keywords:	Image reconstruction, ADMM; Alternating direction method of multipliers; conver-gence analysis; Convergence theorem; Plug and play; Regularization; Regularization terms; Restoration methods, Iterative methods
Department/Centre:	Division of Electrical Sciences > Electrical Engineering
Date Deposited:	27 Jan 2020 11:30
Last Modified:	27 Jan 2020 11:30
URI:	http://eprints.iisc.ac.in/id/eprint/64434

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