Abstract
Underwater images play a key role in ocean exploration, but often suffer from severe quality degradation due to light absorption and scattering in water medium. Although major breakthroughs have been made recently in the general area of image enhancement and restoration, the applicability of new methods for improving the quality of underwater images has not specifically been captured. In this paper, we review the image enhancement and restoration methods that tackle typical underwater image impairments, including some extreme degradations and distortions. Firstly, we introduce the key causes of quality reduction in underwater images, in terms of the underwater image formation model (IFM). Then, we review underwater restoration methods, considering both the IFM-free and the IFM-based approaches. Next, we present an experimental-based comparative evaluation of state-of-the-art IFM-free and IFM-based methods, considering also the prior-based parameter estimation algorithms of the IFM-based methods, using both subjective and objective analysis (the used code is freely available at https://github.com/wangyanckxx/Single-Underwater-Image-Enhancement-and-Color-Restoration). Starting from this study, we pinpoint the key shortcomings of existing methods, drawing recommendations for future research in this area. Our review of underwater image enhancement and restoration provides researchers with the necessary background to appreciate challenges and opportunities in this important field.
Highlights
The oceans contain unknown creatures and vast energy resources, playing an important role in the continuation of life on earth [1]
The images enhanced by HE method (Fig. 6 (b)) present an overwhelming red tone and amplify the noises of the original image. Both CLAHE and relative global histogram stretching (RGHS) are based on adaptive parameters to avoid a global histogram stretching or blind pixel redistribution to reduce sharpness
Quality improvement methods of single underwater images based on image enhancement and color restoration are comprehensively reviewed to help researchers better explore this unknown underwater world
Summary
The oceans contain unknown creatures and vast energy resources, playing an important role in the continuation of life on earth [1]. Considering the underwater optical imaging theory, the proposed underwater white balancing aiming at compensating color cast caused by the light with selective attenuation is gamma corrected and sharpened to generate two fusion images and associated weight maps, which are merged based on the standard multi-scale fusion strategy. Their proposed enhanced images and videos are characterized by better exposedness of the dark regions, improved global contrast and edges sharpness. We will introduce the prior-based and CNN-based image restorations, and explain how these recover natural colors of underwater images by estimating BLs and TMs
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