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Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
Images of flank wear before (a) and after (b) contrast enhancement. Gray level histograms that describe the number of pixels corresponding to all gray level values of images (a,b) are plotted in (c,d).