Use of GIMP Retinex filter to enhance the visibility of craquelure: example n. 1

<p>In the image, A represents the original image. It is a detail of the Portrait of a Young Girl, by Petrus Christus, (high&nbsp;resolution), courtesy Wikipedia.&nbsp; B is the grey-tone corresponding image. C is the image that we can obtain using the GIMP thresholding (threshold tone T = 127), applied to B. D is the image that we can have, applying GIMP Retinex on B (level=low, scale=250, scale div.=8, dynamics=4,0). E is the result of the thresholding of D (T = 127). F is the image obtained by means of the use of generic filter Erode on E. G is the result of applying Retinex (the same parameters used to obtain D) on D. H is the result of thresholding (T = 127), and I is the craquelure as we can see it&nbsp;after applying Erode. L is the Retinex filtered G (the same parameters as for G and D), M is the effect of thresholding and N the final result after using Erode. The example shows that the use of a thresholding on a Retinex filtered image gives much better results than the simple thresholding. This approach, that is the use of Retinex filtering, represents a fast manner for enhancing the visibility of craquelure. The iteration of the Retinex filter is also interesting for working on very dark areas.</p>