Exploiting Milano Retinex Contrast to Enhance Images with Strong Changes of Light Intensity

Improving the quality of non uniformly lighted images is particularly hard due to the presence of regions that, having different brightness, requires different degrees of enhancement. The recently published algorithm REK proposes an interesting solution for enhancing images with abrupt changes of light intensity. REK linearly up-scales the image brightness to increase the quality of dark regions and combines the image with up-scaled brightness with the input one to preserve the quality of the bright regions. The up-scaling parameter α is estimated unsupervisedly based on the segmentation of brights and dark regions. This estimate has two main disadvantages: first, it makes REK dependent on the segmentation algorithm; second, the segmentation may be adversely affected by noise often present in badly illuminated images. To overcome these issues, this work proposes a new estimation of α based on the comparison of image Sobel gradient with an enhanced contrast, specifically the Milano Retinex contrast, which - as the name suggest - is inspired by the principles of Retinex theory. The new estimation of α provides good results, with reduced artifacts and saturation effects.