  This paper focuses on finding the most optimal pre-processing methods
considering three common algorithms for image enhancement: Brightening, CLAHE
and Retinex. For the purpose of image training in general, these methods will
be combined to find out the most optimal method for image enhancement. We have
carried out the research on the different permutation of three methods:
Brightening, CLAHE and Retinex. The evaluation is based on Canny Edge detection
applied to all processed images. Then the sharpness of objects will be
justified by true positive pixels number in comparison between images. After
using different number combinations pre-processing functions on images, CLAHE
proves to be the most effective in edges improvement, Brightening does not show
much effect on the edges enhancement, and the Retinex even reduces the
sharpness of images and shows little contribution on images enhancement.
