Improving the contrast of images with various brightness ranges Using wavelet transform

In this article, a contrast enhancement algorithm using wavelet transform is proposed to create natural enhancement in images with various brightness ranges. In the proposed method, the input image is first decomposed into 4 sub-bands by discrete wavelet transformation. Then, in the LL sub band image, the histogram cutting process is performed by the cutting threshold level equal to the average intensities. Next, the cut histogram based on entropy is divided into three parts with approximately equal number of pixels, and before equalization process, each sub-histogram is mapped to the new dynamic range. Finally, we use the inverse discrete wavelet transform to create an improved image. By controlling the enhancement ratio, the proposed method produces an image with maximum details and natural enhancement in the output image. Comparing the performance of the proposed method with previously presented methods, in terms of entropy as well as visual quality based on the mean opinion score, shows the superiority of the proposed algorithm.