Contrast Enhancement for JPEG Images in the Compressed Domain

Abstract

With the increase in digitization, there has been a great demand for data storage with effective techniques for data computation. As these days, a lot of data is being transferred over internet in the form of images, data storage is a prime concern, for which there is a requirement of image compression without losing the important details of the image. Digital image compression finds its applications in various fields like Medical, Automation, Defense, Photography etc. which also requires that the image produced should be visibly pleasing with sharp and clear details. The latter is achieved by a pre-processing technique called Image Enhancement.This research project is based upon the contrast enhancement of the color
Images, where each color R-G-B channel is separately analyzed in the Y-Cb-Cr channel, in the compressed domain. The Discrete Cosine Transform is used as the compressed domain and further analysis is made on the block coefficients of the DCT where the block size considered is 8x8. Each DCT block contains one DC coefficient and 63 AC coefficients. The DCT coefficients are analyzed on the basis of their statistical behaviour. It is seen that the DC coefficient of each block DCT follow Gaussian distribution and the AC coefficients follow the Laplacian distribution .The DC coefficient being the mean value of the block DCT, is observed to be affecting the illumination of the image whereas the remaining 63 coefficients i.e. AC coefficients of the block DCT affected the contrast of the image. This thesis investigates a novel method for enhancing the image contrast based on the statistical behaviour of the block DCT coefficients. Furthermore, we use the concept of coefficient of variation (Cv) for arriving at a DC scaling factor required to modify the original DC coefficient value of each block. We also evaluate AC scaling factor by band analysis of each block based upon their contrast and entropy bands. The proposed work analyses both the DC coefficient and the 63 AC coefficients of each block separately.