Image contrast enhancement by constrained local histogram equalization

Histogram equalization is a widely used image contrast enhancement method. While global histogram equalization enhances the contrast of the whole imag e, local histogram equalization can enhance many image details by taking di fferent transformation of the same gray level at different places in the or iginal image. However, the local histogram equalization process often resul ts in unacceptable modification of the original image appearance. In this p aper, a constrained local histogram equalization method is proposed to bala nce the conflicting requirements: enhancement of the image details and the maintenance of the overall image appearance. Our method uses the variationa l form of histogram equalization so that a constraint condition, which forc es the local gray level transformations to change continuously in the spati al domain, can be introduced into the equalization process. Experimental re sults of different kinds of images show the effect of our method. (C) 1999 Academic Press.