Abstract: Histogram equalization (HE) makes the average of equalized image close to gray median regardless the original image.The gray levels of HE are degenerated and the detail of the image is lost,and the over enhancement and unnatural effect is occurred easily.For bi-histogram equalization,original image is decomposed into two sub-images based on an appropriate threshold and the sub-images are equalized independently.However,the threshold is the key issue.The iterative method for choosing the threshold of the bi-histogram is proposed previously,i.e.iterative threshold based brightness preserving bi-histogram equalization (IBBHE).Experimental results on dim medical image show that this method not only keeps the image brightness,but avoids the phenomenon of over enhancement.Based on partially overlapped sub-block histogram equalization algorithm (POSHE),an iterative threshold based partially overlapped sub-blocks bi-histogram equalization algorithm is proposed (IPOSBHE) and the iterative method is adopted in the process of sub-block histogram equalization.Experimental results on both dim medical image and bright scene image show that the local image details are enhanced,and the input image brightness is also preserved effectively.The blocking effect on the image is eliminated and the equalized image shows a more natural visual impression.

Key words: image enhancement, histogram equalization, bi-histogram equalization, local histogram equalization