The problem of gray level incorporation during image histogram modification is analyzed in the paper. And two histogram modification methods which can prevent image details losing are presented. One is addressed by expanding the gray-level range of image

so less gray levels are integrated and more details of the image can be hold. The other method is realized by modifying local gray level. According to the method

the image's histogram is modified by way of general histogram modification method firstly. Then the gradient of modified image and original image is calculated and compared. The pixels whose gradient decreased markedly are found. And their gray levels are modified according to the gradient of the original image finally. So the details lost during histogram modification can be resumed. Both the methods can enhance the image contrast and hold the details simultaneously. Further more

the proposed approach is simple and easy to be performed. Experimental results are given at the end of the paper. Four images are adopted to demonstrate the performance of the methods.