At the first, the algorithm unifies the all of the pixel domain within the rectangular pixels to . At the same time, it gave threshold value of the gradient amplitude. Then it calculates the gradient amplitude of each pixel. Finally, by comparing and , the isolated noise pixel is identified. For the noise pixel, we deal it with the median-filtering algorithm. And for the non-noise pixel, we remain them unchanged. So the algorithm did not lose the information of the original image.