n order to intuitively understand this conclusion, a standard grayscale test image named Lena whose resolution is 512×512 is taken as an example, shown as Fig. 2. This image is initially filtered by a frequency domain Gaussian filter Hi(u,v), with DI=π/5, and then, another Gaussian filter HD(u,v) is utilized to obtain frequency residual DF(u,v), with DD=π/3. As shown in Fig. 3(a), the frequency residual DF(u,v) has two symmetrical peaks around low frequency area, while the high frequency is almost eliminated, complying with the conclusion that the frequency residual function of the filtered image is a band-pass filter. While Lena is directly filtered with the detection filter, i.e. HD(u,v), with DD=π/3, the frequency residual is shown in Fig 3. (b), which has rich high frequency energy. For better understanding the distribution of DF(u,v), the horizontal data will be shown alone in Fig. 4. Fig. 4(a) shows the result of the filtered image, where the amplitude of high frequencies is completely suppressed, comparing with the result of the original image shown as Fig. 4(b).