Bandpass filters are a combination of both lowpass and highpass filters. They attenuate all frequencies smaller than a frequency Eqn:eqnfreq3 and higher than a frequency Eqn:eqnfreq6, while the frequencies between the two cut-offs remain in the resulting output image. We obtain the filter function of a bandpass by multiplying the filter functions of a lowpass and of a highpass in the frequency domain, where the cut-off frequency of the lowpass is higher than that of the highpass.