The image of a cat:













The spectra of the image obtained by DFT, DCT and WHT, respectively:













The low-pass filtered images (by DFT and DCT, respectively). Note the ringing effect caused by the ideal filter.













The low-pass filtered images (by DFT, DCT and WHT, respectively). The ringing effect no longer exists due to the Butterworth filter.













These are high-pass filtered images.