The document discusses frequency filtering of images. It explains that the Fourier transform allows analyzing the frequency content of a signal. The discrete Fourier transform (DFT) is used to transform images into the frequency domain. Low pass filters suppress high frequencies to blur an image, while high pass filters remove low frequencies. The ideal low pass filter causes ringing artifacts due to the ripples in the sinc function impulse response. To avoid ringing, the Butterworth filter can be used as it has a flatter frequency response.