This is an interactive demo that lets you explore how to remove certain wavelength-related (frequency) content from an image. By wavelength, I mean intensities

This is an interactive demo that lets you explore how to remove certain wavelength-related (frequency) content from an image. By wavelength, I mean intensities that vary by some rate-of-change of pixels either across horizontally or up-and-down vertically in the image. The low-cut slider sets up a threshold below which content is removed, and the high-cut slider sets up an equivalent threshold above which content is removed. I think it is interesting because it reveals a couple things. First, that most of the content we find essential to the image is in the low-frequencies (the high-cut slider can be moved very far left before the image appears altered). Second, it demonstrates that a sharp cutoff of frequencies results in ringing (oscillations) in the image (the opposite is also true -- a sharp edge in the image will produce oscillating wavelength magnitudes in the frequency plot). This method of removing wavelength-related content (called Fourier Filtering) is very precise in terms of cutting out specific frequencies, but the effect can be undesirable in terms of producing images that appear smooth and artifact-free (no blemishes). If you want to produce a smoother image which has certain periodic content removed, you would have to filter it out gradually in the Fourier representation (called filter "roll-off") instead of a sharp cut-off.