It's so classical, it can be done without a digital computer at all, using fourier optics! The true fastest fourier transform, done at the speed of light.

https://en.wikipedia.org/wiki/Fourier_optics

At the focal plane of a lens, an image is converted into its spatial fourier components. By adding blocking elements at that focal plane (eg. an aperture, or a piece of glass with marker dots), unwanted frequency components can be cut out from the image.