I always liked the imagemagick introductions to fourier transform filtering of images: http://www.imagemagick.org/Usage/fourier/ Here's another one for ad

I always liked the imagemagick introductions to fourier transform filtering of images: http://www.imagemagick.org/Usage/fourier/ Here's another one for advanced uses, including sharpening through FFT division: http://www.imagemagick.org/Usage/fourier/fft_math/