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Is 'frequency' 1d or 2d? Also, gain transfer function looks like a spline, why not try an FIR?

It would cool to add a few images with periodicity. I expected the lake bed image to show some quasi-periodic lines, but couldn't make that out in the spectrum.

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In images frequency is 2d. However in that page (and many other image processing applications) we only care about effectively what is 1d frequency (all the frequencies with the same euclidian distance from the center are binned as one).

You've got directional frequencies in 2d - you'd measure it in the direction of some vector. With sinusoids, cos(wx+vy), the "natural" frequency is along the (w,v) direction. The 1D frequency along this direction is |(w,v)|^2. It's like how with the gradient, you have directional derivatives.

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