What do "fftshift" do exactly for an image ?
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Image Analyst on 16 Feb 2015
Normally the "center" or the 0 frequency location of the spectrum is at the first and last element, and the "ends" of the spectrum (highest frequencies) are at the center of the array. That's just how the fft works. But that's confusing for display because the low frequencies are appearing in two disconnected locations. So fftshift() moves the zero frequency location to the middle of the array so that when you display it with imshow() or plot(), it looks like you'd expect.