FFT Shift

The FFT Shift post-processes the output frequency graph of the FFT and centers 0 in the middle of the array, rather than at the beginning, making it easier to interpret. By default frequencies of zero are on the left, then positives, then negatives.

$$[k_0,{\vec{k}}_{+},{\vec{k}}_{-}]\to [{\vec{k}}_{-},k_0,{\vec{k}}_{+}]$$