How can I make FFTW put the origin (zero frequency) at the center of its output?
For human viewing of a spectrum, it is often convenient to put the origin in frequency space at the center of the output array, rather than in the zero-th element (the default in FFTW). If all of the dimensions of your array are even, you can accomplish this by simply multiplying each element of the input array by (-1)^(i + j + …), where i, j, etcetera are the indices of the element.
