title: How should I interpret the output of numpy.fft.rfft2?
body: <p>Obviously the rfft2 function simply computes the discrete fft of the input matrix. However how do I interpret a given index of the output? Given an index of the output, which Fourier coefficient am I looking at?<br>
  I am especially confused by the sizes of the output. For an n by n matrix, the output seems to be an n by (n/2)+1 matrix (for even n). Why does a square matrix ends up with a non-square fourier transform?</p>