title: Select axes DFT of multi-dimensional array (using FFTW)
body: <p>I have data that is ordered as follows</p>
  
  <pre><code>[ z, y, x, i, j ]
  </code></pre>
  
  <p>where <code>[z,y,x]</code> represent <em>physical dimensions</em> and <code>[i,j]</code> represent <em>tensor dimensions</em>. I.e. it stores a second-order tensor at points in grid with shape <code>[z,y,x]</code>. </p>
  
  <p>I would like to calculate the Fourier transform of each tensor component <code>[i,j]</code> along the physical dimensions <code>[z,y,x]</code>, <strong>using FFTW in C</strong>. </p>
  
  <p>The NumPy-equivalent of this operation would be</p>
  
  <pre><code>np.fft.fftn(a, axes=(0, 1, 2))
  </code></pre>
  
  <p>Neither the documentation nor Google have provided the answer.</p>