fftw help

[wysota]

If this is a 2D transform then the lowest frequencies are towards the middle of the transform, thus DC will be exactly in the middle (0,0). X axis gives you one dimension of FT and Y axis gives you the other one, so (0, nx/2) gives you the constant for one dimension interfered with "nx/2" frequency for the other. At least that's what I think I remember about FT

And all that of course means you only have the quarter of the result and not half of it.