`fft` is a simple 2d FFT on images. In forward transform, it takes
single images in input and produces a cube containing two images in
output: either (real,imaginary) or (amplitude,phase). The reverse
transform works exactly the other way round.

For the moment, `fft` only supports images which size is a power
of 2. If anyone has a request for other image sizes in astronomy, drop
a mail to the authors. There are FFT libraries covering readily any
kind of signal size, but no one has ever requested such a
functionality. Furthermore, there is an FFT command in virtually any data
analysis environment so this command is really there for completeness.

Nicolas Devillard 2002-04-01