fft - fft 2d on an image

     fft [options] <in> <out>

     fft computes a Fast Fourier Transform  on  an  input  image.
     fft  can  also  compute  an  inverse  FFT.  Results can have
     swapped quadrants, and can be expressed in polar  or  carte-
     sian coordinates.

     Take  care  about  the   formats   :   polar/cartesian   and
     swapped/unswapped.  The default procedure is:

     In input of a forward FFT, an image is required. It is taken
     as  the real part (in cartesian coordinates) of a complex 2d
     signal which imaginary part is set to zeros. The output of a
     forward  FFT is a complex 2d signal, i.e. 2 image planes. By
     default,  the  output  is  converted  to  polar  coordinates
     (modulus,  phase),  and  then  quadrants are swapped in both
     modulus and phase to put low frequencies at  the  center  of
     the  images.   To prevent this default behaviour, use the -n
     and -c options described below.

     In input of an inverse FFT, a cube containing  2  images  is
     required.  It  is  taken  as a complex 2d signal which first
     plane contains the modulus and  second  plane  contains  the
     phase.  Before  the  inverse  FFT is computed, a swapping of
     quadrants occurs, and then a conversion to cartesian  (real,
     imaginary)  coordinates  is  performed.  To prevent quadrant
     swapping or cartesian conversion, use the -n and -c  options
     described below.

     The output of an inverse FFT is a cube containing  2  images
     in  cartesian  unswapped  format,  first  one being the real
     part, second  one  being  the  imaginary  part  (meaningless

     fft uses the Danielson-Lanczos lemma, in a code based on one
     originally  written by N. M. Brenner, described in Numerical
     Recipes in C.

     -c  or  --noconv
          Switches to cartesian mode the following data:
          output for a forward FFT (default is polar),  or  input
          for an inverse FFT.

     -i  or  --inverse
          Inverse FFT. The input is a cube containing two  planes
          only.  Default  I/O  format for these 2 planes is polar
          coordinates (modulus, phase). It  will  output  a  cube
          containing  1 plane only (imaginary part is meaningless
          imagewise).  The input  will  be  swapped  before  FFT,
          unless the -n option is used.

     -n  or  --noswap
          This option prevents fft from swapping the output of  a
          forward FFT and the input of an inverse FFT.

     -s  or  --swaponly
          This option can be used to apply the swapping and quit.

     Swapping is done according to the following rule :
     1 2
     4 3
     becomes then
     3 4
     2 1
     which puts then the lowest frequencies at the center of  the

     Input files shall all comply with FITS format.