```NAME
fft - fft 2d on an image

SYNOPSIS
fft [options] <in> <out>

DESCRIPTION
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
imagewise).

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

OPTIONS
-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
image.

FILES
Input files shall all comply with FITS format.
```