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NAME

       im_fwfft, im_invfft, im_invfftr - forward and inverse fft on an image

SYNOPSIS

       #include <vips/vips.h>

       int im_fwfft(in, out)
       IMAGE *in, *out;

       int im_invfft(in, out)
       IMAGE *in, *out;

       int im_invfftr(in, out)
       IMAGE *in, *out;

DESCRIPTION

       im_fwfft()  performs a forward fast Fourier Transform on the image held
       by the  image  descriptor  in  and  writes  the  result  to  the  image
       descriptor  out.  The image can be in any format and have any number of
       bands. The output is always complex.

       If VIPS has been built with  support  for  libfftw,  a  high-speed  FFT
       library,  then  fftwnd_one()  is  used  to  compute the transform. This
       produces a double precision complex result. The first transformation at
       a  particular  image size will be very slow as libfftw optimises itself
       for your machine, but subsequent transforms of images of that size  are
       extremely  fast.   Unfortunately,  libfftw  does  not have good out-of-
       memory behaviour. If you try to transform  a  very  large  image,  your
       program will exit abruptly.

       If  VIPS has not been built with libfftw support, VIPS uses its own fft
       routines. These are rather slow, are single  precision  only,  and  can
       only transform images whose sides are a power of two.

       im_invfft()  performs  the  reverse transform.  The input image must be
       complex, the output is always complex. The image may have any number of
       bands.

       Again,  if  libfftw was present when VIPS was compiled, that library is
       used to calculate the transform.

       im_invfftr() performs the reverse transform.  The input image  must  be
       complex,  the  output  is always real. The image may have any number of
       bands. It is about 2 x faster than im_invfft().

       Again, if libfftw was present when VIPS was compiled, that  library  is
       used to calculate the transform.

RETURN VALUE

       The function returns 0 on success and -1 on error.

SEE ALSO

       im_rotquad(3), im_c2ps(3), im_scaleps(3), im_disp_ps(3).

COPYRIGHT

       1995, National Gallery and Birkbeck College

                                  14 May 1991                      IM_FWFFT(3)