1IM_FWFFT(3)                Library Functions Manual                IM_FWFFT(3)
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NAME

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

9       #include <vips/vips.h>
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11       int im_fwfft(in, out)
12       IMAGE *in, *out;
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14       int im_invfft(in, out)
15       IMAGE *in, *out;
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17       int im_invfftr(in, out)
18       IMAGE *in, *out;
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DESCRIPTION

22       im_fwfft()  performs a forward fast Fourier Transform on the image held
23       by the image descriptor in and writes the result to the image  descrip‐
24       tor  out.  The image can be in any format and have any number of bands.
25       The output is always complex.
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27       If VIPS has been built with  support  for  libfftw,  a  high-speed  FFT
28       library,  then fftwnd_one() is used to compute the transform. This pro‐
29       duces a double precision complex result. The first transformation at  a
30       particular image size will be very slow as libfftw optimises itself for
31       your machine, but subsequent transforms of  images  of  that  size  are
32       extremely  fast.  Unfortunately, libfftw does not have good out-of-mem‐
33       ory behaviour. If you try to transform a very large image, your program
34       will exit abruptly.
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36       If  VIPS has not been built with libfftw support, VIPS uses its own fft
37       routines. These are rather slow, are single  precision  only,  and  can
38       only transform images whose sides are a power of two.
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40       im_invfft()  performs  the  reverse transform.  The input image must be
41       complex, the output is always complex. The image may have any number of
42       bands.
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44       Again,  if  libfftw was present when VIPS was compiled, that library is
45       used to calculate the transform.
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47       im_invfftr() performs the reverse transform.  The input image  must  be
48       complex,  the  output  is always real. The image may have any number of
49       bands. It is about 2 x faster than im_invfft().
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51       Again, if libfftw was present when VIPS was compiled, that  library  is
52       used to calculate the transform.
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RETURN VALUE

56       The function returns 0 on success and -1 on error.
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SEE ALSO

59       im_rotquad(3), im_c2ps(3), im_scaleps(3), im_disp_ps(3).
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62       1995, National Gallery and Birkbeck College
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66                                  14 May 1991                      IM_FWFFT(3)
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