.TH IM_FWFFT 3 "14 May 1991"
.SH NAME
im_fwfft, im_invfft, im_invfftr \- forward and inverse fft on an image
.SH SYNOPSIS
.B #include <vips/vips.h>

.B int im_fwfft(in, out)
.br
.B IMAGE *in, *out;

.B int im_invfft(in, out)
.br
.B IMAGE *in, *out;

.B int im_invfftr(in, out)
.br
.B IMAGE *in, *out;

.SH DESCRIPTION
.B 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.

.B 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.

.B 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
.B im_invfft().

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

.SH RETURN VALUE
The function returns 0 on success and -1 on error.
.SH SEE ALSO
im_rotquad(3), im_c2ps(3), im_scaleps(3), im_disp_ps(3).
.SH COPYRIGHT
1995, National Gallery and Birkbeck College