61 lines
2.0 KiB
Groff
61 lines
2.0 KiB
Groff
.TH IM_FWFFT 3 "14 May 1991"
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.SH NAME
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im_fwfft, im_invfft, im_invfftr \- forward and inverse fft on an image
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.SH SYNOPSIS
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.B #include <vips/vips.h>
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.B int im_fwfft(in, out)
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.br
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.B IMAGE *in, *out;
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.B int im_invfft(in, out)
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.br
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.B IMAGE *in, *out;
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.B int im_invfftr(in, out)
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.br
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.B IMAGE *in, *out;
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.SH DESCRIPTION
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.B im_fwfft()
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performs a forward fast Fourier Transform on the image held by the
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image descriptor in and writes the result to the image descriptor out.
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The image can be in any format and have any number of bands. The output is
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always complex.
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If VIPS has been built with support for libfftw, a high-speed FFT library,
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then fftwnd_one() is used to compute the transform. This produces a double
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precision complex result. The first transformation at a particular image
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size will be very slow as libfftw optimises itself for your machine,
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but subsequent transforms of images of that size are extremely fast.
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Unfortunately, libfftw does not have good out-of-memory behaviour. If you
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try to transform a very large image, your program will exit abruptly.
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If VIPS has not been built with libfftw support, VIPS uses its own fft
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routines. These are rather slow, are single precision only, and can only
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transform images whose sides are a power of two.
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.B im_invfft()
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performs the reverse transform.
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The input image must be complex, the output is always complex. The image may
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have any number of bands.
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Again, if libfftw was present when VIPS was
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compiled, that library is used to calculate the transform.
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.B im_invfftr()
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performs the reverse transform.
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The input image must be complex, the output is always real. The image may
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have any number of bands. It is about 2 x faster than
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.B im_invfft().
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Again, if libfftw was present when VIPS was
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compiled, that library is used to calculate the transform.
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.SH RETURN VALUE
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The function returns 0 on success and -1 on error.
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.SH SEE ALSO
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im_rotquad(3), im_c2ps(3), im_scaleps(3), im_disp_ps(3).
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.SH COPYRIGHT
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1995, National Gallery and Birkbeck College
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