| .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 |
