BFFT-Takes the reverse FFT of a complex data structure

Usage:

bfft frequency_data spatial_data

Description:

These Figaro functions take the FFT of the data in a file. FFT performs a forward transform, BFFT performs an inverse transform. The input file must contain a complex data structure, i.e. one with IMAGINARY and DATA components.

The data may be multi-dimensional; if it is, a multi-dimensional FFT is performed. Note that the Figaro routine R2CMPLX will turn an existing real data structure into a complex one acceptable to this routine. FFT does not perform any cosine belling or other tapering of the data, nor does it reduce it to a zero mean.

Parameters:

CDATA

CDATA is the name of a complex data structure. Such structures for the spatial domain are most easily produced using the R2CMPLX command. For the frequency domain, such data were usually created by R2CMPLX and transformed by FFT.

OUTPUT

OUTPUT is the name of the resulting Fourier transformed data. If OUTPUT is the same as CDATA then the transform is performed in situ; otherwise, a new file is created.

Notes:

The fourier transform routines available in the various math libraries (NAG, IMSL, etc) all have slightly different characteristics, which show up in the programs that use them. This routine has been written around the NAG library (mainly the routines C06FAF and C06FJF), so many of its characteristics may be deduced by reading the relevant parts of the NAG manuals. In version 5.0 this routine was changed to use the PDA library, effectively FFTPACK routines. The data is re-ordered by FFT after the transform so that the zero frequency component is in the center of the resulting array, and this re-ordering is reversed by BFFT before the transform. This means that after FFT has been run, the various axes all go from -N to +N where N is the Nyquist frequency. New axis data structures that reflect this are created by FFT and will be deleted by BFFT.

See also:

FIGARO: FFT, COSBELL, CMPLX2I, CMPLX2R, CMPLX2M, I2CMPLX, R2CMPLX.
KAPPA: CONVOLVE, LUCY, MEM2D, WIENER.

Authors:

ks: Keith Shortridge (AAO)

jm: Jo Murray (RAL, Starlink)

jms: ??? (AAO)

hme: Horst Meyerdierks (UoE, Starlink)