NAME

       i.fft  - Fast Fourier Transform (FFT) for image processing.

KEYWORDS

       imagery, transformation, Fast Fourier Transform

SYNOPSIS

       i.fft
       i.fft --help
       i.fft input=name real=name imaginary=name  [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       input=name [required]
           Name of input raster map

       real=name [required]
           Name for output real part arrays stored as raster map

       imaginary=name [required]
           Name for output imaginary part arrays stored as raster map

DESCRIPTION

       i.fft  is  an  image  processing  program  based  on the FFT algorithm given by Frigo et al. (1998), that
       processes a single input raster  map  layer  (input)  and  constructs  the  real  and  imaginary  Fourier
       components in frequency space.

NOTES

       The  real  and  imaginary  components are stored into the real and imaginary raster map layers.  In these
       raster map layers the low frequency components are in the center and the high  frequency  components  are
       toward the edges.  The input need not be square. A color table is assigned to the resultant map layer.

       The  current  geographic region and mask settings are respected when reading the input file. The presence
       of nulls or a mask will make the resulting fast Fourier transform invalid.

EXAMPLE

       North Carolina example:
       g.region raster=lsat7_2002_70
       i.fft input=lsat7_2002_70 real=lsat7_2002_70.real imaginary=lsat7_2002_70.imag
       # set region to resulting FFT output map (due to new FFT coordinate space):
       g.region raster=lsat7_2002_70.real -p
       d.mon x0
       d.rast lsat7_2002_70.real
       d.rast lsat7_2002_70.imag

REFERENCES

           •   M. Frigo and S. G. Johnson (1998): "FFTW: An Adaptive Software Architecture  for  the  FFT".  See
               www.FFTW.org:  FFTW  is a C subroutine library for computing the Discrete Fourier Transform (DFT)
               in one or more dimensions, of both real and complex data, and of arbitrary input size.

           •   John A. Richards, 1986. Remote Sensing Digital Image Analysis, Springer-Verlag.

           •   Personal communication, between program author and Ali R. Vali, Space Research Center, University
               of Texas, Austin, 1990.

SEE ALSO

        i.cca, g.gui.iclass, i.ifft, i.pca

AUTHOR

       David Satnik, GIS Laboratory, Central Washington University
       Glynn Clements (FFTW support)

SOURCE CODE

       Available at: i.fft source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual