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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)

       Last changed: $Date: 2018-05-05 14:35:31 +0200 (Sat, 05 May 2018) $

SOURCE CODE

       Available at: i.fft source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.6.1 Reference Manual