NAME

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

KEYWORDS

       imagery, FFT

SYNOPSIS

       i.fft
       i.fft help
       i.fft input_image=name real_image=name imaginary_image=name  [range=integer]   [--overwrite]  [--verbose]
       [--quiet]

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

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       input_image=name
           Name of input raster map

       real_image=name
           Name for output real part arrays stored as raster map

       imaginary_image=name
           Name for output imaginary part arrays stored as raster map

       range=integer
           Range of values in output display files
           Default: 255

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_image) and constructs the  real  and  imaginary  Fourier
       components in frequency space.

NOTES

       The  real and imaginary components are stored as arrays of doubles in the cell_misc directory (for use in
       the inverse transform program, i.ifft), and are  also  scaled  and  formatted  into  the  real_image  and
       imaginary_image  raster  map  layers  for  inspection,  masking, etc.  In these raster map layers the low
       frequency components are in the center and the high frequency  components  are  toward  the  edges.   The
       input_image  need not be square;  before processing, the X and Y dimensions of the input_image are padded
       with zeroes to the next highest power of two in extent (i.e., 256 x 256 is processed at  that  size,  but
       200  x 400 is padded to 256 x 512).  The cell category values for viewing, etc., are calculated by taking
       the natural log of the actual values then rescaling to 255, or whatever optional range is  given  on  the
       command line, as suggested by Richards (1986).  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 a mask will, in general, make the resulting fast Fourier transform invalid, or at least  difficult  to
       interpret.

EXAMPLE

       North Carolina example:
       g.region rast=lsat7_2002_70
       i.fft input_image=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 rast=lsat7_2002_70.real -p
       d.mon x0
       d.rast lsat7_2002_70.real
       d.rast lsat7_2002_70.imag

SEE ALSO

                      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 progam author and Ali R.  Vali,  Space  Research  Center,  University  of
       Texas, Austin, 1990.

       i.cca
       i.class
       i.ifft
       i.pca

AUTHOR

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

       Last changed: $Date: 2011-11-08 03:29:50 -0800 (Tue, 08 Nov 2011) $

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       © 2003-2013 GRASS Development Team

GRASS 6.4.3                                                                                        i.fft(1grass)