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


       imagery, FFT


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

           Allow output files to overwrite existing files

           Verbose module output

           Quiet module output

           Name of input raster map

           Name for output real part arrays stored as raster map

           Name for output imaginary part arrays stored as raster map

           Range of values in output display files
           Default: 255


       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.


       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.


       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


                      M. Frigo and S. G. Johnson (1998): "FFTW: An Adaptive Software Architecture
                     for the FFT". See 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-

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



       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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