1i.fft(1)                      Grass User's Manual                     i.fft(1)
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NAME

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

9       imagery
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SYNOPSIS

12       i.fft
13       i.fft help
14       i.fft   input_image=string   real_image=string   imaginary_image=string
15       [range=integer]   [--overwrite]  [--verbose]  [--quiet]
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17   Flags:
18       --overwrite
19           Allow output files to overwrite existing files
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21       --verbose
22           Verbose module output
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24       --quiet
25           Quiet module output
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27   Parameters:
28       input_image=string
29           Input raster map being fft
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31       real_image=string
32           Output real part arrays stored as raster map
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34       imaginary_image=string
35           Output imaginary part arrays stored as raster map
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37       range=integer
38           Range of values in output display files
39           Default: 255
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DESCRIPTION

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

48       The  real  and  imaginary components are stored as arrays of doubles in
49       the cell_misc directory (for use  in  the  inverse  transform  program,
50       i.ifft),  and  are  also  scaled  and formatted into the real_image and
51       imaginary_image raster map layers for  inspection,  masking,  etc.   In
52       these  raster map layers the low frequency components are in the center
53       and  the  high  frequency  components  are  toward  the   edges.    The
54       input_image  need not be square;  before processing, the X and Y dimen‐
55       sions of the input_image are padded with zeroes  to  the  next  highest
56       power  of two in extent (i.e., 256 x 256 is processed at that size, but
57       200 x 400 is padded to 256 x 512).  The cell category values for  view‐
58       ing,  etc., are calculated by taking the natural log of the actual val‐
59       ues then rescaling to 255, or whatever optional range is given  on  the
60       command  line,  as  suggested  by  Richards  (1986).   A color table is
61       assigned to the resultant map layer.
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63       The current geographic region and  mask  settings  are  respected  when
64       reading  the input file.  The presence of a mask will, in general, make
65       the resulting fast Fourier transform invalid, or at least difficult  to
66       interpret.
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SEE ALSO

69       M.  Frigo  and S. G. Johnson (1998): "FFTW: An Adaptive Software Archi‐
70       tecture for the FFT". See www.FFTW.org: FFTW is a C subroutine  library
71       for  computing  the  Discrete  Fourier  Transform  (DFT) in one or more
72       dimensions, of both real and complex data, and of arbitrary input size.
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74       Remote Sensing Digital Image Analysis, by John A.  Richards,  Springer-
75       Verlag, 1986.
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77       Personal  communication,  between  progam author and Ali R. Vali, Space
78       Research Center, University of Texas, Austin, 1990.
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80       i.cca
81       i.class
82       i.ifft
83       i.pca
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AUTHOR

86       David Satnik, GIS Laboratory, Central Washington University
87       Glynn Clements (FFTW support)
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89       Last changed: $Date: 2003-04-17 16:51:33 +0200 (Thu, 17 Apr 2003) $
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91       Full index
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93       © 2003-2008 GRASS Development Team
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97GRASS 6.3.0                                                           i.fft(1)