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]
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17   Flags:
18       --overwrite
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20   Parameters:
21       input_image=string
22           input raster file being fft
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24       real_image=string
25           output real part arrays stored as raster file
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27       imaginary_image=string
28           output imaginary part arrays stored as raster file
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30       range=integer
31           Range of values in output display files Default: 255
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DESCRIPTION

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

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

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

78       David Satnik, GIS Laboratory, Central Washington University
79       Glynn Clements (FFTW support)
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81       Last changed: $Date: 2003/04/17 14:49:02 $
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83       Full index
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87GRASS 6.2.2                                                           i.fft(1)