1i.fft(1) Grass User's Manual i.fft(1)
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6 i.fft - Fast Fourier Transform (FFT) for image processing.
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9 imagery
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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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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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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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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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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)