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)
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