NAME
i.fft - Fast Fourier Transform (FFT) for image processing.
(GRASS Image Processing Program)
SYNOPSIS
i.fft
i.fft help
i.fft input_image=name real_image=name
imaginary_image=name [range=value]
DESCRIPTION
i.fft is an image processing program based on the
algorithm given by Press (1988), with enhancements by Vali
(1990), that processes a single input raster map layer
(input_image) and constructs the real and
imaginary Fourier components in frequency space.
OPTIONS
Parameters:
- input_image=name
- Input raster map layer on which the fast Fourier
transform is run.
- real_image=name
- Output real part arrays stored as raster map layer.
- imaginary_image=name
- Output imaginary part arrays stored as raster map layer.
- range=value
- Range of values used during fast Fourier transformation.
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.
SEE ALSO
Numerical Recipes in C,
by William H. Press,
Cambridge University Press, 1988.
Remote Sensing Digital Image Analysis,
by John A. Richards,
Springer-Verlag, 1986.
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