NAME
i.ifft - Inverse Fast Fourier Transform (ifft) for image processing.
(GRASS Image Processing Program)
SYNOPSIS
i.ifft
i.ifft help
i.ifft real_image=name
imaginary_image=name output_image=name
DESCRIPTION
i.ifft is an image processing program based on the algorithm given
by Frigo et al. (1998), that converts real and imaginary frequency space
images (produced by
i.fft) into a normal image.
OPTIONS
Parameters:
- real_image=name
- Input raster map layer for inversion fast Fourier transform, real part.
- imaginary_image=name
- Input raster map layer for inversion fast Fourier transform, imaginary.
- output_image=name
- Output inversion raster map layer after fast Fourier transformation.
NOTES
The current mask is respected when reading the real and
imaginary component files; thus,
r.mask become a primary program for
selecting the portion of the frequency space data to be included in the
inverse transform. The GRASS program r.digit can be used to create
masks while viewing the real or imaginary component image. Alternatively
r.circle can be used to generate high-, low- and donut filters
specifying the DC point as circle/ring center.
When i.ifft is executed, it (automatically) uses the same GRASS
region definition setting that was used during the original transformation
done with i.fft.
The real and imaginary components are read from arrays of
doubles in the cell_misc directory (produced by
the forward transform program,
i.fft),
and the reconstructed image will preserve the cell value
scaling of the original image processed by
i.fft. No color
table is assigned to the output map; one should be created
before viewing the output_image.
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.
Remote Sensing Digital Image Analysis,
by John A. Richards,
Springer-Verlag, 1986.
Personal communication,
between program author and Ali R. Vali, Space Research Center,
University of Texas, Austin, 1990.
i.cca,
i.class,
i.fft,
i.pca,
r.circle
AUTHOR
David Satnik, GIS Laboratory,
Central Washington University
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