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<h2>NAME</h2>
<em><b>i.ifft</b></em> - Inverse Fast Fourier Transform (IFFT) for image processing.
<h2>KEYWORDS</h2>
imagery, FFT
<h2>SYNOPSIS</h2>
<b>i.ifft</b><br>
<b>i.ifft help</b><br>
<b>i.ifft</b> <b>real_image</b>=<em>name</em> <b>imaginary_image</b>=<em>name</em> <b>output_image</b>=<em>name</em> [--<b>overwrite</b>] [--<b>verbose</b>] [--<b>quiet</b>]
<h3>Flags:</h3>
<DL>
<DT><b>--overwrite</b></DT>
<DD>Allow output files to overwrite existing files</DD>
<DT><b>--verbose</b></DT>
<DD>Verbose module output</DD>
<DT><b>--quiet</b></DT>
<DD>Quiet module output</DD>
</DL>
<h3>Parameters:</h3>
<DL>
<DT><b>real_image</b>=<em>name</em></DT>
<DD>Name of input raster map (image fft, real part)</DD>
<DT><b>imaginary_image</b>=<em>name</em></DT>
<DD>Name of input raster map (image fft, imaginary part</DD>
<DT><b>output_image</b>=<em>name</em></DT>
<DD>Name for output raster map</DD>
</DL>
<h2>DESCRIPTION</h2>
<em>i.ifft</em> 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
<em><a href="i.fft.html">i.fft</a></em>) into a normal image.
<h2>NOTES</h2>
The current mask is respected when reading the real and
imaginary component files; thus, creating a mask is a
primary step for
selecting the portion of the frequency space data to be included in the
inverse transform. The GRASS program <em>r.digit</em> can be used to create
masks while viewing the real or imaginary component image. Alternatively
<em>r.circle</em> can be used to generate high-, low- and donut filters
specifying the DC point as circle/ring center.
When <em>i.ifft</em> is executed, it (automatically) uses the same GRASS
region definition setting that was used during the original transformation
done with <em><a href="i.fft.html">i.fft</a></em>.
<p>
The real and imaginary components are read from arrays of
doubles in the <em>cell_misc</em> directory (produced by
the forward transform program,
<em><a href="i.fft.html">i.fft</a></em>),
and the reconstructed image will preserve the cell value
scaling of the original image processed by
<em><a href="i.fft.html">i.fft</a></em>. No color
table is assigned to the output map; one should be created
before viewing the <em>output_image</em>.
<h2>SEE ALSO</h2>
M. Frigo and S. G. Johnson (1998): "FFTW: An Adaptive Software Architecture
for the FFT". See <a href=>www.FFTW.org</a>: 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.
<p>
<b>Remote Sensing Digital Image Analysis</b>,
by John A. Richards,
Springer-Verlag, 1986.
<p>
Personal communication,
between program author and Ali R. Vali, Space Research Center,
University of Texas, Austin, 1990.
<p>
<em><a href="i.cca.html">i.cca</a></em>,
<em><a href="i.class.html">i.class</a></em>,
<em><a href="i.fft.html">i.fft</a></em>,
<em><a href="i.pca.html">i.pca</a></em>,
<em><a href="r.circle.html">r.circle</a></em>
<h2>AUTHOR</h2>
David Satnik, GIS Laboratory,
Central Washington University
<br>
Glynn Clements (FFTW support)
<p><i>Last changed: $Date: 2011-11-08 01:42:51 -0800 (Tue, 08 Nov 2011) $</i>
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