Geografic-Information-System/imagery/i.fft/i.fft.html

<h2>DESCRIPTION</h2>

<em>i.fft</em> is an image processing program based on the FFT algorithm
given by Frigo et al. (1998), that processes a single input raster map layer
(<b>input_image</b>) and constructs the real and imaginary Fourier
components in frequency space.

<h2>NOTES</h2>

The real and imaginary components are stored into the
<b>real_image</b> and <b>imaginary_image</b> raster map
layers.  In these raster map
layers the low frequency components are in the center and
the high frequency components are toward the edges.  The
<b>input_image</b> need not be square. A
color table is assigned to the resultant map layer.


<p>
The current geographic region and mask settings are respected when
reading the input file. The presence of nulls or a mask will make the
resulting fast Fourier transform invalid.

<h2>EXAMPLE</h2>

North Carolina example:

<div class="code"><pre>
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
</pre></div>

<h2>SEE ALSO</h2>

<ul>
<li> M. Frigo and S. G. Johnson (1998): "FFTW: An Adaptive Software Architecture
for the FFT". See <a href="http://www.fftw.org">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.

<li> John A. Richards, 1986. Remote Sensing Digital Image Analysis, Springer-Verlag.
</ul>

<p>Personal communication, between progam author and Ali R. Vali,
Space Research Center, <A HREF="http://www.utexas.edu">University of Texas</a>, Austin, 1990.

<p>
<em>
<a href="i.cca.html">i.cca</a>,
<a href="g.gui.iclass.html">g.gui.iclass</a>,
<a href="i.class.html">i.class</a>,
<a href="i.ifft.html">i.ifft</a>,
<a href="i.pca.html">i.pca</a>
</em>

<h2>AUTHOR</h2>

David Satnik, GIS Laboratory, 
Central Washington University
<br>
Glynn Clements (FFTW support)

<p><i>Last changed: $Date$</i>