<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <title>GRASS GIS: i.ifft</title> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <link rel="stylesheet" href="grassdocs.css" type="text/css"> </head> <body bgcolor="white"> <img src="grass_logo.png" alt="GRASS logo"><hr align=center size=6 noshade> <h2>NAME</h2> <em><b>i.ifft</b></em> - Inverse Fast Fourier Transform (IFFT) for image processing. <h2>KEYWORDS</h2> imagery <h2>SYNOPSIS</h2> <b>i.ifft</b><br> <b>i.ifft help</b><br> <b>i.ifft</b> <b>real_image</b>=<em>string</em> <b>imaginary_image</b>=<em>string</em> <b>output_image</b>=<em>string</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>string</em></DT> <DD>Input raster map (image fft, real part)</DD> <DT><b>imaginary_image</b>=<em>string</em></DT> <DD>Input raster map (image fft, imaginary part</DD> <DT><b>output_image</b>=<em>string</em></DT> <DD>Output inverse raster map after IFFT</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: 2006-04-13 20:50:10 +0200 (Thu, 13 Apr 2006) $</i> <HR> <P><a href="index.html">Main index</a> - <a href="imagery.html">imagery index</a> - <a href="full_index.html">Full index</a></P> <P>© 2003-2008 <a href="http://grass.osgeo.org">GRASS Development Team</a></p> </body> </html>
