.TH i.ifft 1grass "" "GRASS 6.4.4" "Grass User's Manual"
.SH NAME
\fI\fBi.ifft\fR\fR  - Inverse Fast Fourier Transform (IFFT) for image processing.
.SH KEYWORDS
imagery, FFT
.SH SYNOPSIS
\fBi.ifft\fR
.br
\fBi.ifft help\fR
.br
\fBi.ifft real_image\fR=\fIname\fR \fBimaginary_image\fR=\fIname\fR \fBoutput_image\fR=\fIname\fR  [\-\-\fBoverwrite\fR]  [\-\-\fBverbose\fR]  [\-\-\fBquiet\fR] 
.SS Flags:
.IP "\fB\-\-overwrite\fR" 4m
.br
Allow output files to overwrite existing files
.IP "\fB\-\-verbose\fR" 4m
.br
Verbose module output
.IP "\fB\-\-quiet\fR" 4m
.br
Quiet module output
.PP
.SS Parameters:
.IP "\fBreal_image\fR=\fIname\fR" 4m
.br
Name of input raster map (image fft, real part)
.IP "\fBimaginary_image\fR=\fIname\fR" 4m
.br
Name of input raster map (image fft, imaginary part
.IP "\fBoutput_image\fR=\fIname\fR" 4m
.br
Name for output raster map
.PP
.SH DESCRIPTION
\fIi.ifft\fR 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
\fIi.fft\fR) into a normal image.
.SH NOTES
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 \fIr.digit\fR can be used to create
masks while viewing the real or imaginary component image. Alternatively
\fIr.circle\fR can be used to generate high-, low- and donut filters
specifying the DC point as circle/ring center.
When \fIi.ifft\fR is executed, it (automatically) uses the same GRASS
region definition setting that was used during the original transformation
done with \fIi.fft\fR.
.PP
The real and imaginary components are read from arrays of
doubles in the \fIcell_misc\fR directory (produced by
the forward transform program,
\fIi.fft\fR),
and the reconstructed image will preserve the cell value
scaling of the original image processed by
\fIi.fft\fR.  No color
table is assigned to the output map;  one should be created
before viewing the \fIoutput_image\fR.
.SH 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.
.PP
\fBRemote Sensing Digital Image Analysis\fR,
by John A. Richards,
Springer-Verlag, 1986.
.PP
Personal communication,
between program author and Ali R. Vali, Space Research Center,
University of Texas, Austin, 1990.
.PP
\fIi.cca\fR, 
\fIi.class\fR, 
\fIi.fft\fR, 
\fIi.pca\fR, 
\fIr.circle\fR
.SH AUTHOR
David Satnik, GIS Laboratory, 
Central Washington University
.br
Glynn Clements (FFTW support)
.PP
\fILast changed: $Date: 2011-11-08 10:42:51 +0100 (Tue, 08 Nov 2011) $\fR
.PP
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.PP
© 2003-2014 GRASS Development Team