.TH i.fft 1grass "" "GRASS 7.8.5" "GRASS GIS User's Manual"
.SH NAME
\fI\fBi.fft\fR\fR  \- Fast Fourier Transform (FFT) for image processing.
.SH KEYWORDS
imagery, transformation, Fast Fourier Transform
.SH SYNOPSIS
\fBi.fft\fR
.br
\fBi.fft \-\-help\fR
.br
\fBi.fft\fR \fBinput\fR=\fIname\fR \fBreal\fR=\fIname\fR \fBimaginary\fR=\fIname\fR  [\-\-\fBoverwrite\fR]  [\-\-\fBhelp\fR]  [\-\-\fBverbose\fR]  [\-\-\fBquiet\fR]  [\-\-\fBui\fR]
.SS Flags:
.IP "\fB\-\-overwrite\fR" 4m
.br
Allow output files to overwrite existing files
.IP "\fB\-\-help\fR" 4m
.br
Print usage summary
.IP "\fB\-\-verbose\fR" 4m
.br
Verbose module output
.IP "\fB\-\-quiet\fR" 4m
.br
Quiet module output
.IP "\fB\-\-ui\fR" 4m
.br
Force launching GUI dialog
.SS Parameters:
.IP "\fBinput\fR=\fIname\fR \fB[required]\fR" 4m
.br
Name of input raster map
.IP "\fBreal\fR=\fIname\fR \fB[required]\fR" 4m
.br
Name for output real part arrays stored as raster map
.IP "\fBimaginary\fR=\fIname\fR \fB[required]\fR" 4m
.br
Name for output imaginary part arrays stored as raster map
.SH DESCRIPTION
\fIi.fft\fR is an image processing program based on the FFT algorithm
given by Frigo et al. (1998), that processes a single input raster map layer
(\fBinput\fR) and constructs the real and imaginary Fourier
components in frequency space.
.SH NOTES
The real and imaginary components are stored into the
\fBreal\fR and \fBimaginary\fR 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
\fBinput\fR need not be square. A
color table is assigned to the resultant map layer.
.PP
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.
.SH EXAMPLE
North Carolina example:
.br
.nf
\fC
g.region raster=lsat7_2002_70
i.fft input=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 raster=lsat7_2002_70.real \-p
d.mon x0
d.rast lsat7_2002_70.real
d.rast lsat7_2002_70.imag
\fR
.fi
.SH REFERENCES
.RS 4n
.IP \(bu 4n
M. Frigo and S. G. Johnson (1998): \(dqFFTW: An Adaptive Software Architecture
for the FFT\(dq. 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.
.IP \(bu 4n
John A. Richards, 1986. Remote Sensing Digital Image Analysis, Springer\-Verlag.
.IP \(bu 4n
Personal communication, between program author and Ali R. Vali,
Space Research Center, University of Texas, Austin, 1990.
.RE
.SH SEE ALSO
\fI
i.cca,
g.gui.iclass,
i.ifft,
i.pca
\fR
.SH AUTHOR
David Satnik, GIS Laboratory,
Central Washington University
.br
Glynn Clements (FFTW support)
.SH SOURCE CODE
.PP
Available at: i.fft source code (history)
.PP
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.PP
© 2003\-2020
GRASS Development Team,
GRASS GIS 7.8.5 Reference Manual