NAME

       i.ifft  - Inverse Fast Fourier Transform (IFFT) for image processing.

KEYWORDS

       imagery, FFT

SYNOPSIS

       i.ifft
       i.ifft help
       i.ifft  real_image=name imaginary_image=name output_image=name  [--overwrite]  [--verbose]
       [--quiet]

   Flags:
       --overwrite
           Allow output files to overwrite existing files

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       real_image=name
           Name of input raster map (image fft, real part)

       imaginary_image=name
           Name of input raster map (image fft, imaginary part

       output_image=name
           Name for output raster map

DESCRIPTION

       i.ifft 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 i.fft) into a normal
       image.

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 r.digit can  be  used  to  create
       masks  while  viewing the real or imaginary component image. Alternatively r.circle can be
       used to generate high-, low- and donut filters specifying  the  DC  point  as  circle/ring
       center.  When i.ifft is executed, it (automatically) uses the same GRASS region definition
       setting that was used during the original transformation done with i.fft.

       The real and imaginary components are  read  from  arrays  of  doubles  in  the  cell_misc
       directory  (produced by the forward transform program, i.fft), and the reconstructed image
       will preserve the cell value scaling of the original image processed by i.fft.   No  color
       table  is  assigned  to  the  output  map;   one  should  be  created  before  viewing the
       output_image.

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.

       Remote Sensing Digital Image Analysis, by John A. Richards, Springer-Verlag, 1986.

       Personal  communication,  between  program  author and Ali R. Vali, Space Research Center,
       University of Texas, Austin, 1990.

       i.cca, i.class, i.fft, i.pca, r.circle

AUTHOR

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

       Last changed: $Date: 2011-11-08 01:42:51 -0800 (Tue, 08 Nov 2011) $

       Full index

       © 2003-2013 GRASS Development Team