i.ifft <main>	      GRASS Reference Manual	    <main> i.ifft



NAME
     i.ifft - Inverse Fast Fourier Transform (ifft) for image
     processing.
     (GRASS Image Processing Program)

SYNOPSIS
     i.ifft
     i.ifft help
     i.ifft real_image=name imaginary_image=name
     output_image=name

DESCRIPTION
     i.ifft is an image processing program based on the algorithm
     given by Press (1988), and modified by Vali (1990), that
     converts real and imaginary frequency space images (produced
     by i.fft) into a normal image.


     Parameters:

     real_image=name	      Input raster map layer for
			      inversion fast Fourier transform,
			      real part.

     imaginary_image=name     Input raster map layer for
			      inversion fast Fourier transform,
			      imaginary.

     output_image=name	      Output inversion raster map layer
			      after fast Fourier transformation.


     The current mask is respected when reading the real and
     imaginary component files;	 thus, r.mask become a primary
     program for selecting the portion of the frequency space
     data to be included in the inverse transform.  The GRASS
     program d.digit can be used to create masks while viewing
     the real or imaginary component image.  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
     Numerical Recipes in C, by William H. Press, Cambridge



GRASS 5.0beta6	      GRASS Development Team			1






i.ifft <main>	      GRASS Reference Manual	    <main> i.ifft



     University Press, 1988.

     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

AUTHOR
     David Satnik, GIS Laboratory, Central Washington University










































2		      GRASS Development Team	   GRASS 5.0beta6