i.ifft(1)		      Grass User's Manual		     i.ifft(1)



NNAAMMEE
       ii..iifffftt  - Inverse Fast Fourier Transform (ifft) for image processing.
       _(_G_R_A_S_S _I_m_a_g_e _P_r_o_c_e_s_s_i_n_g _P_r_o_g_r_a_m_)

SSYYNNOOPPSSIISS
       ii..iifffftt
       ii..iifffftt hheellpp
       ii..iifffftt rreeaall__iimmaaggee==_n_a_m_e iimmaaggiinnaarryy__iimmaaggee==_n_a_m_e oouuttppuutt__iimmaaggee==_n_a_m_e

DDEESSCCRRIIPPTTIIOONN
       _i_._i_f_f_t  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_._f_f_t) into a normal image.

OOPPTTIIOONNSS
   PPaarraammeetteerrss::
       rreeaall__iimmaaggee==_n_a_m_e
	      Input  raster  map  layer  for inversion fast Fourier transform,
	      real part.

       iimmaaggiinnaarryy__iimmaaggee==_n_a_m_e
	      Input raster map layer for  inversion  fast  Fourier  transform,
	      imaginary.

       oouuttppuutt__iimmaaggee==_n_a_m_e
	      Output inversion raster map layer after fast Fourier transforma-
	      tion.


NNOOTTEESS
       The current mask is respected when reading the real and imaginary  com-
       ponent  files;  thus, _r_._m_a_s_k become a primary program for selecting the
       portion of the frequency space data  to	be  included  in  the  inverse
       transform.  The GRASS program _r_._d_i_g_i_t can be used to create masks while
       viewing the real or imaginary component image.  Alternatively  _r_._c_i_r_c_l_e
       can be used to generate high-, low- and donut filters specifying the DC
       point as circle/ring center.  When _i_._i_f_f_t is  executed,	it  (automati-
       cally) uses the same GRASS region definition setting that was used dur-
       ing the original transformation done with _i_._f_f_t.


       The real and imaginary components are read from arrays  of  doubles  in
       the  _c_e_l_l___m_i_s_c  directory  (produced  by the forward transform program,
       _i_._f_f_t), and the reconstructed image will preserve the cell value  scal-
       ing  of	the  original  image  processed  by  _i_._f_f_t.  No color table is
       assigned to the output map;  one should be created before  viewing  the
       _o_u_t_p_u_t___i_m_a_g_e.

SSEEEE AALLSSOO
       M.  Frigo  and S. G. Johnson (1998): "FFTW: An Adaptive Software Archi-
       tecture 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.


       RReemmoottee  SSeennssiinngg	DDiiggiittaall IImmaaggee AAnnaallyyssiiss, 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_._c_c_a, _i_._c_l_a_s_s, _i_._f_f_t, _i_._p_c_a, _r_._c_i_r_c_l_e

AAUUTTHHOORR
       David Satnik, GIS Laboratory, Central Washington University

       _L_a_s_t _c_h_a_n_g_e_d_: _$_D_a_t_e_: _2_0_0_2_/_0_1_/_2_5 _0_5_:_4_5_:_3_3 _$



GRASS 5.0							     i.ifft(1)
