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     oouutt--
       ppuutt__iimmaaggee==_n_a_m_e

DDEESSCCRRIIPPTTIIOONN
       _i_._i_f_f_t  is  an image processing program based on the algo-
       rithm 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 transformation.


NNOOTTEESS
       The  current  mask  is respected when reading the real and
       imaginary component 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 (automatically) uses the same
       GRASS region definition setting that was used  during  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  for-
       ward  transform	program,  _i_._f_f_t),  and	the reconstructed
       image will preserve the cell value scaling 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 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.


       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  Univer-
       sity

       _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)
