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



NNAAMMEE
       ii..fffftt  - Fast Fourier Transform (FFT) 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..fffftt
       ii..fffftt hheellpp
       ii..fffftt	 iinnppuutt__iimmaaggee==_n_a_m_e     rreeaall__iimmaaggee==_n_a_m_e	  iimmaaggiinnaarryy__iimmaaggee==_n_a_m_e
       [rraannggee==_v_a_l_u_e]

DDEESSCCRRIIPPTTIIOONN
       _i_._f_f_t is an image processing program based on the FFT  algorithm  given
       by  Frigo et al. (1998), that processes a single input raster map layer
       (iinnppuutt__iimmaaggee) and constructs the real and imaginary Fourier  components
       in frequency space.

OOPPTTIIOONNSS
   PPaarraammeetteerrss::
       iinnppuutt__iimmaaggee==_n_a_m_e
	      Input  raster  map  layer on which the fast Fourier transform is
	      run.

       rreeaall__iimmaaggee==_n_a_m_e
	      Output real part arrays stored as raster map layer.

       iimmaaggiinnaarryy__iimmaaggee==_n_a_m_e
	      Output imaginary part arrays stored as raster map layer.

       rraannggee==_v_a_l_u_e
	      Range of values used during fast Fourier transformation.


NNOOTTEESS
       The real and imaginary components are stored as arrays  of  doubles  in
       the  _c_e_l_l___m_i_s_c  directory  (for	use  in the inverse transform program,
       _i_._i_f_f_t), and are also scaled and  formatted  into  the  rreeaall__iimmaaggee  and
       iimmaaggiinnaarryy__iimmaaggee	raster	map  layers  for inspection, masking, etc.  In
       these raster map layers the low frequency components are in the	center
       and   the   high  frequency  components	are  toward  the  edges.   The
       iinnppuutt__iimmaaggee need not be square;	before processing, the X and Y	dimen-
       sions  of  the  iinnppuutt__iimmaaggee  are padded with zeroes to the next highest
       power of two in extent (i.e., 256 x 256 is processed at that size,  but
       200  x 400 is padded to 256 x 512).  The cell category values for view-
       ing, etc., are calculated by taking the natural log of the actual  val-
       ues  then  rescaling to 255, or whatever optional range is given on the
       command line, as suggested  by  Richards  (1986).   A  color  table  is
       assigned to the resultant map layer.


       The  current  geographic  region  and  mask settings are respected when
       reading the input file.	The presence of a mask will, in general,  make
       the  resulting fast Fourier transform invalid, or at least difficult to
       interpret.

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  progam author and Ali R. Vali, Space
       Research Center, University of Texas, Austin, 1990.


       _i_._c_c_a

       _i_._c_l_a_s_s

       _i_._i_f_f_t

       _i_._p_c_a

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

       _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.fft(1)
