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



NNAAMMEE
       ii..fffftt   -  Fast Fourier Transform (FFT) for image process-
       ing.
       _(_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	   iimmaaggii--
       nnaarryy__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	transfor-
	      mation.


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 for-
       matted 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 dimensions 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  viewing,  etc.,
       are  calculated	by  taking  the natural log of the actual
       values 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 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 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 Univer-
       sity
       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)
