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



NNAAMMEE
       ii..zzcc  - Zero-crossing "edge detection" raster function 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..zzcc
       ii..zzcc hheellpp
       ii..zzcc  iinnppuutt__mmaapp==_n_a_m_e  zzcc__mmaapp==_n_a_m_e  [wwiiddtthh==_v_a_l_u_e]  [tthhrreesshh--
       oolldd==_v_a_l_u_e] [oorriieennttaattiioonnss==_v_a_l_u_e]

DDEESSCCRRIIPPTTIIOONN
       _i_._z_c  is  an image processing program used for edge detec-
       tion.  The raster  map  produced  shows	the  location  of
       "boundaries"  on  the  input  map.   Boundaries tend to be
       found in regions of changing cell values and tend  to  run
       perpendicular  to  the  direction of the slope.	The algo-
       rithm used for edge detection is one of	the  "zero-cross-
       ing" algorithms and is discussed briefly below.


       This  program  will be run interactively if the user types
       ii..zzcc without program arguments on the  command  line.   In
       this event, the program will prompt the user for parameter
       values using the standard interface described in the  man-
       ual  entry  for _p_a_r_s_e_r.	Alternately, the user can run the
       program non-interactively by specifying program	parameter
       values on the command line.

OOPPTTIIOONNSS
   PPaarraammeetteerrss::
       iinnppuutt__mmaapp==_n_a_m_e
	      Name of input raster map layer.

       zzcc__mmaapp==_n_a_m_e
	      Name of raster map layer to be used for zero-cross-
	      ing values.

       wwiiddtthh==_v_a_l_u_e
	      This parameter determines the  x-y  extent  of  the
	      Gaussian	filter.   The default value is 9;  higher
	      and  lower  values  can  be  tested  by  the  user.
	      Increasing the width will result in finding "edges"
	      representing more gradual changes in cell values.
	      Default:	9

       tthhrreesshhoolldd==_v_a_l_u_e
	      This parameter determines the "sensitivity" of  the
	      Gaussian	filter.  The default value is 10;  higher
	      and  lower  values  can  be  tested  by  the  user.
	      Increasing the threshold value will result in fewer
	      edges being found.
	      Default:	10

       oorriieennttaattiioonnss==_v_a_l_u_e
	      This value is the number of azimuth directions  the
	      cells  on  the  output raster map layer are catego-
	      rized into (similar to the aspect raster map  layer
	      produced by the _r_._s_l_o_p_e_._a_s_p_e_c_t program).	For exam-
	      ple, a value of 16 would result in  detected  edges
	      being  categorized into one of 16 bins depending on
	      the direction of the edge at that point.
	      Default:	1

       The  current  region  definition  and  mask  settings  are
       respected when reading the input map.

NNOOTTEESS
       The  procedure to find the "edges" in the image is as fol-
       lows:

	       The Fourier transform of the image is taken,

	       The Fourier transform of the Laplacian of  a  two-
	      dimensional Gaussian function is used to filter the
	      transformed image,

	       The result  is  run  through  an  inverse  Fourier
	      transform,

	       The  resulting  image  is  traversed  in search of
	      places where the image  changes  from  positive  to
	      negative or from negative to positive,

	       Each  cell in the map where the value crosses zero
	      (with a change in value greater than the	threshold
	      value)  is  marked as an edge and an orientation is
	      assigned to it.  The resulting raster map layer  is
	      output.


SSEEEE AALLSSOO
       _i_._f_f_t

       _i_._i_f_f_t

       _r_._m_a_p_c_a_l_c

       _r_._m_f_i_l_t_e_r

       _r_._s_l_o_p_e_._a_s_p_e_c_t

       _p_a_r_s_e_r

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.zc(1)
