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



NNAAMMEE
       ii..zzcc   -  Zero-crossing "edge detection" raster function for image pro-
       cessing.
       _(_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] [tthhrreesshhoolldd==_v_a_l_u_e] [oorriieenn--
       ttaattiioonnss==_v_a_l_u_e]

DDEESSCCRRIIPPTTIIOONN
       _i_._z_c  is  an  image  processing	program  used for edge detection.  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-crossing" 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  manual entry for _p_a_r_s_e_r.  Alternately, the user can
       run the program non-interactively by specifying program parameter  val-
       ues 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-crossing 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 fil-
	      ter.   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 categorized  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 example, 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 follows:

	       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 pos-
	      itive,

	       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 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.zc(1)
