NAME

       i.zc  - Zero-crossing "edge detection" raster function for image processing.

KEYWORDS

       imagery, edges

SYNOPSIS

       i.zc
       i.zc --help
       i.zc       input=string       output=string       [width=integer]        [threshold=float]
       [orientations=integer]   [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       input=string [required]
           Name of input raster map

       output=string [required]
           Zero crossing raster map

       width=integer
           x-y extent of the Gaussian filter
           Default: 9

       threshold=float
           Sensitivity of Gaussian filter
           Default: 10

       orientations=integer
           Number of azimuth directions categorized
           Default: 1

DESCRIPTION

       i.zc 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 algorithm used for edge detection is one of the "zero-crossing" algorithms and
       is discussed briefly below.

OPTIONS

   Parameters:
       input_map=name
           Name of input raster map layer.

       zc_map=name
           Name of raster map layer to be used for zero-crossing values.

       width=value
           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

       threshold=value
           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

       orientations=value
           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.slope.aspect 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.

NOTES

       The procedure to find the "edges" in the image is as follows:

       1      The Fourier transform of the image is taken,

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

       3      The result is run through an inverse Fourier transform,

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

       5      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.

SEE ALSO

       i.fft, i.ifft, r.mapcalc, r.mfilter, r.slope.aspect

AUTHOR

       David Satnik, GIS Laboratory, Central Washington University

       Last changed: $Date: 2011-11-08 22:24:20 +0100 (Tue, 08 Nov 2011) $

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