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]  [--verbose]  [--quiet]

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

       --verbose
           Verbose module output

       --quiet
           Quiet module output

   Parameters:
       input=string
           Name of input raster map

       output=string
           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 03:29:50 -0800 (Tue, 08 Nov 2011) $

       Full index

       © 2003-2013 GRASS Development Team