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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: 1.0

       orientations=integer
           Number of azimuth directions categorized
           Default: 1

DESCRIPTION

       i.zc is an image processing module 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.

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.

       The width= 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.

       The threshold= parameter determines the "sensitivity" of the Gaussian filter.  The default
       value  is 1;  higher and lower values can be tested by the user.  Increasing the threshold
       value will result in fewer edges being found.

       The orientations= 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
       r.slope.aspect.  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.

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

SEE ALSO

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

AUTHOR

       David Satnik, GIS Laboratory, Central Washington University

       Last changed: $Date: 2017-02-27 13:07:47 +0100 (Mon, 27 Feb 2017) $

SOURCE CODE

       Available at: i.zc source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.6.1 Reference Manual