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

SOURCE CODE

       Available at: i.zc source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual