xenial (1) i.zc.1grass.gz

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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