1i.zc(1)                     GRASS GIS User's Manual                    i.zc(1)
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NAME

6       i.zc   -  Zero-crossing "edge detection" raster function for image pro‐
7       cessing.
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KEYWORDS

10       imagery, edges
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SYNOPSIS

13       i.zc
14       i.zc --help
15       i.zc input=string  output=string   [width=integer]    [threshold=float]
16       [orientations=integer]       [--overwrite]     [--help]     [--verbose]
17       [--quiet]  [--ui]
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19   Flags:
20       --overwrite
21           Allow output files to overwrite existing files
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23       --help
24           Print usage summary
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26       --verbose
27           Verbose module output
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29       --quiet
30           Quiet module output
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32       --ui
33           Force launching GUI dialog
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35   Parameters:
36       input=string [required]
37           Name of input raster map
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39       output=string [required]
40           Zero crossing raster map
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42       width=integer
43           x-y extent of the Gaussian filter
44           Default: 9
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46       threshold=float
47           Sensitivity of Gaussian filter
48           Default: 1.0
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50       orientations=integer
51           Number of azimuth directions categorized
52           Default: 1
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DESCRIPTION

55       i.zc is an image processing module used for edge detection.  The raster
56       map  produced  shows  the  location  of  "boundaries" on the input map.
57       Boundaries tend to be found in regions of changing cell values and tend
58       to run perpendicular to the direction of the slope.  The algorithm used
59       for edge detection is one of the "zero-crossing" algorithms and is dis‐
60       cussed briefly below.
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NOTES

63       The procedure to find the "edges" in the image is as follows:
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65       1      The Fourier transform of the image is taken,
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67       2      The  Fourier  transform  of  the  Laplacian of a two-dimensional
68              Gaussian function is used to filter the transformed image,
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70       3      The result is run through an inverse Fourier transform,
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72       4      The resulting image is traversed in search of places  where  the
73              image changes from positive to negative or from negative to pos‐
74              itive,
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76       5      Each cell in the map where the value crosses zero (with a change
77              in  value greater than the threshold value) is marked as an edge
78              and an orientation is assigned to it.  The resulting raster  map
79              layer is output.
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81       The  width= parameter determines the x-y extent of the Gaussian filter.
82       The default value is 9; higher and lower values can be  tested  by  the
83       user.  Increasing the width will result in finding "edges" representing
84       more gradual changes in cell values.
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86       The threshold= parameter determines the "sensitivity" of  the  Gaussian
87       filter.  The default value is 1;  higher and lower values can be tested
88       by the user.  Increasing the threshold value will result in fewer edges
89       being found.
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91       The  orientations=  value is the number of azimuth directions the cells
92       on the output raster map layer are categorized  into  (similar  to  the
93       aspect  raster  map  layer  produced by r.slope.aspect.  For example, a
94       value of 16 would result in detected edges being categorized  into  one
95       of 16 bins depending on the direction of the edge at that point.
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97       The  current  region  definition  and  mask settings are respected when
98       reading the input map.
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SEE ALSO

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

104       David Satnik, GIS Laboratory, Central Washington University
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SOURCE CODE

107       Available at: i.zc source code (history)
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109       Main index | Imagery index | Topics index | Keywords index |  Graphical
110       index | Full index
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112       © 2003-2020 GRASS Development Team, GRASS GIS 7.8.5 Reference Manual
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116GRASS 7.8.5                                                            i.zc(1)
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