1i.zc(1) Grass User's Manual i.zc(1)
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6 i.zc - Zero-crossing "edge detection" raster function for image pro‐
7 cessing.
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10 imagery, edges
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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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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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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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101 i.fft, i.ifft, r.mapcalc, r.mfilter, r.slope.aspect
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104 David Satnik, GIS Laboratory, Central Washington University
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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 © 2003-2019 GRASS Development Team, GRASS GIS 7.8.2 Reference Manual
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116GRASS 7.8.2 i.zc(1)