1i.zc(1) Grass User's Manual i.zc(1)
2
6 i.zc - Zero-crossing "edge detection" raster function for image pro‐
7 cessing.
8
10 imagery
11
13 i.zc
14 i.zc help
15 i.zc input=string output=string [width=integer] [threshold=float]
16 [orientations=integer] [--overwrite] [--verbose] [--quiet]
17 Flags:
19 --overwrite
20 Allow output files to overwrite existing files
21 --verbose
23 Verbose module output
24 --quiet
26 Quiet module output
27 Parameters:
29 input=string
30 Name of input raster map
31 output=string
33 Zero crossing raster map
34 width=integer
36 x-y extent of the Gaussian filter
37 Default: 9
38 threshold=float
40 Sensitivity of Gaussian filter
41 Default: 10
42 orientations=integer
44 Number of azimuth directions categorized
45 Default: 1
46
48 i.zc is an image processing program used for edge detection. The
49 raster map produced shows the location of "boundaries" on the input
50 map. Boundaries tend to be found in regions of changing cell values
51 and tend to run perpendicular to the direction of the slope. The algo‐
52 rithm used for edge detection is one of the "zero-crossing" algorithms
53 and is discussed briefly below.
54 This program will be run interactively if the user types i.zc without
56 program arguments on the command line. In this event, the program will
57 prompt the user for parameter values using the standard interface
58 described in the manual entry for parser. Alternately, the user can
59 run the program non-interactively by specifying program parameter val‐
60 ues on the command line.
61
63 Parameters:
64 input_map=name
65 Name of input raster map layer.
66 zc_map=name
68 Name of raster map layer to be used for zero-crossing values.
69 width=value
71 This parameter determines the x-y extent of the Gaussian filter.
72 The default value is 9; higher and lower values can be tested
73 by the user. Increasing the width will result in finding
74 "edges" representing more gradual changes in cell values.
75 Default: 9
76 threshold=value
78 This parameter determines the "sensitivity" of the Gaussian fil‐
79 ter. The default value is 10; higher and lower values can be
80 tested by the user. Increasing the threshold value will result
81 in fewer edges being found.
82 Default: 10
83 orientations=value
85 This value is the number of azimuth directions the cells on the
86 output raster map layer are categorized into (similar to the
87 aspect raster map layer produced by the r.slope.aspect program).
88 For example, a value of 16 would result in detected edges being
89 categorized into one of 16 bins depending on the direction of
90 the edge at that point.
91 Default: 1
92 The current region definition and mask settings are respected when
94 reading the input map.
95
97 The procedure to find the "edges" in the image is as follows: The
98 Fourier transform of the image is taken, The Fourier transform of the
99 Laplacian of a two-dimensional Gaussian function is used to filter the
100 transformed image, The result is run through an inverse Fourier trans‐
101 form, The resulting image is traversed in search of places where the
102 image changes from positive to negative or from negative to positive,
103 Each cell in the map where the value crosses zero (with a change in
104 value greater than the threshold value) is marked as an edge and an
105 orientation is assigned to it. The resulting raster map layer is out‐
106 put.
107
109 i.fft, i.ifft, r.mapcalc, r.mfilter, r.slope.aspect
110
112 David Satnik, GIS Laboratory, Central Washington University
113 Last changed: $Date: 2003-04-17 16:51:33 +0200 (Thu, 17 Apr 2003) $
115 Full index
117 © 2003-2008 GRASS Development Team
119GRASS 6.3.0 i.zc(1)