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]
17 Flags:
19 --overwrite
20 Parameters:
22 input=string
23 input raster map
24 output=string
26 zero crossing raster map
27 width=integer
29 x-y extent of the Gaussian filter Default: 9
30 threshold=float
32 sensitivity of Gaussian filter Default: 10
33 orientations=integer
35 number of azimuth directions categorized Default: 1
36
38 i.zc is an image processing program used for edge detection. The
39 raster map produced shows the location of "boundaries" on the input
40 map. Boundaries tend to be found in regions of changing cell values
41 and tend to run perpendicular to the direction of the slope. The algo‐
42 rithm used for edge detection is one of the "zero-crossing" algorithms
43 and is discussed briefly below.
44 This program will be run interactively if the user types i.zc without
46 program arguments on the command line. In this event, the program will
47 prompt the user for parameter values using the standard interface
48 described in the manual entry for parser. Alternately, the user can
49 run the program non-interactively by specifying program parameter val‐
50 ues on the command line.
51
53 Parameters:
54 input_map=name
55 Name of input raster map layer.
56 zc_map=name
58 Name of raster map layer to be used for zero-crossing values.
59 width=value
61 This parameter determines the x-y extent of the Gaussian filter.
62 The default value is 9; higher and lower values can be tested
63 by the user. Increasing the width will result in finding
64 "edges" representing more gradual changes in cell values.
65 Default: 9
66 threshold=value
68 This parameter determines the "sensitivity" of the Gaussian fil‐
69 ter. The default value is 10; higher and lower values can be
70 tested by the user. Increasing the threshold value will result
71 in fewer edges being found.
72 Default: 10
73 orientations=value
75 This value is the number of azimuth directions the cells on the
76 output raster map layer are categorized into (similar to the
77 aspect raster map layer produced by the r.slope.aspect program).
78 For example, a value of 16 would result in detected edges being
79 categorized into one of 16 bins depending on the direction of
80 the edge at that point.
81 Default: 1
82 The current region definition and mask settings are respected when
84 reading the input map.
85
87 The procedure to find the "edges" in the image is as follows: The
88 Fourier transform of the image is taken, The Fourier transform of the
89 Laplacian of a two-dimensional Gaussian function is used to filter the
90 transformed image, The result is run through an inverse Fourier trans‐
91 form, The resulting image is traversed in search of places where the
92 image changes from positive to negative or from negative to positive,
93 Each cell in the map where the value crosses zero (with a change in
94 value greater than the threshold value) is marked as an edge and an
95 orientation is assigned to it. The resulting raster map layer is out‐
96 put.
97
99 i.fft, i.ifft, r.mapcalc, r.mfilter, r.slope.aspect
100
102 David Satnik, GIS Laboratory, Central Washington University
103 Last changed: $Date: 2003/04/17 14:51:24 $
105 Full index
107GRASS 6.2.2 i.zc(1)