i.zc(1grass) GRASS GIS User's Manual i.zc(1grass)
NAME
i.zc - Zero-crossing "edge detection" raster function for image pro-
cessing.
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: 1.0
orientations=integer
Number of azimuth directions categorized
Default: 1
DESCRIPTION
i.zc is an image processing module 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 dis-
cussed briefly below.
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 pos-
itive,
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.
The width= 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.
The threshold= parameter determines the "sensitivity" of the Gaussian
filter. The default value is 1; higher and lower values can be tested
by the user. Increasing the threshold value will result in fewer edges
being found.
The orientations= value is the number of azimuth directions the cells
on the output raster map layer are categorized into (similar to the as-
pect raster map layer produced by r.slope.aspect. 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.
The current region definition and mask settings are respected when
reading the input map.
SEE ALSO
i.fft, i.ifft, r.mapcalc, r.mfilter, r.slope.aspect
AUTHOR
David Satnik, GIS Laboratory, Central Washington University
SOURCE CODE
Available at: i.zc source code (history)
Accessed: unknown
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GRASS 7.8.7 i.zc(1grass)
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