r.thin(1grass) GRASS GIS User's Manual r.thin(1grass)
NAME
r.thin - Thins non-null cells that denote linear features in a raster
map layer.
KEYWORDS
raster, geometry
SYNOPSIS
r.thin
r.thin --help
r.thin input=name output=name [iterations=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=name [required]
Name of input raster map
output=name [required]
Name for output raster map
iterations=integer
Maximal number of iterations
Default: 200
DESCRIPTION
r.thin scans the named input raster map layer and thins non-NULL cells
that denote linear features into linear features having a single cell
width. Raster lines often need to be thinned (skeletonizing raster fea-
tures) to a single pixel width before they can be transformed to vector
data.
r.thin will thin only the non-NULL (no data) raster cells of the named
input raster map layer within the current geographic region settings.
The cell width of the thinned output raster map layer will be equal to
the cell resolution of the currently set geographic region. All of the
thinned linear features will have the width of a single cell.
r.thin will create a new output raster data file containing the thinned
linear features. r.thin assumes that linear features are encoded with
positive values on a background of NULL’s in the input raster data
file, hence it creates a NULL/1 output map.
NOTES
r.thin only creates raster map layers. In order to create a vector map,
the user will need to run r.to.vect on the resultant raster map.
r.thin may create small spurs or "dangling lines" during the thinning
process. These spurs may be removed (after creating a vector map
layer) by v.clean (rmdangle tool).
This code implements the thinning algorithm described in "Analysis of
Thinning Algorithms Using Mathematical Morphology" by Ben-Kwei Jang and
Ronlad T. Chin in Transactions on Pattern Analysis and Machine Intelli-
gence, vol. 12, No. 6, June 1990. The definition Jang and Chin give of
the thinning process is "successive removal of outer layers of pixels
from an object while retaining any pixels whose removal would alter the
connectivity or shorten the legs of the sceleton."
The sceleton is finally thinned when the thinning process converges;
i.e., "no further pixels can be removed without altering the connectiv-
ity or shortening the sceleton legs" (p. 541). The authors prove that
the thinning process described always converges and produces one-pixel
thick sceletons. The number of iterations depends on the original
thickness of the object. Each iteration peels off the outside pixels
from the object. Therefore, if the object is <= n pixels thick, the
algorithm should converge in <= iterations.
EXAMPLE
To vectorize the raster map streams_derived in the North Carolina sam-
ple dataset that represents the stream network derived from the 10m
resolution DEM by r.watershed, run:
g.region raster=elevation -p
# create flow accumulation map
r.watershed elevation=elevation accumulation=accum_50K thresh=50000
# extract streams from flow accumulation map
r.mapcalc "streams_from_flow = if(abs(accum_50K) > 1000, 1, null())"
# skeletonize map
r.thin streams_from_flow out=streams_thin
d.mon wx0
d.rast streams_from_flow
d.erase
d.rast streams_thin
Raster feature thinning (skeletonizing)
The resulting map cabe optionally vectorized:
r.to.vect streams_thin output=streams_thin type=line
# visualize
d.rast accum_50K
d.vect streams_thin color=red width=2
Vectorized stream network after thinning extracted from flow accumula-
tion map
SEE ALSO
g.region, r.to.vect, v.clean, wxGUI vector digitizer, v.build
AUTHOR
Olga Waupotitsch, U.S.Army Construction Engineering Research Laboratory
The code for finding the bounding box as well as input/output code was
written by Mike Baba (DBA Systems, 1990) and Jean Ezell (USACERL,
1988).
SOURCE CODE
Available at: r.thin source code (history)
Accessed: unknown
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