## NAME

r.thin - Thins non-zero cells that denote linear features in a raster map layer.

## SYNOPSIS

r.thin
r.thin help
r.thin input=string output=string

### Parameters:

input=string
  Name of existing raster map
output=string
  Name of output raster file

## DESCRIPTION

r.thin scans the named input raster map layer and thins non-zero cells that denote linear features into linear features having a single cell width.

r.thin will thin only the non-zero 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 0's in the input raster data file.

## NOTE

r.thin only creates raster map layers. You will need to run r.to.vect on the resultant raster file to create a vector (v.digit) map layer.

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.

r.thin creates a 0/1 output map.

## NOTE

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 Intelligence, 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 connectivity 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.

## SEE ALSO

g.region, r.to.vect, v.clean, v.digit, 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).

Last changed: $Date: 2004/08/10 08:35:01 $

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