.TH r.thin
.SH NAME
\fIr.thin\fR \- Thins non-zero cells that denote linear features
in a raster map layer.
.br
.I "(GRASS Raster Program)"
.SH SYNOPSIS
.B r.thin
.br
\fBr.thin help\fR
.br
\fBr.thin input\*=\fIname \fBoutput\*=\fIname\fR
.SH DESCRIPTION
.I r.thin
scans the named \fIinput\fR raster map layer and thins non-zero cells that 
denote linear features into linear features having a single cell width.

.I r.thin
will thin only the non-zero cells of the named \fIinput\fR raster map layer
within the current geographic region settings.
The cell width of the thinned \fIoutput\fR 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.  

.I r.thin
will create a new \fIoutput\fR raster data file containing
the thinned linear features.  
.I r.thin 
assumes that linear features are encoded with positive values on a
background of 0's in the \fIinput\fR raster data file.
.LP
.LP
\fBParameters:\fR
.IP \fBinput\*=\fIname\fR 18
Name of a raster map layer containing data to be thinned.
.IP \fBoutput\*=\fIname\fR 18
Name of the new raster map layer to hold thinned program output.
.LP
.LP
The user can run this program either non-interactively or interactively.
The program will be run non-interactively if the user specifies
program arguments on the command line, using the form:
.LP
.RS
\fBr.thin input\*=\fIname \fBoutput\*=\fIname\fR
.RE
.LP
Alternately, the user can simply type:
.LP
.RS
\fBr.thin\fR
.RE
.LP
on the command line, without program arguments.  In this case,
the user will be prompted for needed parameter values using the standard
GRASS parser interface described in the manual entry for \fIparser\fR.
.SH NOTE
.I r.thin 
only creates raster map layers.  You will need to run 
.I r.line
on the resultant raster file to create a vector (\fIv.digit\fR) map layer.
.LP
.I 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 \fIv.trim\fR.
.LP
\fIr.thin\fR creates a 0/1 output map.
.SH 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 \fITransactions on Pattern Analysis and Machine
Intelligence\fR, 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."
.LP
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.
.SH "SEE ALSO"
.I g.region,
.I r.line,
.I v.digit,
.I v.support,
.I v.trim,
and
.I parser
.SH "AUTHOR"
.nf
Olga Waupotitsch, U. S. Army Construction Engineering Research  Laboratory
.br 

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).