Divide a polygon by a line?

Hi.

 I've create very simple example "cutting polygon by line
c(0,5,10,5)" using by gpclib.

library(maptools)
library(gpclib)

 x1<-t(structure(c(0,0,10,0,10,10,0,10,0,0),dim=c(2,5))) #
simple square


### method  using buffer

 min_buffer_dist <- 1e-10 # set minimum value

x2<-t(structure(c(-1,5,11,5+min_buffer_dist,11,5-min_buffer_dist,-1,5),dim=c(2,4)))
# buffered similar line as polygon
# x2<-t(structure(c(-1,5,11,5,11,5,-1,5),dim=c(2,4))) # 
this line can't cut square, just vanished.
 plot(setdiff(as(x1,"gpc.poly"),as(x2,"gpc.poly"))) # plot
result




 Regards.


--- Roger Bivand <Roger.Bivand at nhh.no> wrote:

On Tue, 30 Oct 2007, Agustin Lobo wrote:

Is it possible to divide a polygon by a line
into 2 polygons?

No, in general. All computational geometry ends up
in having to deal with 
the type of polygon. A convex polygon is one thing,
and can be divided 
into two parts, but a general polygon may be
concave, and the number of 
parts does not have to be two - the line could cross
it multiple times. 
The approach taken in the Rgshhs function in
maptools is to overlay a 
rectangular box over the polygon(s), and see what
comes out, but quite 
often it isn't two (or in that case a single part
within the rectangle).

Roger

Agus

-- 
Roger Bivand
Economic Geography Section, Department of Economics,
Norwegian School of
Economics and Business Administration, Helleveien
30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no

------------------------------------------------------------------------