
I think that the oft-quoted exhortation ``RTFM'' applies here.  The
help for dirichlet() clearly says:

> This function computes the Dirichlet tessellation (within the original 
> window of |X|). 

Hence your second tessellation (within the smaller polygonal window) is the
intersection of the first tessellation (within the larger rectangular 
window)
with the aforesaid polygonal window.

The reason that you get a coloured pixellated plot for the second 
tessellation
is that taking the intersection of polygons requires (currently) the use of
the "gpclib" package, which has an unfortunately restrictive license and 
which
is therefore by default disabled.  The user must explicitly ask for it 
to be available.

See license.polygons().

When "gpclib" is disabled, intersections of polygons are calculated 
(approximately)
via pixellation.

To change this behaviour, execute

     spatstat.options(gpclib=TRUE)

     cheers,

         Rolf Turner

On 24/05/11 04:10, Mathieu Rajerison wrote:
> Hi,
>
>
> I have a set of points.
>
> I generated two different sets of ppp objets, each with a different window
> (but same points)
>
> The first window corresponds to the extent of the data, the second to the
> administrative unit contour (polygonal window).
>
>
>     1. bastia.ppp is the first set of points.
>     2. bastia.ppp.polwin is the second.
>
>
> I have applied dirichlet function to each of them and plotted it.
>
>
>     1. plot(dirichlet(bastia.ppp)) =>  gives me lines
>     2. plot(dirichlet(bastia.ppp.polwin)) =>  gives me coloured map.
>
>
> Why do I get different plotting results?
>
> Also, I don't notice differences in voronoi polygons contours between the
> two sets. So does dirichlet take into account polygon windows or does it
> always generate voronoi given a rectangular window?
>
>
> Thanks in advance for the answer!
>
> 	[[alternative HTML version deleted]]
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo@r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo


	[[alternative HTML version deleted]]

