[R-sig-Geo] SPATSTAT: converting owin(mask) to owin (polygon)

[Adrian.Baddeley at csiro.au]

Alexandre VILLERS wrote:

> I would like to create a new window for a point pattern in order to get 
> a tess object (dirichlet tessellation) from that same point pattern.
> For the moment, here is an example of what I do:

> PPP<-  rpoispp(100) #generate a point pattern
> wind1<-density(PPP, sigma=0.1) #smoothed object of class im
> wind2<-wind1 #copy the object
> wind2$v<-ifelse(wind2$v<150,  NA, wind1$v) #replace with NAs the value 
> # of pixel I don't want to keep (those whose intensity is < 150)
>  M<-as.owin(wind2) #convert the im object to a window
> PPPd<-ppp(PPP$x,PPP$y, window=M) #the new point pattern with the new window
> plot(dirichlet(PPPd)) #the result which did not convince me !

This is a bug in plot.tess. The dirichlet command has correctly computed a tessellation, but the plot method for tessellations has included some strange black lines. To suppress the lines, use
    plot(dirichlet(PPPd), axes=FALSE)
However it will take some more work to get a nice appearance for this plot. I will repair plot.tess in the next version (1.16-0) which will be out soon.

Incidentally, the following lines ...

> wind2 <- wind1
> wind2$v<-ifelse(wind2$v<150,  NA, wind1$v) 
> M<-as.owin(wind2) #convert the im object to a window

... could be replaced by

  M <- levelset(wind1, 150, "<")

Adrian Baddeley