[R] Clip contour or image

[Pierre Kleiber]

   I was going to suggest masking the ouside of a selected contour
line using the function antipolygon(), which I have written.  However,
I see that contour() in R does not have the ability to return contour
lines as x, y vectors as it does in Splus.  This is a feature I found
useful before I switched to R.  Question for the R gurus... Would it
be difficult to add this feature to R's version of coutour()?

   By the way... this is my antipolygon function:

     antipolygon <- function(x,y,col=0){
       u <- par("usr")
       x <- c(x,x[1],u[1],u[1],u[2],u[2],u[1])
       y <- c(y,y[1],u[3],u[4],u[4],u[3],u[3])
       polygon(x,y,border=col,col=col)
       box()
     }  

But I should add that it has not been thoroughly tested.  

   Cheers, Pierre


Thomas Petzoldt wrote:
> 
> Han Lai schrieb:
> >
> > Hi,
> >
> > I am almost new to R.  Would any one show me (please include an example)
> > how to clip a contour or image figure so that I only see the image or
> > lines inside a selected area?
> 
> Hi,
> 
> I had a similar problem some weeks before. My problem was to clip an
> interpolated (kriged) surface image of a lake to its _irregular_shaped_
> shoreline. My solution was some kind of "image manipulation". I
> "stamped" the shoreline into the image matrix and filled the region
> outside with the help of the seedfill algorithm, which I implemented as
> an external shared library (runs as Win32-DLL and .so on Linux).
> 
> You will need the compiled C library and some R functions. If you or
> another one who would be interested on my R and C functions, I can give
> it to you.
> 
> Maybe it would be worth to create a package with "basic image
> manipulation procedures" for matrices. If there is anybody, who has a
> better (or easier) solution, I would be interested too.
> 
> Thomas Petzoldt
> 
> --
> Thomas Petzoldt                    (Limnology and Ecological Modelling)
> Institute for Hydrobiology         http://www.tu-dresden.de/fghhihb/
> Dresden University of Technology
> mailto:petzoldt at rcs.urz.tu-dresden.de
> 01062 Dresden
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-- 
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Pierre Kleiber             Email: pkleiber at honlab.nmfs.hawaii.edu
Fishery Biologist                     Tel: 808 983-5399/737-7544
NOAA FISHERIES - Honolulu Laboratory         Fax: 808 983-2902
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