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Filling polygons with points

5 messages · Wolf, Michael, Duncan Murdoch, Andy Bunn +2 more

Original
Wolf, Michael
# 
Dear list,

are there any possibilities to fill a polygon with a point pattern or with a symbol pattern like '+' oder '-' instead of shading lines?

Thanks in advance

Dr. Michael Wolf
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E-Mail: michael.wolf at bezreg-muenster.nrw.de
Duncan Murdoch
# 
On Thu, 14 Oct 2004 15:16:30 +0200, "Wolf, Michael"
<Michael.Wolf at bezreg-muenster.nrw.de> wrote :
I don't think the graphics devices know how to do that, but you could
program it yourself:

Supposed interior(x,y) is a function that determines whether the
points (x,y) are in the polygon, then just create a grid of locations,
subset when interior(x,y) is true, and plot using pch='+'.

I made a quick search and was unable to find a general implementation
of the "interior" function for an arbitrary polygon; I'm a bit
surprised about that.  Hopefully someone else can point to one,
otherwise please write one, and document it and contribute it to R.
It's a relatively standard algorithm, and would be useful.

Duncan Murdoch
Andy Bunn
# 
A good place to start might be with the pip function in splancs. That should
do it with a little fussing, I think.

require(splancs)
data(bodmin)
pts <- gen(bodmin$poly,5000)
pts.inside <- pip(pts, bodmin$poly)
plot(bodmin$poly, asp=1, type="n")
pointmap(pts.inside, add=TRUE)
polymap(bodmin$poly, add=TRUE)

-Andy
PIKAL Petr
#
On 14 Oct 2004 at 10:00, Duncan Murdoch wrote:

            

      
      
      
    

        
Hi

I think that you can find such functionity in splancs package 
probably function pip() find if points are inside or outside of the 
polygon.

Cheers
Petr

            

      
      
      
    

        

      
      
    

        
Petr Pikal
petr.pikal at precheza.cz

  
  


  


      

    

    

      
      

        


  

        

      Roger Bivand
    

    

      
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On Thu, 14 Oct 2004, Petr Pikal wrote:

        

            

      
      
      
    

        
Going a bit further, you could use areapl() and gridpts() in splancs to 
return plotting points for the symbols that would give an even symbol per 
unit area coverage. This is a very similar question to:

https://stat.ethz.ch/pipermail/r-help/2004-September/056351.html

which was about dot density maps. A function to do that will be in the 
next maptools, but I think this is a bit different and is more like a 
hatching but with arbitrary symbols and chosen spacing - it can be done, 
but do you need this kind of symbology?

Roger