R help - May 2004 - How to draw holes generated by gpclib using plot function

Hi.

 I've tried to create a polygon with one hole by gpclib using following
example script.

 holepoly <- read.polyfile(system.file("poly-ex/hole-poly.txt",
package
="gpclib"), nohole = FALSE)
 area.poly(holepoly)
 plot(holepoly,poly.args=list(col="red",border="blue"))

 And I noticed plot function couldn't draw polygons with holes correctly.

 Does anyone know how to solve this situation?

 Regards.

Hisaji ONO wrote:
> Hi.
> 
>  I've tried to create a polygon with one hole by gpclib using following
> example script.
> 
>  holepoly <-
read.polyfile(system.file("poly-ex/hole-poly.txt", package
> ="gpclib"), nohole = FALSE)
>  area.poly(holepoly)
>  plot(holepoly,poly.args=list(col="red",border="blue"))
> 
>  And I noticed plot function couldn't draw polygons with holes
correctly.
> 
>  Does anyone know how to solve this situation?
This is basic constraint in the R graphics system.  Polygons must
consist of a single (possibly self-intersecting) ring.  It would be
possible to implement a primitive which is bounded by several
non-intersecting rings by joining the interior holes to the outer
boundary to create a simply connected shape. Then you could draw
the interior with the existing polygon primitive.  You can find a
more precise description of the process in the FIST (fast
industrial-strength triangulation) paper:

 M. Held (2001):
``FIST: Fast Industrial-Strength Triangulation of Polygons''.
Algorithmica 30(4): 563-596, 2001.

It would be VERY useful to have an implementation of this (hint, hint!)

Alternatively, I think that gpc has an option to return a triangulated
version of the polygon.  If you get hold of this you could just draw the
triangles. though this might be slow for complex polygons.


-- 
Ross Ihaka                         Email:  ihaka at stat.auckland.ac.nz
Department of Statistics           Phone:  (64-9) 373-7599 x 85054
University of Auckland             Fax:    (64-9) 373-7018
Private Bag 92019, Auckland
New Zealand

Sorry, but this limitation in plotting is something that I probably 
should have mentioned in the help page.  At the time when I wrote this 
plotting wasn't really a priority so I never thought much of it.

The GPC C library has a function has a function to return a collection 
of tristrips but this is not implemented in the R package.  It 
shouldn't be too difficult to include and I will hopefully include it 
with the next release.

-roger

Hisaji ONO wrote:
> Hi.
> 
>  I've tried to create a polygon with one hole by gpclib using following
> example script.
> 
>  holepoly <-
read.polyfile(system.file("poly-ex/hole-poly.txt", package
> ="gpclib"), nohole = FALSE)
>  area.poly(holepoly)
>  plot(holepoly,poly.args=list(col="red",border="blue"))
> 
>  And I noticed plot function couldn't draw polygons with holes
correctly.
> 
>  Does anyone know how to solve this situation?
> 
>  Regards.
> 
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