plot polygons - transparent holes/island

I'd like to plot a SPDF with two objects, one of them buffering the other
"island". How can I set the "island" transparent?
Similarly, keeping the outer polygon only, how can the hole still
transparent?

Thanks in advance,

Paulo E. Cardoso

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Roger,

I really thank you for you comments. You are always available to explain
everything, and that's a fortune for forum users.
But I still with the same problem. I did,'t understand how to leave the
inner hole or the inner polygon transparent, in order to see any other layer
plotter at first.
Should I send a simple example?

2010/1/30 Roger Bivand <Roger.Bivand at nhh.no>

On Sat, 30 Jan 2010, Paulo Eduardo Cardoso wrote:

 I'd like to plot a SPDF with two objects, one of them buffering the other

"island". How can I set the "island" transparent?
Similarly, keeping the outer polygon only, how can the hole still
transparent?

Polygons objects are plotted largest to smallest, as are Polygon objects in
each Polygons object. Each Polygons object and all its constituent Polygon
objects are painted the colour chosen for that Polygons object.

If you want the holes Polygon objects to overpaint their containing Polygon
objects, in the base graphics plot() method, you set the pbg= argument to
the background colour (not "transparent", this will let you see the painted
Polygon below). In the spplot method, this is handled internally by reading
the trellis.par.get()$background value, and using it if not "transparent",
or "white" if it is "transparent".

Both approaches depend on the hole slot of the Polygon object being
correctly set.

R graphics do not analyse objects leaving out say holes, they overpaint.
This is because the holes are not "known" in advance by the functions being
executed.

Hope this helps,

Roger

Thanks in advance,

Paulo E. Cardoso

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--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no