[R-sig-Geo] plot polygons - transparent holes/island

[Roger Bivand]

On Sun, 31 Jan 2010, Paulo Eduardo Cardoso wrote:

> 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?

Please do, either code for a provided data set, or code and a link to the 
URL where the data set is available.

Roger

>
> 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
>>>
>>>        [[alternative HTML version deleted]]
>>>
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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
>>
>>
>

-- 
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