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

[Roger Bivand]

On Mon, 1 Feb 2010, Paulo Eduardo Cardoso wrote:

> Hi,
> Code exemple
> library(maptools)
> load(file='D:\\R\\transparencia\\TransparentPolygon.Rdata')
> areas <- readShapeSpatial('D:\\R\\transparencia\\Areas.shp')
> pontos <- readShapeSpatial('D:\\R\\transparencia\\Points.shp')
> plot(pontos,pch=19,col=pontos$point)
> plot(areas,pbg="transparent",bg="red",add=TRUE)
>
> data can be downloaded from
>
> http://rapidshare.com/files/344532718/TransparentPolygon.Rdata.html
>
> example image can be downloaded grom
>
> http://rapidshare.com/files/344533300/ExampleTransparentPolygon.png.html
>

The image cannot be reproduced with your plotting order, because the 
polygons overpaint the points. Crucially, the big square is completely 
painted. You also have two Polygons objects, one a square with a round 
hole, the other an island filling the hole. With your objects, you could 
do:

plot(areas[1,], pbg="white", col="grey85", border="grey50") 
# set up background polygon and paint hole
plot(areas[2,], angle=45, density=8, border="grey50", col="grey50",
   add=TRUE)
# hatch the island polygon
points(pontos[pontos$point=="in",], pch=16)
# add the in points
points(pontos[pontos$point=="out",], pch=22, col="grey50", bg="grey50")
# and the out points

If your output devive uses and alpha channel, you may be able to find 
other ways forward, but they will not be portable across devices.

Hope this helps,

Roger

>
>
>
>
> 2010/1/31 Roger Bivand <Roger.Bivand at nhh.no>
>>
>> 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]]
>>>>>
>>>>> _______________________________________________
>>>>> R-sig-Geo mailing list
>>>>> R-sig-Geo at stat.math.ethz.ch
>>>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>>>
>>>>>
>>>> --
>>>> 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
>>
>

-- 
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