[R] fill the area outside a polygon

[Michael Sumner]

On Wed, 2 Dec 2015 at 23:10 Adrian Dușa <dusa.adrian at unibuc.ro> wrote:

> Dear All,
>
> I know how to fill a polygon, using a basic R graphics device:
>
> par(mai=c(0, 0, 0, 0))
> plot(1:100, type="n")
> polygon(c(20, 80, 80, 20), c(20, 20, 80, 80), col="lightblue")
>
>
> But let's say I have an outer polygon like this:
> polygon(c(0,100,100,0), c(0,0,100,100))
>
> Is it possible to fill the area between the two polygons?
>
>
Try polypath with the evenodd rule (vs. the winding rule):


p1 <- list(x = c(20, 80, 80, 20), y= c(20, 20, 80, 80))
p2 <- list(x = c(0,100,100,0), y = c(0,0,100,100))
plot(p2, type="n")
polypath(x = c(p1$x, NA_real_, p2$x), y = c(p1$y, NA_real_, p2$y), col =
"lightblue", rule = "evenodd")

Cheers, Mike.


> I realise there is a quick and dirty solution to fill the outer polygon,
> then draw the inner polygon colored with white, but I would like to
> actually "clip" the outer polygon to the boundaries of the inner one.
>
> Of course, there are various packages which draw maps (i.e. maptools), but
> I need this in base R graphics. In other words, is it possible to define a
> polygon with a hole inside, in base R?
>
> Thank you,
> Adrian
>
> --
> Adrian Dusa
> University of Bucharest
> Romanian Social Data Archive
> Soseaua Panduri nr.90
> 050663 Bucharest sector 5
> Romania
>
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