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I want to plot objects of class SpatialPolygons with a transparent filling color. Slightly changed example from the help pages of SpatialPolygons-Class:

# simple example, from vignette("sp"):
Sr1 = Polygon(cbind(c(2,4,4,1,2),c(2,3,5,4,2)))
Sr2 = Polygon(cbind(c(5,4,2,5),c(2,3,2,2)))
Sr3 = Polygon(cbind(c(4,4,5,10,4),c(5,3,2,5,5)))
Sr4 = Polygon(cbind(c(5,6,6,5,5),c(4,4,3,3,4)), hole = TRUE)

Srs1 = Polygons(list(Sr1), "s1")
Srs2 = Polygons(list(Sr2), "s2")
Srs3 = Polygons(list(Sr3, Sr4), "s3/4")
SpP = SpatialPolygons(list(Srs1,Srs2,Srs3), 1:3)
plot(1,1,type="n",ylim=c(0,10),xlim=c(0,10),xlab="",ylab="",yaxt="n",xaxt="n")
plot(SpP, col = paste0(c("#FF0000","#00FF00","#0000FF"),"55"), pbg="white",add=TRUE)

works fine. However, if I want to zoom in, the SpatialPolygons-Class method of plot can't handle transparency anymore:

plot(1,1,type="n",ylim=c(0,7),xlim=c(0,7),xlab="",ylab="",yaxt="n",xaxt="n")
plot(SpP, col = paste0(c("#FF0000","#00FF00","#0000FF"),"55"), pbg="white",add=TRUE)

Is there a way to get this to work? (I want to keep the classical plot function if possible)

Chris
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1 Answers1

2

One way to solve the problem is to clip the polygons. 

c1 <- Polygon(cbind(c(0,7,7,0, 0),c(0,0,7,7,0)))
c2 <- Polygons(list(c1), "sclip")
Pclip <- SpatialPolygons(list(c2))
library(rgeos)
SpPclip <- gIntersection(SpP, Pclip, byid = T)
plot(SpPclip, col = paste0(c("#FF0000","#00FF00","#0000FF"),"55"), pbg="white", 
     ylim = c(-1,10), xlim = c(-1,10))
plot(Pclip, add = T)

The output looks like this: Output of the clipping

loki
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    Thank you! That fixed it. Note: To get the same plot as in my example, one has to extend the `Pclip` Polygon by the default 4% extension of the plotting region: `c1 <- Polygon(cbind(c(-0.28,7.28,7.28,-0.28, -0.28),c(-0.28,-0.28,7.28,7.28,-0.28)))` and plot without limits. – Chris Apr 22 '16 at 06:50