An example of how to fill a SpatialPolygons object with a png image

Fill_SpatialPolygons_with_image.r

## Required libraries for this example
library(png)
library(sp)
library(maptools)
library(raster)

## A function as a wrapper around 4 of the 5 steps
## described in the main text of the blog....
## As input it requires a SpatialPolygons object
## and the URL to the png image that will be used
## to fill the SpatialPolygons object. Additional
## arguments are passed onto the plotRGB function
## from the raster package.
plot.masked.image <- function(sp, png.url,  ...) {
  
  ###############################################
  ##                  STEP 2                   ##
  ###############################################
  ## First step is to read the png file, in this
  ## case from an url...
  ## Note that step 1 is shown below and is executed
  ## before calling this function...
  
  ## open an url connection to the png file
  u <- url(png.url, "rb")

  ## I'm assuming here that the png file is less
  ## then 10 MiB (just increase the number if it is larger).
  img.dat <- readBin(u, "raw", 10*1024*1024)

  ## close the url connection...
  close(u)

  ## convert the raw data read from the file into a
  ## raster object:
  img.dat <- readPNG(img.dat)

  ###############################################
  ##                  STEP 3                   ##
  ##    Georeference the image, such that it   ##
  ##    matches the location of the country    ##
  ###############################################
  ## convert the raster object into
  ## a raster brick object and georeference
  ## it using the extent and CRS of the
  ## SpatialPolygons object 'sp'.
  img <- brick(img.dat,
               xmn = extent(sp)@xmin,
               xmx = extent(sp)@xmax,
               ymn = extent(sp)@ymin,
               ymx = extent(sp)@ymax,
               crs = CRS(proj4string(sp)))
  
  ###############################################
  ##                  STEP 4                   ##
  ##   Use the 'over' function to determine    ##
  ## which pixels are inside the country shape ##
  ###############################################
  indices <- over(as(img, "SpatialGrid"), sp)

  ## set the RGB values that do not overlap with
  ## SpatialPolygons object sp to NA
  img[is.na(indices)] <- NA

  ###############################################
  ##                  STEP 5                   ##
  ##   Plot the pixels of the image that are   ##
  ##  inside the country and plot the outline  ##
  ###############################################
  ## This is done by plotting the resulting
  ## rasterbrick as RGB.
  ## 'bgalpha = 0' is important as otherwise
  ## you may interfere with previously plotted
  ## shapes...
  plotRGB(img, 1, 2, 3, 1, bgalpha = 0, ...)
}

###############################################
##                  STEP 1                   ##
##          Download the country shape(s)    ##
###############################################

## Download several country's outlines from gadm.org:
nld <- raster::getData("GADM", country = "NLD", level = 1)
nld <- unionSpatialPolygons(nld, nld@data$ENGTYPE_1 != "Water body")[2]
uk  <-  raster::getData("GADM", country = "GBR", level = 0)
bel <- raster::getData("GADM", country = "BEL", level = 0)
fr  <- raster::getData("GADM", country = "FRA", level = 0)
de  <- raster::getData("GADM", country = "DEU", level = 0)
lux <- raster::getData("GADM", country = "LUX", level = 0)

## specify a bounding box, to initialise the base plot
box <- as(extent(-5.5, 8, 48, 58.5), "SpatialPolygons")
proj4string(box) <- proj4string(nld)

## open a graphics device for the resulting plot and save as png:
png("flagmap.png", 400, 500, type = "cairo")

## Get rid of the margins
par(mar = c(0, 0, 0, 0))

## Initialise the base plot
plot(box, border = NA, xaxs = "i", yaxs = "i")

## Call the magical function defined above.
## For each country use the flag image as
## available from wikipedia.
## The resolution of the flag-image can be
## increased, simply by increasing the 800px
## bit in the url into for instance 1200px...
plot.masked.image(nld, "https://upload.wikimedia.org/wikipedia/commons/thumb/2/20/Flag_of_the_Netherlands.svg/800px-Flag_of_the_Netherlands.svg.png", add = T)
plot(nld, add = T, lwd = 2)
plot.masked.image(bel, "https://upload.wikimedia.org/wikipedia/commons/thumb/6/65/Flag_of_Belgium.svg/800px-Flag_of_Belgium.svg.png", add = T)
plot(bel, add = T, lwd = 2)
plot.masked.image(lux, "https://upload.wikimedia.org/wikipedia/commons/thumb/d/da/Flag_of_Luxembourg.svg/800px-Flag_of_Luxembourg.svg.png", add = T)
plot(lux, add = T, lwd = 2)
plot.masked.image(uk, "https://upload.wikimedia.org/wikipedia/en/thumb/a/ae/Flag_of_the_United_Kingdom.svg/800px-Flag_of_the_United_Kingdom.svg.png", add = T)
plot(uk, add = T, lwd = 2)
plot.masked.image(fr, "https://upload.wikimedia.org/wikipedia/en/thumb/c/c3/Flag_of_France.svg/800px-Flag_of_France.svg.png", add = T)
plot(fr, add = T, lwd = 2)
plot.masked.image(de, "https://upload.wikimedia.org/wikipedia/en/thumb/b/ba/Flag_of_Germany.svg/800px-Flag_of_Germany.svg.png", add = T)
plot(de, add = T, lwd = 2)

## make sure to turn off the graphical device:
dev.off()