Plotting country outlines is really easy in R, but making those plots a bit more fancy can be frustrating. I thought it would be nice to fill the country outlines with an image rather than with a solid colour. How hard could it be? After some googling it appears that there is very little documentation on this topic. The only document that I found was by Paul Murrel (2011). He plots a black filled country shape first and then captures the rasterized shape using the grid.cap function from the grid package. This captured raster is then used as a mask on the image that needs to be plotted in the shape of a country.
I'm having difficulties with this approach for two reasons: one, I can't seem to get grid.cap working as it should; two, I don't like having to plot the shape of a country first in order to create a mask. The first matter is probably my own wrongdoing, but all the grid.cap function returns me is a matrix of white pixels. I grew tired of figuring out what I was doing wrong. For the second aspect, I think a more direct approach should be possible by using the ‘over’ function from the sp package.
So these are the steps that I took to tackle the problem:
- Download the country shape
- Download a suitable png image
- Georeference the image, such that it matches the location of the country
- Use the ‘over’ function to determine which pixels are inside the country shape
- Plot the pixels of the image that are inside the country and plot the outline
In this post I will show you how to create the image shown below, where each country is filled with an image of their respective flag. Below I will explain in a bit more detail how this image was created, following the steps listed above. I've also provided the script used to create the image below, where each of the steps are also clearly marked.
The country shapes for step 1 are downloaded from GADM using the getData function from the raster package. Of course you can use any SpatialPolygons object from any other source.
The country flag png images are downloaded from wikipedia (step 2). So this example shows how to download png image and turn it into a usable format. If you like, you can use any image format like jpg or tif, but that will require some modification of the code presented here.
To understand the next step (3), I'm using georeferencing and I may need to explain what that means. Basically, this is telling where and how the image should be positioned in the coordinate reference system (CRS) of the SpatialPolygons objects (the country outlines). In this step I'm instructing the system to stretch the image to the bounding box of the respective country. This also means that the aspect ratio of the original image is probably messed up. If you would like to keep the original aspect ratio, you need to modify this step. The image was downloaded as an array with the red, green and blue component in separate dimensions. With the brick function from the raster package this array is turned into a correctly georeferenced brick object.
Now that we have properly positioned the image, we need to determine which pixels are located inside the country outlines (step 4). For this purpose, we use the ‘over’ function from the sp package. This function will not accept a raster brick object as input. The raster brick object is therefore cast into a SpatialGrid format. The function will return a dataframe with the country shape element that matches with a specific grid cell. From this information it can be derived whether the pixel is situated inside or outside the country outline. The pixel values for those situated outside the country outlines are set to NA. Note that this can be a time-consuming step. Especially when the resolution of your image is high, and/or the country outline is complex (i.e., contains details, a lot of islands and/or holes), so either be patient. Alternatively, you can speed up the process by either lowering the resolution of your image (hint: focal) or simplifying the country outline (hint: gSimplify).
All there is left to do is to plot the image and the outline (step 5). I use the plotRGB function from the raster package to plot the image. Don't forget to set the ‘bgalpha’ argument to 0, to ensure that NA values are plotted as transparent pixels. Otherwise, it will plot white pixels over anything that has previously been plotted.
I've wrapped four of the five steps in a function, such that I could easily repeat these steps for different countries. Hopefully this post will help you to create your own cool graphics in R. Good luck, and please let me know in the comments if any of the steps are not clear...
