[This article was first published on Minding the Brain, and kindly contributed to R-bloggers]. (You can report issue about the content on this page here)
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

This brings together two of my favorite (professional) things: R and visual illusions. Aside from being an extremely impressive application of R, it’s a cool way of making it clear that the illusion is, in fact, an illusion. Here’s a simple example:

library(grid)
grid.newpage()
grid.rect(c(1,3,1,3)/4, c(3,3,1,1)/4, 1/2, 1/2,
gp = gpar(col = NA, fill = gray(1:4/5)))
grid.rect(c(1,3,1,3)/4, c(3,3,1,1)/4, 1/6, 1/6,
gp = gpar(col = NA, fill = gray(0.5)))

Which creates the image below. The first call to grid.rect makes a set of four squares of different shades of gray, the second call inserts smaller squares inside those larger squares. The smaller squares are all the same shade of gray – which is obvious from the R code – but they appear to be different: the one in the upper left appears lightest and the one in the lower right appears darkest.