footstep illusion by R

gistfile1.r

library(grid)
library(animation)

saveGIF({
  ani.options(interval = 0.1, nmax = 200)
  for (x in seq(0, 1, length = 200)) {
    grid.newpage()
    grid.rect(gp = gpar(fill = "grey", col = NA))
    grid.rect(seq(0, 1, 1/50), 1/4, 1/50/2, 1/2, gp = gpar(fill = "grey10", col = NA))
    grid.rect(x, c(0.15, 0.3, 0.7, 0.85), 1/50, 0.1, gp = gpar(fill = c("black", "white"), col = NA))
  }
}, movie.name = "footstep.gif")