The Pinna illusion http://www.scholarpedia.org/article/Pinna_illusion

Move your head back and forth while viewing the figure.

library(grid)
grid.newpage()
grid.rect(gp=gpar(col=NA, fill="grey50"))
N = 32; R0 = 0.8; Rf = 0.8; NR = 5
lwd = 2
cols = c("gray20", "gray80")
for(j in 1:NR) {
  r = R0*Rf^(j-1)
  th = seq(0, 2*pi, len = (N*2+1))[-c(1)]
  if (j%%2==1) th = rev(th)
  px1 = cos(th)*r
  px2 = c(matrix(cos(th)*(r*0.90), 2)[2:1,])
  py1 = sin(th)*r
  py2 = c(matrix(sin(th)*(r*0.90), 2)[2:1,])
  grid.polyline(0.5*c(px1, px2[seq(1, length(px2), by=2)])+0.5, 
                0.5*c(py1, py2[seq(1, length(py2), by=2)])+0.5,
                id = c(rep(1:(N), each=2), 1:(N)),
                gp = gpar(col = cols[2], lwd = lwd))
  grid.polyline(0.5*c(px2, px1[seq(1, length(px2), by=2)])+0.5, 
                0.5*c(py2, py1[seq(1, length(py2), by=2)])+0.5,
                id = c(rep(1:(N), each=2), 1:(N)),
                gp = gpar(col = cols[1], lwd = lwd))
}

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