x <- -6:16
     op <- par(mfrow = c(2, 2))
     contour(outer(x, x), method = "edge", vfont = c("sans serif", "plain"))
     z <- outer(x, sqrt(abs(x)), FUN = "/")
     ## Should not be necessary:
     z[!is.finite(z)] <- NA
     image(x, x, z)
     contour(x, x, z, col = "pink", add = TRUE, method = "edge",
             vfont = c("sans serif", "plain"))
     contour(x, x, z, ylim = c(1, 6), method = "simple", labcex = 1)
     contour(x, x, z, ylim = c(-6, 6), nlev = 20, lty = 2, method = "simple")
     par(op)

pause()
     ## Persian Rug Art:
     x <- y <- seq(-4*pi, 4*pi, len = 27)
     r <- sqrt(outer(x^2, y^2, "+"))
     opar <- par(mfrow = c(2, 2), mar = rep(0, 4))
     for(f in pi^(0:3))
       contour(cos(r^2)*exp(-r/f),
               drawlabels = FALSE, axes = FALSE, frame = TRUE)
pause()
     data("volcano")
     rx <- range(x <- 10*1:nrow(volcano))
     ry <- range(y <- 10*1:ncol(volcano))
     ry <- ry + c(-1,1) * (diff(rx) - diff(ry))/2
     tcol <- terrain.colors(12)
     par(opar); opar <- par(pty = "s", bg = "lightcyan")
     plot(x = 0, y = 0,type = "n", xlim = rx, ylim = ry, xlab = "", ylab = "")
     u <- par("usr")
     rect(u[1], u[3], u[2], u[4], col = tcol[8], border = "red")
     contour(x, y, volcano, col = tcol[2], lty = "solid", add = TRUE,
             vfont = c("sans serif", "plain"))
     title("A Topographic Map of Maunga Whau", font = 4)
     abline(h = 200*0:4, v = 200*0:4, col = "lightgray", lty = 2, lwd = 0.1)
     par(opar)