Profile Rainbow in R

library('jpeg'); library('png'); library('gplots')
file <- file.choose()
if (length(grep(pattern = '\\.png$', x = file, ignore.case = TRUE)) > 0) {
  imgdat <- readPNG(source = file)
} else if (length(grep(pattern = '\\.jp[e]?g$', x = file, ignore.case = TRUE)) > 0) {
  imgdat <- readJPEG(source = file)
}

n <- 6; xlim <- c(0, dim(imgdat)[2]); ylim <- c(0, dim(imgdat)[1])
rb <- paste(col2hex(cname = c('red', 'orange', 'yellow', 'green', 'blue', 'purple')), '77', sep = '')

barx <- rep(c(xlim[2], xlim[2], xlim[1], xlim[1]), times = n)
bary <- seq(ylim[2], ylim[1], length.out = n+1)[c(0:1, 1:0) + rep(1:n, each = 4)]
rectcoords <- cbind(matrix(barx, nc = 4, byrow = TRUE), matrix(bary, nc = 4, byrow = TRUE), 1:n)

png(filename = 'rainbow_overlay.png', width = xlim[2], height = ylim[2])
par(bty = 'n', mai = rep(0, 4), oma = rep(0, 4), xaxs = 'i', yaxs = 'i')
plot(0, 0, xlim = xlim, ylim = ylim, type = 'n', asp = 1)
rasterImage(image = imgdat, xleft = xlim[1], ybottom = ylim[1], xright = xlim[2], ytop = ylim[2])
apply(rectcoords, MARGIN = 1, FUN = function(x) polygon(x = x[1:4], y = x[5:8], border = NA, col = rb[x[9]]))
dev.off()