#' Pointillism filter
#'
#' @description Convert an image (RGB array) into array of points in a style reminiscent of pointillism
#'
#' @param img Image array.
#' @param k Point size. Bigger k means bigger point / less detail.
#' @param seed Random seed for voxel color sampling.
#'
#' @examples {
#' ## Read image
#' library(jpeg)
#' img <- readJPEG(system.file("img", "Rlogo.jpg", package="jpeg"))
#' ## Plot original image
#' plot.new()
#' lim <- par()
#' rasterImage(img, lim$usr[1], lim$usr[3], lim$usr[2], lim$usr[4])
#' ## Generate and plot pointillist image
#' pointillism(img)
#' }
pointillism <- function(img, k=max(2, round(nrow(img)/100)), seed=0) {
  mat.melt <- do.call(rbind, lapply(seq(1, nrow(img)-k, by=k), function(row) {
      do.call(rbind, lapply(seq(1, ncol(img)-k, by=k), function(col) {
        voxel <- img[row:(row+k), col:(col+k),]
        set.seed(seed)
        randrow <- sample(seq_len(nrow(voxel)), 1)
        randcol <- sample(seq_len(ncol(voxel)), 1)
        c(row, col, voxel[randrow, randcol,], k)
    }))
  }))
  par(mfrow=c(1,1), mar=rep(0,4))
  plot(mat.melt[,2], rev(mat.melt[,1]),
       col=rgb(mat.melt[,c(3,4,5)]),
       pch=16,
       cex=mat.melt[vi,6]/k)
}