A fast Floyd-Steinberg dithering filter in C++ using Rcpp

dither.R

library(Rcpp)

#' Floyd-Steinberg dithering
#' 
#' @description Rcpp C++ function for Floyd-Steinberg dithering 
#'     More info: https://en.wikipedia.org/wiki/Floyd%E2%80%93Steinberg_dithering
#'     Note, this is a C++ translation of the FloydConvolution function by Todos Logos
#'     from https://www.r-bloggers.com/r-is-a-cool-image-editor-2-dithering-algorithms/
#' 
#' @param x Greyscaled 2D image matrix
#' @return Dithered image matrix
#' 
#' @examples {
#' ## Read image
#' library(jpeg)
#' img <- readJPEG(system.file("img", "Rlogo.jpg", package="jpeg"))
#' ## Take red channel as greyscale
#' imgGrey <- img[,,1]
#' dither <- ditherC(imgGrey)
#' ## Plot
#' image(t(apply(dither, 2, rev)), useRaster=TRUE, axes=FALSE, col = grey(seq(0, 1, length = 256)))
#' }
cppFunction('NumericMatrix ditherC(NumericMatrix x) {
  NumericMatrix a = x;
  int nrow = a.nrow(), ncol = a.ncol();

  for (int i = 1; i < nrow-1; i++) {
      for (int j = 1; j < ncol-1; j++) {
          int P = trunc(a(i, j)+0.5);
          double e = a(i, j) - P;
          a(i, j) = P;
          a(i, j+1) = a(i, j+1) + (e * 7/16);
          a(i+1, j-1) = a(i+1, j-1) + (e * 3/16);
          a(i+1, j) = a(i+1, j) + (e * 5/16);
          a(i+1, j+1) = a(i+1, j+1) + (e * 1/16);
      }
  }

  return a;
}')

Sample output