Dither.pde

int index(int x, int y, PImage img) 
{
  return x + y * img.width;
}

void dither(PImage img) {
  img.loadPixels();
  for (int y = 0; y < img.height-1; y++) {
    for (int x = 1; x < img.width-1; x++) {
      color pix = img.pixels[index(x, y, img)];
      float oldR = red(pix);
      float oldG = green(pix);
      float oldB = blue(pix);
      int factor = 15;
      int newR = round(factor * oldR / 255) * (255/factor);
      int newG = round(factor * oldG / 255) * (255/factor);
      int newB = round(factor * oldB / 255) * (255/factor);
      img.pixels[index(x, y, img)] = color(newR, newG, newB);

      float errR = oldR - newR;
      float errG = oldG - newG;
      float errB = oldB - newB;


      int index = index(x, y, img);
      color c = img.pixels[index];
      float r = red(c);
      float g = green(c);
      float b = blue(c);
      r = r + errR * 7/16.0;
      g = g + errG * 7/16.0;
      b = b + errB * 7/16.0;
      img.pixels[index] = color(r, g, b);

      index = index(x, y, img);
      c = img.pixels[index];
      r = red(c);
      g = green(c);
      b = blue(c);
      r = r + errR * 3/16.0;
      g = g + errG * 3/16.0;
      b = b + errB * 3/16.0;
      img.pixels[index] = color(r, g, b);

      index = index(x, y, img);
      c = img.pixels[index];
      r = red(c);
      g = green(c);
      b = blue(c);
      r = r + errR * 5/16.0;
      g = g + errG * 5/16.0;
      b = b + errB * 5/16.0;
      img.pixels[index] = color(r, g, b);


      index = index(x, y, img);
      c = img.pixels[index];
      r = red(c);
      g = green(c);
      b = blue(c);
      r = r + errR * 1/16.0;
      g = g + errG * 1/16.0;
      b = b + errB * 1/16.0;
      img.pixels[index] = color(r, g, b);
    }
  }
  img.updatePixels();
}