Image Dither algorithm

DitherExample.pde

//Image Dithering
//http://www.processing.org/discourse/beta/num_1203512880.html

PImage ditherImage(PImage img) {
  
  PImage ditheredImg = img.get(0, 0, img.width, img.height);
  // tile = 2*2 pixels
  int[] tilePixelColors = new int[4];
  float[] bias = new float[] {
    0.25, .75, 0.5, 0.0
  };
  
  int pixelPosition;
  float valScale = 1.0 / 255.0;

  // rows
  for (int i=0; i<img.height/2.0; i++) {
    // coloumns
    for (int j=0; j<img.width/2.0; j++) {
      pixelPosition = i*2*img.width + j*2;

      tilePixelColors[0] = (int) (bias[0] + valScale*greyValue(img.pixels[pixelPosition]));
      tilePixelColors[1] = (int) (bias[1] + valScale*greyValue(img.pixels[pixelPosition+1]));
      tilePixelColors[2] = (int) (bias[2] + valScale*greyValue(img.pixels[pixelPosition+img.width]));
      tilePixelColors[2] = (int) (bias[3] + valScale*greyValue(img.pixels[pixelPosition+img.width+1]));

      ditheredImg.pixels[pixelPosition] = color(255*tilePixelColors[0]);
      ditheredImg.pixels[pixelPosition+1] = color(255*tilePixelColors[1]);
      ditheredImg.pixels[pixelPosition+img.width] = color(255*tilePixelColors[2]);
      ditheredImg.pixels[pixelPosition+img.width+1] = color(255*tilePixelColors[3]);
    }
  }
  return ditheredImg;
}

int greyValue(int _color) {
  return (int)((red(_color) + green(_color) + blue(_color)) / 3.0) ;
}

PImage imgO, imgD;

void setup(){
  size(800, 600);
  
  imgO = loadImage("Water lilies.jpg");
  imgD = ditherImage(imgO);
}

void draw(){
  image(imgD, 0, 0, width, height);
  
  noLoop();
}