dithering results not perfectly white.

corneel_cannaerts

Hi all,

I'm writing a custom dithering algorithm based on: http://www.tannerhelland.com/4660/dithering-eleven-algorithms-source-code & https://github.com/dpiccone/dithering_algorithms. Somehow the pixels that are supposed to be white are not. It seems to be related to how processing handles colors or brightness. A workaround is commented out in the code, but i'd like to understand why this is happening.

PImage img;

float[][] errorDiffusion = {{1,0,1.0}};
float[][] floydSteinberg = {{-1, 1, 3.0/16}, {0, 1, 5.0/16}, {1, 0, 7.0/16}, {1, 1, 1.0/16}};
float[][] falseFloydSteinberg = {{0, -1, 3.0/8}, {1, 0, 3.0/8}, {1, 1, 2.0/8}};
float[][] atkinson = {{1, 0, 1.0/8}, {2, 0, 1.0/8}, {-1, 1, 1.0/8}, {0, 1, 1.0/8}, {1, 1, 1.0/8}, {0, 2, 1.0/8}};

void setup() {
  size(800, 800, JAVA2D);
  img = loadImage("mona.jpg");
  img = dither(img, falseFloydSteinberg);
  noStroke();
  noSmooth();
}

void draw() {
  background(255, 0, 0);
  image(img, 0, 0);

  color c = img.get(mouseX, mouseY);
  println(brightness(c));  
}

PImage dither(PImage inp, float[][] matrix) {
  PImage src = inp.copy();
  for (int x = 0; x < src.width; x++) {
    for (int y = 0; y < src.height; y++) {
      color oldpixel = src.get(x, y);
      color newpixel = findClosestColor(oldpixel);

      src.set(x, y, newpixel);

      float quant_error = brightness(oldpixel) - brightness(newpixel);

      for (int k=0; k<matrix.length; k++) {   
        int xi = int(x+matrix[k][0]);
        int yi = int(y+matrix[k][1]);       
        src.set(xi, yi, color(brightness(src.get(xi, yi)) + matrix[k][2]* quant_error));
      }
    }
  }
//doing a second pass gives a workaround  
/*
  for (int x = 0; x < src.width; x++) {
    for (int y = 0; y < src.height; y++) {
      color oldpixel = src.get(x, y);
      color newpixel = findClosestColor(oldpixel);
      src.set(x, y, newpixel);
    }
  }
*/
  return src;
}

color findClosestColor(color c) {
  color r;
  if (brightness(c) < 127) {
    r = color(0);
  } else {
    r = color(255);
  }
  return r;
}