PImage img;

void setup() {
  size(100, 100);
  noStroke();

  img = loadImage("eye.png");
  noLoop();
}

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

  // Graustufen
  for (int x = 0; x < width; x += 1) {
    for (int y = 0; y < height; y += 1) {
      color col = get(x, y);
      fill(brightness(col));
      rect(x, y, 1, 1);
    }
  }
}