PImage img;

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

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

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

  // Gerasteret und invertiert
  for (int x = 0; x < width; x += 10) {
    for (int y = 0; y < height; y += 10) {
      color col = get(x, y);
      col = invertColor(col);
      fill(col);
      rect(x, y, 10, 10);
    }
  }
}

color invertColor(color c) {
  return color(255 - red(c), 255 - green(c), 255 - blue(c));
}