PImage img;
PFont font;

void setup() {
  size(600, 400);
  img = loadImage("robotmarko-g.jpg");
  loadPixels(); 
  img.loadPixels();
  font = createFont("Arial", 16, true);
  textFont(font, 22);
  fill(0); 
}

void draw() {
  for (int y = 0; y < img.height; y++)
    for (int x = 0; x < img.width; x++) {
      int imgIndex = x + y * img.width;
      float r = brightness(img.pixels[imgIndex]);
      pixels[imgIndex] =  color(r);
  }
  updatePixels();
  
  int ind = mouseX + mouseY * width;
  float r = brightness(pixels[ind]);
  if (mouseX < width / 2)
    textAlign(LEFT);
  else 
    textAlign(RIGHT);
  text(int(r), mouseX, mouseY + ((mouseY < 22) ? 22: 0));
}