PImage img = loadImage("robotmarko-g.jpg");
size(600, 400);
loadPixels(); 
img.loadPixels();

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]);
    float s = 255 - r;
    pixels[imgIndex] =  color(s);
  }
  
updatePixels();