PImage img = loadImage("robotmarko-g.jpg");
size(680, 460);
loadPixels(); 
img.loadPixels();
int x0 = 40;
int y0 = 30; 

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]);
    int displayIndex = (x + x0) + (y + y0) * width;      
    pixels[displayIndex] =  color(r);
  }
  
updatePixels();