PImage img = loadImage("robotmarko-g-low-contrast.jpg");
float rmin = 255, rmax = 0;
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]);
    if (rmin > r) rmin = r;
    if (rmax < r) rmax = r;
  }

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 - rmin) / (rmax - rmin);      
    pixels[imgIndex] =  color(s);
  }
  
updatePixels();