PImage img = loadImage("robotmarko-g-low-contrast.jpg");
size(600, 400);
loadPixels(); 
img.loadPixels();
int[] h = new int[256];
int[] heq = new int[256];

for (int y = 0; y < img.height; y++)
  for (int x = 0; x < img.width; x++) {
    int imgIndex = x + y * img.width;
    int r = int(brightness(img.pixels[imgIndex]));
    h[r]++;
  }

int temp = 0;
for(int i = 0; i < 256; i++){
  temp += h[i];
  heq[i] = (int) 255 * temp / (img.width * img.height);
}

for (int y = 0; y < img.height; y++)
  for (int x = 0; x < img.width; x++) {
    int imgIndex = x + y * img.width;
    int r = int(brightness(img.pixels[imgIndex]));
    pixels[imgIndex] =  color(heq[r]);
  }
  
updatePixels();