// allegedly the best method for rgb to grayscale conversion:
// Z=0.2126×R+0.7152G+0.0722B

static int rgbToLuminosity(int rgb) {
  int r = (rgb >> 16) & 0xFF;
  int g = (rgb >> 8) & 0xFF;
  int b = rgb & 0xFF;
  ret iround(r*0.2126+g*0.7152+b*0.0722);
}