static float rgbDiff(RGB a, RGB b) {
  return (float) (Math.abs(a.r-b.r) + Math.abs(a.g-b.g) + Math.abs(a.b-b.b))/3;
}

// 0xRRGGBB - result is between 0 and 1
static float rgbDiff(int a, int b) {
  int r1 = (a >> 16) & 0xFF;
  int g1 = (a >> 8) & 0xFF;
  int b1 = a & 0xFF;
  int r2 = (b >> 16) & 0xFF;
  int g2 = (b >> 8) & 0xFF;
  int b2 = b & 0xFF;
  ret (Math.abs(r1-r2) + Math.abs(g1-g2) + Math.abs(b1-b2))
    // hilarious micro-optimisation instead of just dividing
    // (99.9% sure HotSpot does this anyway!)
    *(1/(3f*255));
}