// returns diff (not similarity)
static float bwImageSectionsSimilarity(BWImage big, BWImage pat, int x, int y, float max) {
  int wp = pat.getWidth(), hp = pat.getHeight();
  int w = big.getWidth(), h = big.getHeight();
  float diff = 0;
  for (int yy = 0; yy < hp; yy++)
    for (int xx = 0; xx < wp; xx++) {
      diff += Math.abs(big.getPixel(x+xx, y+yy)-pat.getPixel(xx, yy));
      if (diff > max) ret max+1;
    }
  ret diff;
}