// returns diff (not similarity), not divided by pixel number (same for maxError)
static float rgbImageSectionsSimilarity(RGBImage big, RGBImage pat, int x, int y, float maxError) {
  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 += rgbDiff(big.getInt(x+xx, y+yy), pat.getInt(xx, yy));
      if (diff > maxError) ret maxError+1;
    }
  ret diff;
}

Began life as a copy of #1005394