static float bwImagesSimilar_similarity = 0.99f;

static bool bwImagesSimilar(BWImage big, BWImage pat) {
  ret bwImagesSimilar(big, pat, bwImagesSimilar_similarity);
}

static bool bwImagesSimilar(BWImage big, BWImage pat, float similarity) {
  int wp = pat.getWidth(), hp = pat.getHeight();
  int w = big.getWidth(), h = big.getHeight();
  if (wp != w || hp != h) false;
  float max = (1f-similarity)*wp*hp;
  float diff = 0;
  for (int yy = 0; yy < hp; yy++)
    for (int xx = 0; xx < wp; xx++) {
      diff += Math.abs(big.getPixel(xx, yy)-pat.getPixel(xx, yy));
      if (diff > max) false;
    }
  true;
}

Began life as a copy of #1005394