// works in similarity
static float bwImageSectionsSimilarity2_partial(BWImage big, BWImage pat, int x, int y, int px1, int py1, int wp, int hp, float similarity) {
  float maxError = (1f-similarity)*wp*hp;
  float diff = 0;
  for yy to hp:
    for xx to wp: {
      diff += Math.abs(big.getPixel(x+xx, y+yy)-pat.getPixel(px1+xx, py1+yy));
      if (diff > maxError) ret similarity-0.001f;
    }
  ret 1f-diff/wp/hp;
}

Began life as a copy of #1005919