static BWImage bwFoldImage(BWImage big, BWImage pat) {
  int wp = pat.getWidth(), hp = pat.getHeight();
  int w = big.getWidth(), h = big.getHeight();
  BWImage out = new(w-wp, h-hp);
  for (int y = 0; y < h-hp; y++)
    for (int x = 0; x < w-wp; x++)
      out.setPixel(x, y, bwImageSectionsSimilarity2(big, pat, x, y, 0));
  ret out;
}