static Pair<BWImage, S> unsubtitle(BWImage img) {
  int h = img.getHeight(), y1 = h-12;
  if (y1 < 0) null;
  int w = img.getWidth();
  int x1 = w, x2 = 0;
  for (int x = 0; x < w; x++)
    if (img.getPixel(x, y1) < 0.5) {
      x1 = min(x1, x);
      x2 = max(x2, x+1);
      if (img.getPixel(x, h-1) >= 0.5) null;
    } else
      if (img.getPixel(x, h-1) < 0.5) null;
      
  if (x1 > x2) null;
  BWImage sub = img.clip(x1-1, y1, x2-x1+2, h-y1);
  //showBWImage(sub);
  S text = imageToUtf(sub);
  ret pair(img.clip(0, 0, w, y1), text);
}