static L<Rect> horizontalAutoSplitThenAutoCrop(BWImage img) {
  ret horizontalAutoSplitThenAutoCrop(img, horizontalAutoSplit_threshold);
}

static L<Rect> horizontalAutoSplitThenAutoCrop(BWImage img, float threshold) {
  new L<Rect> l;
  for (Rect r : horizontalAutoSplit(img, threshold))
    l.add(translateRect(autoCropOfBWImage(img.clip(r)), r.x, r.y));
  ret l;
}