static L<Rect> ocr_vsplit(RGBImage img, bool[] split) {
  int w = img.w(), h = img.h();
  int y1 = 0, y = 0;
  new L<Rect> l;
  while (y < h) {
    while (y < h && split[y]) y++;
    if (y > y1) {
      l.add(new Rect(0, y1, w, y-y1));
      y1 = y;
    }
    while (y < h && !split[y]) y++;
    if (y > y1) {
      l.add(new Rect(0, y1, w, y-y1));
      y1 = y;
    }
  }
  ret l;
}