static L<Rect> ocr_hsplit(RGBImage img, bool[] split) {
  int w = img.w(), h = img.h();
  int x1 = 0, x = 0;
  new L<Rect> l;
  while (x < w) {
    while (x < w && split[x]) x++;
    if (x > x1) {
      l.add(new Rect(x1, 0, x-x1, h));
      x1 = x;
    }
    while (x < w && !split[x]) x++;
    if (x > x1) {
      l.add(new Rect(x1, 0, x-x1, h));
      x1 = x;
    }
  }
  ret l;
}