static L<Rect> ocr_highBlack_hsplit(RGBImage rgb) {
  int w = rgb.w(), h = rgb.h();
  *float[w] data;
  for x to w:
    data[x] = l_highBlack(rgb, x, h/2);
  float threshold = max(data)/2;
  *bool[w] split;
  for x to w:
    split[x] = data[x] >= threshold;
  ret ocr_hsplit(rgb, split);
}