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