static float img_upperWidthToLowerWidth(BWImage image) {
  float upper = img_averageWidth(image, 0, (int) (image.getHeight()/3f));
  float lower = img_averageWidth(image, (int) (image.getHeight()*2/3f), image.getHeight());
  float ratio = safeRatio(upper, lower);
  //print("upper: " + upper + ", lower: " + lower + ", ratio: " + ratio);
  return ratio;
}