static float img_leftishness(BWImage image) {
  float middle = image.getWidth()/2f;
  float sum = 0;
  for (int y = 0; y < image.getHeight(); y++) {
    int x1 = img_leftmostPoint(image, y);
    int x2 = img_rightmostPoint(image, y);
    if (x2 > x1) {
      float center = (x1+x2)/2f;
      sum += (middle-center)/middle;
    }
  }
  ret sum/image.getHeight();
}