static float img_averageWidth(BWImage image) {
  ret img_averageWidth(image, 0, image.getHeight());
}

static float img_averageWidth(BWImage image, int y1, int y2) {
  float sum = 0;
  for (int y = y1; y < y2; y++) {
    sum += img_width(image, y);
  }
  return sum/(y2-y1);
}