static float bwSimpleContrastValue(BWImage bw) {
  if (bw == null) ret 0f;
  int w = bw.getWidth(), h = bw.getHeight();
  float min = 1, max = 0;
  for y to h: for x to w: {
    float f = bw.getPixel(x, y);
    max = max(max, f);
    min = min(min, f);
  }
  ret max-min;
}