// white = certain (either completely black or completely white)
// black = uncertain (exactly 50% gray)
static BWImage certaintyImage(IBWImage img) {
  ret bwImageFromFunction(img.getWidth(), img.getHeight(),
    (x, y) -> {
      double pix = img.getFloatPixel(x, y);
      ret absdiff(pix, 0.5)*2;
    });
}