static BWImage bwImageTo2Bit(BWImage img, float threshold) {
  int w = img.w(), h = img.h();
  BWImage img2 = new BWImage(w, h);
  for y to h: for x to w:
    img2.setPixel(x, y, img.getPixel(x, y) >= threshold ? 1f : 0f);
  ret img2;
}