static BWImage bwImageTo2Bit_inner(BWImage img, float threshold1, float threshold2) {
  int w = img.w(), h = img.h();
  BWImage img2 = new BWImage(w, h);
  for y to h: for x to w: {
    float f = img.getPixel(x, y);
    img2.setPixel(x, y, f < threshold1 || f > threshold2 ? 0f : 1f);
  }
  ret img2;
}