static BWImage bwInverted(BWImage bw) {
  if (bw == null) ret bw;
  int w = bw.getWidth(), h = bw.getHeight();
  BWImage bw2 = new BWImage(w, h);
  for y to h: for x to w:
    bw2.setInt(x, y, 255-bw.getInt(x, y));
  ret bw2;
}