static int bwImage_minOfArea(BWImage bw, Rect r) {
  if (bw == null) ret 0;
  int w = r.w, h = r.h;
  int min = 255;
  for y to h: for x to w: {
    int v = bw.getInt(r.x+x, r.y+y);
    min = min(min, v);
  }
  ret min;
}