static bool[] markImageThroughLua(final RGBImage img, S lua) {
  Sandbox s = luaSandbox();
  final int w = img.getWidth(), h = img.getHeight();
  final bool[] mark = new bool[w*h];
  s.setOuter("w", Lua.value(w));
  s.setOuter("h", Lua.value(h));
  s.setOuter("getInt", new TwoArgFunction {
    public LuaValue call(LuaValue x, LuaValue y) {
      ret LuaValue.valueOf(img.getInt(x.toint(), y.toint()));
    }
  });
  s.setOuter("markPixel", new TwoArgFunction {
    public LuaValue call(LuaValue _x, LuaValue _y) {
      int x = _x.toint(), y = _y.toint();
      if (x >= 0 && x < w && y >= 0 && y < h)
        mark[y*w+x] = true;
      ret Lua.NIL;
    }
  });
  luaEval(s, lua);
  ret mark;
}