protected void checkAlpha() {
    if (pixels == null) return;

    for (int i = 0; i < pixels.length; i++) {
      // since transparency is often at corners, hopefully this
      // will find a non-transparent pixel quickly and exit
      if ((pixels[i] & 0xff000000) != 0xff000000) {
        format = ARGB;
        break;
      }
    }
  }