code: public void begin (float fbufWidth, float fbufHeight, boolean flip) {
      if (begun) throw new IllegalStateException(getClass().getSimpleName() + " mismatched begin()");
      begun = true;
    }
docs: Must be called before this batch is used to accumulate and send drawing commands.
  
  @param flip whether or not to flip the y-axis. This is generally true when rendering to the
  default frame buffer (the screen), and false when rendering to textures.