我正在使用LibGDX为android构建一个简单的游戏（基于flappy bird）。

当鸟（演员）移动时，它会在屏幕上保留旧图像，比如：

我不知道为什么会这样。。。

这是我的GameplayScreen类（代表游戏屏幕）

public class GameplayScreen extends ScreenAdapter{

    private FlappyBird _game;
    private OrthographicCamera _camera;

    private Stage _gameplayStage;
    private Bird _bird;
    private Image _background;


    public GameplayScreen(FlappyBird game){
        _game = game;

        _camera = new OrthographicCamera(FlappyBird.WIDTH, FlappyBird.HEIGHT);

        _gameplayStage = new Stage(new StretchViewport(FlappyBird.WIDTH, FlappyBird.HEIGHT, _camera));

        _background = new Image(Assets.background);
        _gameplayStage.addActor(_background);

        _bird = new Bird();
        _bird.setPosition(FlappyBird.WIDTH * 0.25f, FlappyBird.HEIGHT/2, Align.center);
        _gameplayStage.addActor(_bird);

    }

    @Override
    public void render(float delta){
        _gameplayStage.act();
        _gameplayStage.draw();
    }

    /*Resizes the camera when the screen is resized*/
    @Override
    public void resize(int width, int height){
        _camera.setToOrtho(false, width, height);
        Assets.batch.setProjectionMatrix(_camera.combined);
        _gameplayStage.getViewport().update(width, height, true);
    }
}

有人有线索吗？