关于cocos2dx RenderTexture的newImage为空

Sprite* spr1 = Sprite::create("spr1.png");spr1->retain();//在3.0此处必须写上这个        auto render = RenderTexture::create(visibleSize.width, visibleSize.height, Texture2D::PixelFormat::RGBA8888);render->retain();render->beginWithClear(0, 0, 0, 255);spr1->visit();render->end();Director::getInstance()->getRenderer()->render();//在3.0此处必须写上这个，否则newImage整张图片都为黑色,或者在下一帧获取char jpg2[20];sprintf(jpg2, "image-%d.jpg", 123);std::string fullpath1 = FileUtils::getInstance()->getWritablePath() + jpg2;Image* image1 = render->newImage();image1->saveToFile(fullpath1, true);  //设置保存目录和文件名image1->release();