后视镜功能，如何得到camera的图像？

Cocos2d-x
#1

使用cocos2dx3.x，我想实现后视镜的功能，将后视镜的图像渲染出来。创建一个新的camera后，怎么从camera中得到它所看到的图像呢？

#2

使用rendertexure

#3

有什么参考资料么？我试了下
_camera = Camera::createPerspective(60, (GLfloat)DESIGN_RESOLUTION_WIDTH / DESIGN_RESOLUTION_HEIGHT, 1, 2000);
_camera->setCameraFlag(CameraFlag::USER1);
this->addChild(_camera);

_renderTexture = RenderTexture::create(DESIGN_RESOLUTION_WIDTH, DESIGN_RESOLUTION_HEIGHT, Texture2D::PixelFormat::RGBA8888);
_renderTexture->retain();

_renderTexture->begin();
_camera->visit();
_renderTexture->end();

Sprite* sprite = Sprite::createWithTexture(_renderTexture->getSprite()->getTexture());
sprite->setPosition(width, height);
this->addChild(sprite);

看不到相应的图像…