如何使用openCV CvVideoCamera捕获第一帧

Question

我设法用swift连接openCV CvVideoCamera，但我目前想要保存第一帧，以便我以后可以屏蔽它。 以下是我目前在.mm文件中所做的一些示例

- (id)initWithController:(UIViewController<CvCameraDelegate>*)c andImageView:(UIImageView*)iv 
{
    delegate = c;
    imageView = iv;
    videoCamera = [[CvVideoCamera alloc] initWithParentView:imageView];
    self->videoCamera.defaultAVCaptureVideoOrientation =  AVCaptureVideoOrientationPortrait;
    videoCamera.defaultFPS = 30; 
    videoCamera.delegate = self;

    return self;
}

- (void)processImage:(cv::Mat &)img {
    //Here I want to convert only the first frame to ycrcb
    cv::cvtColor(firstFrame, ycrcb, COLOR_BGR2YCrCb);

    //...rest of my code on all the other frames then masking it with first frame
}

ViewController.swift代码

 override func viewDidLoad() {
        super.viewDidLoad()
        camera = OpenCVWrapper(controller: self, andImageView: imgView)
}
@IBAction func startTranslating(_ sender: UIButton) {
    camera.start()
}