C#调用摄像头拍的照片是黑的

private void Page_Loaded(object sender, RoutedEventArgs e) {

        this.videoCaptureElement = new VideoCaptureElement();
        this.videoCaptureElement.BeginInit();
        this.videoCaptureElement.Width = 568;
        this.videoCaptureElement.Height = 364;

                    //videoCaptureElement position
                    Canvas.SetTop(videoCaptureElement, 200);
        Canvas.SetLeft(videoCaptureElement, 200);

        string[] inputNames = MultimediaUtil.VideoInputNames;
        this.videoCaptureElement.VideoCaptureSource = inputNames[0];
        this.videoCaptureElement.EndInit();
        this.videoCaptureElement.Play();
        this.CameraRenderLayer.Children.Add(this.videoCaptureElement);

        //button event

        TakePic.MouseLeftButtonDown += new MouseButtonEventHandler(takePhoto);

    }
    private void pagePrevious(object sender, RoutedEventArgs e) {

        this.NavigationService.Content = new BirthdayPage();
    }
    private void takePhoto(object sender, RoutedEventArgs e) {




        RenderTargetBitmap bmp = new RenderTargetBitmap((int)this.videoCaptureElement.ActualWidth, (int)this.videoCaptureElement.ActualHeight, 96, 96, PixelFormats.Pbgra32);
        bmp.Render(this.videoCaptureElement);
        BitmapEncoder encoder = new JpegBitmapEncoder();
        encoder.Frames.Add(BitmapFrame.Create(bmp));
        string now = DateTime.Now.Year + "" + DateTime.Now.Month + "" + DateTime.Now.Day + "" + DateTime.Now.Hour + "" + DateTime.Now.Minute + "" + DateTime.Now.Second;
        string filename = "D:\\" + UserInfoSingleton.sharedInstance().UserName + "pic.jpg";
        FileStream fstream = new FileStream(filename, FileMode.Create);
        encoder.Save(fstream);
        fstream.Close();
        videoCaptureElement.Stop();

    }

如果我把Canvas.SetTop(videoCaptureElement, 200); Canvas.SetLeft(videoCaptureElement, 200);这两句注释掉，就能正常拍出照片，但是一旦调整了videoCaptureElement的位置拍出的照片就是黑色的