in Open CV Sharp, how can I preview a webcam at one resolution and save the image at a higher resolution?

Question

I am creating a web camera capture user control for use in a WPF application. Previously I was using WPF Media Toolkit for the camera capture. That works, but the frame rate I am getting from Open CV is better Also, using Media Toolkit, I haven't been able to save the current frame at high res but I can in Open CV. My Usercontrol is working well with my Logitech Brio web cam. Setting the resolution to 1920 x 1080, my frame rate is very good. When I set it to 3840 x 2160, it is a bit choppy.

What I want to do is view the camera live preview at 1920, bu when I want to capture the current frame, do so at full resolution. I tried stopping the camera, setting the resolution and restarting, but that takes way too long. I can’t seem to change the resolution on the fly.

Any ideas?

I was thinking of setting the image source in the capture loop, and setting another image source less often, after changing its size, but I doubt that is a very good solution.

   public async Task Start()
        {
            // Never run two parallel tasks for the webcam streaming
            if (_previewTask != null && !_previewTask.IsCompleted)
                return;

            var initializationSemaphore = new SemaphoreSlim(0, 1);

            _cancellationTokenSource = new CancellationTokenSource();
            _previewTask = Task.Run(async () =>
            {
                try
                {
                    // Creation and disposal of this object should be done in the same thread 
                    // because if not it throws disconnectedContext exception

                    if (!videoCapture.Open(CameraDeviceId))
                    {
                        throw new ApplicationException("Cannot connect to camera");
                    }
                    SetResolution();
                    using (frame)
                    {
                        while (!_cancellationTokenSource.IsCancellationRequested)
                        {
                            videoCapture.Read(frame);
                            if (!frame.Empty())
                            {
                                // Releases the lock on first not empty frame
                                if (initializationSemaphore != null)
                                    initializationSemaphore.Release();
                                _lastFrame = FlipHorizontally
                                    ? BitmapConverter.ToBitmap(frame.Flip(FlipMode.Y))
                                    : BitmapConverter.ToBitmap(frame);
                                _lastFrame.SetResolution(800, 600);
                                var lastFrameBitmapImage = _lastFrame.ToBitmapSource();
                                lastFrameBitmapImage.Freeze();
                                webcamPreview.Dispatcher.Invoke(
                                       () => webcamPreview.Source = lastFrameBitmapImage);
                            }
                            //  RenderGreenScreenMask(frame, backGroundImage);
                            // 30 FPS
                            await Task.Delay(FrameDelay);
                        }
                    }

                    videoCapture?.Dispose();
                }
                finally
                {
                    if (initializationSemaphore != null)
                        initializationSemaphore.Release();
                }

            }, _cancellationTokenSource.Token);

            // Async initialization to have the possibility to show an animated loader without freezing the GUI
            // The alternative was the long polling. (while !variable) await Task.Delay
            await initializationSemaphore.WaitAsync();
            initializationSemaphore.Dispose();
            initializationSemaphore = null;

            if (_previewTask.IsFaulted)
            {
                // To let the exceptions exit
                await _previewTask;
            }
        }