C# OpenCV VideoWriter saves in unexpected colors

Question

I am using Intel Realsense RGB camera to show live stream on WPF window as well as save the stream into a file. What I am showing on window has correct colors but when I save it, the video colors are off (more purple). Please see screenshot: https://ibb.co/txy9Sgd

I am using a EmguCV video writer to save the video. I don't have much knowledge about formats. I am guessing I am doing something wrong with Format24bppRgb format?

private Pipeline pipeline = new Pipeline(); // Create and config the pipeline to strem color and depth frames.
private CancellationTokenSource tokenSource;
private VideoWriter writer = null;

public StartTests()
{
    InitializeComponent();
}

private void Window_Loaded(object sender, RoutedEventArgs e)
{
    tokenSource = new CancellationTokenSource();
    int fcc = VideoWriter.Fourcc('M', 'P', '4', 'V'); //'M', 'J', 'P', 'G'
    float fps = 15F;
    writer = new VideoWriter("testttt.mp4", fcc, fps, new System.Drawing.Size(640, 480), true);

    Config cfg = new Config();
    cfg.EnableStream(Stream.Color, 640, 480, format: Format.Rgb8);
    PipelineProfile pp = pipeline.Start(cfg);
    StartRenderFrames(pp);
}

private void StartRenderFrames(PipelineProfile pp)
{
    // Allocate bitmaps for rendring. Since the sample aligns the depth frames to the color frames, both of the images will have the color resolution
    using (VideoStreamProfile p = pp.GetStream(Stream.Color) as VideoStreamProfile)
    {
        imgColor.Source = new WriteableBitmap(p.Width, p.Height, 96d, 96d, PixelFormats.Rgb24, null);
    }
    Action<VideoFrame> updateColor = UpdateImage(imgColor);

    Task.Factory.StartNew(() =>
    {
        while (!tokenSource.Token.IsCancellationRequested)
        {
            using (FrameSet frames = pipeline.WaitForFrames()) // Wait for the next available FrameSet
            {
                VideoFrame colorFrame = frames.ColorFrame.DisposeWith(frames);

                // Save frames to file here...
                System.Drawing.Bitmap ColorImg = new System.Drawing.Bitmap(colorFrame.Width, colorFrame.Height, colorFrame.Stride, System.Drawing.Imaging.PixelFormat.Format24bppRgb, colorFrame.Data);
                Image<Bgr, Byte> imageCV = new Image<Bgr, byte>(ColorImg); //Image Class from Emgu.CV
                Mat matFrame = imageCV.Mat;
                writer.Write(matFrame);

                // Render to WPF window here...
                Dispatcher.Invoke(DispatcherPriority.Render, updateColor, colorFrame);
            }
        }
    }, tokenSource.Token);
}

static Action<VideoFrame> UpdateImage(Image img)
{
    WriteableBitmap wbmp = img.Source as WriteableBitmap;
    return new Action<VideoFrame>(frame =>
    {
        using (frame)
        {
            var rect = new Int32Rect(0, 0, frame.Width, frame.Height);
            wbmp.WritePixels(rect, frame.Data, frame.Stride * frame.Height, frame.Stride);
        }
    });
}

private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
    tokenSource.Cancel();
    tokenSource.Dispose();
    pipeline.Stop();
    writer.Dispose();
    tokenSource = new CancellationTokenSource();
}

I want to see consistent colors in the saved .mp4 file but I am seeing weird colors.

Note - My code is based on this example: https://github.com/IntelRealSense/librealsense/blob/master/wrappers/csharp/cs-tutorial-2-capture/Window.xaml.cs

Answer 1

OpenCV assumes BGR colour format, so if you change the RealSense stream format to Format.Bgra8 and the WriteableBitmap format to PixelFormats.Bgr24, you should be alright.

So you should have:

cfg.EnableStream(Stream.Color, 640, 480, format: Format.Bgr8);

and

new WriteableBitmap(p.Width, p.Height, 96d, 96d, PixelFormats.Bgr24, null);

I don't think you'll need to change the System.Drawing.Bitmap pixelformat as you're only using it to feed the OpenCV mat.