Thread safety when using Aforge.NET and Windows Forms

Question

I'm having a lot of trouble with getting AForge.NET to play together with my Windows Forms application. The application needs a stream of bitmaps to show the video in a custom PictureBox and at the same time use the same stream to track objects using their colors.

I have tried locking the NewFrame event, copying the image from the videosource to a temporary image, and as see below, using monitors.

//Event for when a frame from the video is ready
videoSource.NewFrame += (s, e) =>
{
    if (System.Threading.Monitor.TryEnter(updaterLock, 20))
    {
        Bitmap old = currentImage;

        currentImage = (Bitmap)e.Frame.Clone();
        currentImage.RotateFlip(RotateFlipType.RotateNoneFlipY);

        if (currentImage != null)
        {
            if (ImageUpdated != null)
                ImageUpdated(this, EventArgs.Empty);

            if (old != null)
            {
                old.Dispose();
                old = null;
            }
        }
        else
            currentImage = old;

        System.Threading.Monitor.Exit(updaterLock);
    }
};

The code above is part of a class that returns a singleton instance giving access to the currentImage through a property. The class can be found in its entirety here. In the custom control the Bitmap is accessed like this (the DisplayControl class holds a reference to the instance of RgbStream - the stream of bitmaps):

Control.DisplayControl.Instance.ImageUpdated += (s, e) => this.Image = Control.DisplayControl.Instance.Bitmap;

An exception (InvalidOperationException) is thrown when accessing the Image property of the control (this.Image), which looks like this:

System.InvalidOperationException was unhandled by user code
  HResult=-2146233079
  Message=Cross-thread operation not valid: Control 'gridControl' accessed from a thread other than the thread it was created on.
  Source=System.Windows.Forms
  StackTrace:
       at System.Windows.Forms.Control.get_Handle()
       at System.Windows.Forms.Control.SetBoundsCore(Int32 x, Int32 y, Int32 width, Int32 height, BoundsSpecified specified)
       at System.Windows.Forms.Control.SetBounds(Int32 x, Int32 y, Int32 width, Int32 height, BoundsSpecified specified)
       at System.Windows.Forms.Control.set_Size(Size value)
       at SystemInterface.GUI.Controls.OccupancyGridControl.set_Image(Image value) in c:\Users\Stefan\SW505\root\ProductCode\GUI\Controls\OccupancyGridControl.cs:line 64
       at SystemInterface.GUI.Controls.OccupancyGridControl.<.ctor>b__0(Object s, EventArgs e) in c:\Users\Stefan\SW505\root\ProductCode\GUI\Controls\OccupancyGridControl.cs:line 207
       at Control.DisplayControl.<.ctor>b__0(Object s, EventArgs e) in c:\Users\Stefan\SW505\root\ProductCode\Control\DisplayControl.cs:line 36
       at System.EventHandler.Invoke(Object sender, EventArgs e)
       at Services.CameraServices.RgbStream.<.ctor>b__0(Object s, NewFrameEventArgs e) in c:\Users\Stefan\SW505\root\ProductCode\Services\CameraServices\RgbStream.cs:line 121
       at AForge.Video.DirectShow.VideoCaptureDevice.OnNewFrame(Bitmap image)
       at AForge.Video.DirectShow.VideoCaptureDevice.Grabber.BufferCB(Double sampleTime, IntPtr buffer, Int32 bufferLen)
  InnerException:

Any ideas to how this can be fixed? Thanks :)

Yup. Can't use Bitmap.Clone() either, that bitmap is unlikely to still be valid after the NewFrame event stops running. Create a deep copy with the Bitmap(Image) constructor. If this is a true streaming video source then this is pretty unlikely to play back smoothly. — Hans Passant, Feb 05 '14 at 11:51

score 2 · Accepted Answer · answered Feb 05 '14 at 09:27

I suspect that you GUI is subscribed to the ImageUpdated event. Change your RgbStream method to the following:

private RgbStream(VideoCaptureDevice video)
{
    videoSource = video;
    currentImage = null;
    updaterLock = new object();

    if (videoSource == null)
        return;

    //Start the sensor and wait for it to be ready
    videoSource.Start();
    while (!videoSource.IsRunning) { }

    //Event for when a frame from the video is ready
    videoSource.NewFrame += (s, e) =>
    {
        if (System.Threading.Monitor.TryEnter(updaterLock, 20))
        {
            Bitmap old = currentImage;

            currentImage = (Bitmap)e.Frame.Clone();
            currentImage.RotateFlip(RotateFlipType.RotateNoneFlipY);

            if (currentImage != null)
            {
                if (ImageUpdated != null)
                {
                    SynchronizationContext context = SynchronizationContext.Current ?? new SynchronizationContext();
                    context.Send(s =>
                    {
                        ImageUpdated(this, EventArgs.Empty);

                    }, null);
                }

                if (old != null)
                {
                    old.Dispose();
                    old = null;
                }
            }
            else
                currentImage = old;

            System.Threading.Monitor.Exit(updaterLock);
        }
    };
}

Thank you VERY much for this. Currently I'm not able to test it thoroughly with tracking enabled, but it seems to be working. I will accept your answer as the correct one when I'm able to check later today :) thanks again - your suggestion is very appreciated! — thilemann, Feb 05 '14 at 10:27
I found that the SyncronizationContext didn't solve all problems. So I made a MethodInvoker delegate where the bitmap is assigned to the control. Now it "rarely" throws an exception :P Is there a more appropriate method doing the assignment? — thilemann, Feb 06 '14 at 07:55

Thread safety when using Aforge.NET and Windows Forms

1 Answers1