Using a background worker to efficiently write to a GUI thread

Question

I am using a BackgroundWorker to pull video from a camera and write it to a PictureBox on my WinForms form. In the BW thread I simply pull a frame from the camera, put it into the PictureBox, sleep, and continue:

while (CaptureThreadRunning)
{
    Thread.Sleep(5);
    Image tmp = Camera.GetFrame(500);
    pbCameraFeed.Image = tmp;
    //this.BeginInvoke(new MethodInvoker(delegate { pbCameraFeed.Image = Camera.GetFrame(500); }));
}

The issue is that eventually adjusting or moving the form around my screen will throw the exception System.InvalidOperationException with the message Additional information: Object is currently in use elsewhere. on the line pbCameraFeed.Image = tmp;

I assume that the library is trying to paint something to do with the PictureBox at the same time as my while loop is, so I switched to the this.BeginInvoke implementation that is commented out above. Unfortunately that cuts my framerate significantly. I am running this code on a very slow Mini PC which may be contributing to the issue.

What I really want is a way to update my GUI with the image reliably that doesn't drop my framerate by nearly half. Are there other standard ways to do this? A BW thread seemed perfect for this application, but am I missing something?

Thanks

Answer 1

If I were you I would definitely check out the AForge.NET Framework. No need to reinvent the wheel ;)

http://www.aforgenet.com/framework/samples/video.html

AForge.NET is an open source C# framework designed for developers and researchers in the fields of Computer Vision and Artificial Intelligence - image processing, neural networks, genetic algorithms, fuzzy logic, machine learning, robotics, etc.

The framework is comprised by the set of libraries and sample applications, which demonstrate their features:

AForge.Imaging - library with image processing routines and filters;

AForge.Vision - computer vision library;

AForge.Video - set of libraries for video processing ;

...

Answer 2

I would recommend not to use a PictureBox, and instead directly draw to a UserControl surface. This can be easily done by addig code to the Paint and Invalidate events of a UserControl.

This example below, creates a user control which has a BitMap property that it's drawed to its surface every time the control is invalidated. So, for example, to randomly render JPG images from folder D:\\MyPictures, you can do the following:

using System.Windows.Forms;
using System.Drawing;

void Main()
{
    var pictures = Directory.GetFiles(@"D:\MyPictures", "*.jpg");
    var rnd = new Random();
    var form = new Form();
    var control = new MyImageControl() { Dock = DockStyle.Fill };
    form.Controls.Add(control);

    var timer = new System.Windows.Forms.Timer();
    timer.Interval = 50;
    timer.Tick += (sender, args) => 
    {
        if (control.BitMap != null)
            control.BitMap.Dispose();
        control.BitMap = new Bitmap(pictures[rnd.Next(0, pictures.Length)]);
        control.Invalidate();
    };
    timer.Enabled = true;
    form.ShowDialog();
}

public class MyImageControl : UserControl // or PictureBox
{
    public Bitmap BitMap { get; set; }
    public MyImageControl()
    {
        this.Paint += Graph_Paint;
        this.Resize += Graph_Resize;
    }
    private void Graph_Paint(object sender, PaintEventArgs e)
    {
        if (this.BitMap != null)
        {
            lock (this.BitMap)
            {
                Graphics g = e.Graphics;
                g.DrawImage(this.BitMap, this.ClientRectangle);
            }
        }
    }
    private void Graph_Resize(object sender, System.EventArgs e)
    {
        this.Invalidate();
    } 
}

I think this can be easily changed to render the camera images instead of the picture files.

The code was tested on LinqPad