I'm trying to take photos from my webcam every 3 seconds. But when I try to run the code I always get the above error. I searched through StackOverflow and found some solutions that suggested locking the resource. I thought I did it with a mutex but unfortunately, I still get the error. I think it has something to do with the bitmap but I don't know where and which bitmap.

This is the code:

//Here I declare global variables
FilterInfoCollection filterInfoCollection;
VideoCaptureDevice videoCaptureDevice;

Int16 INTERVAL;
string SAVESPACE;

private static Mutex mutex = new Mutex();

//Here I start the stream of my webcam from where I get my images
private void startNewCamera()
        {
            videoCaptureDevice = new VideoCaptureDevice(filterInfoCollection[comboBox_Cameras.SelectedIndex].MonikerString);
            videoCaptureDevice.NewFrame  = VideoCaptureDevice_NewFrame;
            videoCaptureDevice.Start();
        }

//from my understanding this is where the live stream is being generated
//I also locked this one with a mutex
private void VideoCaptureDevice_NewFrame(object sender, NewFrameEventArgs eventArgs)
        {
            mutex.WaitOne();
            pictureBox_Preview.Image = (Bitmap)eventArgs.Frame.Clone();
            mutex.ReleaseMutex();
        }


//Here I start with the backgroundworker from which I want to take a photo every 3 seconds
private void btn_Start_Click(object sender, EventArgs e)
        {
            //start recording
            if (backgroundWorker_camera.IsBusy != true)
            {
                backgroundWorker_camera.RunWorkerAsync();
            }
        }


//This is where the magic happens
//I lock the whole function but the error occurs nevertheless
private void backgroundWorker_camera_DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
        {
            while (!backgroundWorker_camera.CancellationPending)
            {
                System.Threading.Thread.Sleep(INTERVAL);
                mutex.WaitOne();
                
                //HERE THE ERROR ALWAYS OCCURS
                pictureBox_Capture.Image = pictureBox_Preview.Image;

                var bitmap = new Bitmap(pictureBox_Capture.Width, pictureBox_Capture.Height);
                pictureBox_Capture.Invoke(new Action(() =>
                    {
                        pictureBox_Capture.DrawToBitmap(bitmap, pictureBox_Capture.ClientRectangle);
                    }));

                System.Drawing.Imaging.ImageFormat imageFormat = null;
                imageFormat = System.Drawing.Imaging.ImageFormat.Png;
                mutex.ReleaseMutex();                
            }    
        }

If there is anything I can improve on my question please let me know. Thank you! :)

CodePudding user response：

I figured out the problem thanks to @NIRE GUPTAs' answer. Now I store the image in two separate variables for every thread. The updated functions look like this:

    //New global Image variables for every thread
    Image captureImageTH1;
    Image captureImageTH2;
    
    //The function where Livestream am commes from
    private void VideoCaptureDevice_NewFrame(object sender, NewFrameEventArgs eventArgs)
        {
            captureImageTH1 = (Bitmap)eventArgs.Frame.Clone();
            captureImageTH2 = (Bitmap)eventArgs.Frame.Clone();

            pictureBox_Preview.Image = captureImageTH1;
        }  


     //The function where I capture the Image every 3 seconds
     private void backgroundWorker_camera_DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
        {
            string time;

            while (!backgroundWorker_camera.CancellationPending)
            {
                System.Threading.Thread.Sleep(INTERVAL);
                time = DateTime.Now.ToString("yyyy:MM:dd:HH:mm:ss");

                pictureBox_Capture.Image = captureImageTH2;

                var bitmap = new Bitmap(pictureBox_Capture.Width, pictureBox_Capture.Height);
                pictureBox_Capture.Invoke(new Action(() =>
                    {
                        pictureBox_Capture.DrawToBitmap(bitmap, pictureBox_Capture.ClientRectangle);
                    }));

                System.Drawing.Imaging.ImageFormat imageFormat = null;
                imageFormat = System.Drawing.Imaging.ImageFormat.Png;
                

                captureImageTH2.Save(Path.Combine(SAVESPACE, time   ".png"));
            }  

As you can see I'm now trying to save the captured image. But every time I do It I get the following error:

System.Runtime.InteropServices.ExternalException: "A generic error occurred in GDI ."

What is it that I'm doing wrong now? This time as well the solutions from the internet didn't help.

CodePudding user response：

Images in GDI are not thread-safe. What it means, basically - you cannot use one image from 2 threads simultaneously without some additional changes to your code.