Hangs on creating Capture or grabbing from offline IP-camera

[cbagpipe]

Very simple code to reproduce subject. It completely stucks. I can't even kill thread or cancel task whenever it hangs.

Using EmguCV 3.1.0.1 NuGet package

using Emgu.CV;
using System;

namespace OpenCVTest
{
    class Program
    {
        static void Main(string[] args)
        {
            // if camera offline from start, program stucks here.
            var capture = new Capture(@"rtsp://192.168.123.123/");

            // if camera becomes and stays offline mone than 20-30 seconds, program stucks here
            while (capture.Grab())
            {
                Console.Write("grab.");
            }
        }
    }
}

[kolomietsvv]

If still relevant. I have Inherited the Capture class (v3.1.0.1 the Capture class still had not been renamed to VideoCapture, version from nuget), and I have overrided the Grab function. That is what I've got.

public class VideoCapture : Capture
    {
        public new event EventHandler ImageGrabbed;

        public event EventHandler GrabFailed;

        public VideoCapture(string inputString) : base(inputString) { }

        [HandleProcessCorruptedStateExceptions]
        public override bool Grab()
        {
            try
            {
                if (_ptr == IntPtr.Zero)
                    return false;

                bool grabbed = cveVideoCaptureGrab(Ptr);

                if (grabbed)
                    ImageGrabbed?.Invoke(this, new EventArgs());
                else
                    GrabFailed?.Invoke(this, new EventArgs());
                return grabbed;
            }
            catch
            {
                Thread.Yield();
                GrabFailed?.Invoke(this, new EventArgs());
                return false;
            }
        }

        /// <summary>
        /// Grab a frame
        /// </summary>
        /// <param name="capture">Video capturing structure</param>
        /// <returns>True on success</returns>
        [DllImport("cvextern", CallingConvention = CvInvoke.CvCallingConvention)]
        [return: MarshalAs(CvInvoke.BoolToIntMarshalType)]
        internal static extern bool cveVideoCaptureGrab(IntPtr capture);
}

Sometimes _capture.GrabFailed += ReconnectCamera works, but sometimes it steps into grub, but doesn't reach the GrabFailed?.Invoke(this, new EventArgs());. And stays somwhere in bool grabbed = cveVideoCaptureGrab(Ptr);

[LingboTang]

I don't think this problem has been fixed up till now, when I read the source code, there's no exception handling for Grab, Retrieve and QueryFrame. I'm using the *.dll directly, so I can't simply override that one function. It is really inconvenient, please consider fix it right away.

[LingboTang]

After we get an decoding error like:

h264: cabac decode of qscale diff failed at error decoding

This line:

bool grabbed = CvInvoke.cveVideoCaptureGrab(Ptr);

will always return false because the Ptr will point to null, so cveVideoCaptureGrab will not grab any frames successfully. How I fix this problem right now is every time the decoding error pops up, I'll just restart the capture. But I think EmguCV should handle this problem on your side.