highgui.VideoCapture buffer introducing lag

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If you only display the frames that contain delay you will get a low frame-rate. Not sure if this is what Valdaine already meant, but based on his solution I came up with a way to remove the delay while keeping the original framerate.

First you have to flush the buffer, and then you start capturing the frames faster than the stream framerate (use cv::waitKey(1) or ideally a loop without any interval).

VideoCapture camera("host-address:port");

Mat frame;

flush(camera);

while (cv::waitKey(1) != 27)
{
    camera >> frame;

    cv::imshow("IP Camera", frame);
}

The first frame read from the buffer has a 2ms delay, so the loop must be escaped only after reading the second frame with delay.

void flush(VideoCapture& camera)
{
    int delay = 0;
    QElapsedTimer timer;

    int framesWithDelayCount = 0;

    while (framesWithDelayCount <= 1)   
    {
        timer.start();

        camera.grab();                      

        delay = timer.elapsed();        

        if(delay > 0)
        {
            framesWithDelayCount++;
        }
    }
}

Dear all, I experienced the same problem as InakiE. Thanks to Tytan, I tried the solution to read several images in the buffer. I tried an enhancement by measuring the time it takes to capture the image. I remarked that it takes around 4ms to capture from the buffer and then around 30ms when buffer empty. I wrote the following code (I use Qt library but can be easily adapted):

   int delai=0;
   QTime h;
        do
        {   h.start();//Start the chronometer
            cap>>AcquisitionFrame; //Declarations not included in this example
            delai=h.elapsed(); //Compute delay in ms since the start commande
            qDebug() << delai; //Display the delay for test 
                               // and measurement purposes in debug windows
         }
        while(delai<10);

With that config, the number of frame to loose is independant from the time it takes to process the image. We are "just in time".

If it can help others.