c++ - OpenCV VideoCapture lag due to the capture buffer

Question

I am capturing video through a webcam which gives a mjpeg stream. I did the video capture in a worker thread. I start the capture like this:

const std::string videoStreamAddress = "http://192.168.1.173:80/live/0/mjpeg.jpg?x.mjpeg";
qDebug() << "start";
cap.open(videoStreamAddress);
qDebug() << "really started";
cap.set(CV_CAP_PROP_FRAME_WIDTH, 720);
cap.set(CV_CAP_PROP_FRAME_HEIGHT, 576);

the camera is feeding the stream at 20fps. But if I did the reading in 20fps like this:

if (!cap.isOpened()) return;

        Mat frame;
        cap >> frame; // get a new frame from camera
        mutex.lock();

        m_imageFrame = frame;
        mutex.unlock();

Then there is a 3+ seconds lag. The reason is that the captured video is first stored in a buffer.When I first start the camera, the buffer is accumulated but I did not read the frames out. So If I read from the buffer it always gives me the old frames. The only solutions I have now is to read the buffer at 30fps so it will clean the buffer quickly and there's no more serious lag.

Is there any other possible solution so that I could clean/flush the buffer manually each time I start the camera?

Answer 1

OpenCV Solution

According to this source, you can set the buffersize of a cv::VideoCapture object.

cv::VideoCapture cap;
cap.set(CV_CAP_PROP_BUFFERSIZE, 3); // internal buffer will now store only 3 frames

// rest of your code...

There is an important limitation however:

CV_CAP_PROP_BUFFERSIZE Amount of frames stored in internal buffer memory (note: only supported by DC1394 v 2.x backend currently)

Update from comments. In newer versions of OpenCV (3.4+), the limitation seems to be gone and the code uses scoped enumerations:

cv::VideoCapture cap;
cap.set(cv::CAP_PROP_BUFFERSIZE, 3);

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Hackaround 1

If the solution does not work, take a look at this post that explains how to hack around the issue.

In a nutshell: the time needed to query a frame is measured; if it is too low, it means the frame was read from the buffer and can be discarded. Continue querying frames until the time measured exceeds a certain limit. When this happens, the buffer was empty and the returned frame is up to date.

(The answer on the linked post shows: returning a frame from the buffer takes about 1/8th the time of returning an up to date frame. Your mileage may vary, of course!)

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Hackaround 2

A different solution, inspired by this post, is to create a third thread that grabs frames continuously at high speed to keep the buffer empty. This thread should use the cv::VideoCapture.grab() to avoid overhead.

You could use a simple spin-lock to synchronize reading frames between the real worker thread and the third thread.