OpenCV VideoCapture lag due to the capture buffer

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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);

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!)

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.

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