Why is My OpenCV VideoCapture Lagging, and How Can I Fix It?

OpenCV VideoCapture Lag Due to the Capture Buffer

Capturing video through a webcam can involve a lag due to the capture buffer, where frames are stored before being read. This issue arises when the rate at which frames are read is slower than the rate at which they are captured, leading to an accumulation of frames in the buffer.

OpenCV Solution

In OpenCV, you can set the buffer size of a VideoCapture object using CV_CAP_PROP_BUFFERSIZE:

<code class="cpp">cv::VideoCapture cap;
cap.set(CV_CAP_PROP_BUFFERSIZE, 3); // Store only 3 frames in the buffer</code>

Hackaround 1

If the OpenCV solution doesn't work, you can use this hackaround:

1. Measure the time it takes to query a frame from the buffer.
2. If the time is low, discard the frame as it was read from the buffer.
3. Continue querying frames until the time exceeds a limit, indicating that the buffer is empty and the next frame is up to date.

Hackaround 2

Another hackaround involves using a third thread to grab frames continuously at a high speed to keep the buffer empty:

1. Create a third thread that calls cv::VideoCapture.grab() to fetch frames.
2. Use a spin lock to synchronize the reading of frames between the worker thread and the third thread.

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