How Can I Reduce Buffer Lag When Capturing Video with OpenCV's VideoCapture?

Buffer Lag in OpenCV VideoCapture

Introduction

This article explores a common problem encountered when capturing video using OpenCV's VideoCapture object: lag due to the capture buffer. We present potential solutions, both from OpenCV and as hackarounds.

OpenCV Solution

According to the OpenCV documentation, the capture buffer size can be set. However, this feature is only available with the DC1394 v2.x backend.

<code class="cpp">cv::VideoCapture cap;
cap.set(CV_CAP_PROP_BUFFERSIZE, 3);</code>

In OpenCV versions 3.4 and later, the limitation seems to have been removed:

<code class="cpp">cv::VideoCapture cap;
cap.set(cv::CAP_PROP_BUFFERSIZE, 3);</code>

Hackaround 1

If the OpenCV solution fails, we can employ a hackaround based on measuring the time it takes to retrieve a frame. If it's too short, the frame was likely taken from the buffer and can be discarded. We continue querying frames until the time exceeds a threshold, indicating an up-to-date frame.

Hackaround 2

Another hackaround involves creating a separate thread to continuously grab frames at high speed, using cv::VideoCapture.grab(), to keep the buffer empty. Synchronization between the main and additional threads can be achieved using a spin-lock.

Conclusion

While OpenCV provides a buffer size setting, its availability is limited with certain backends. The hackarounds presented offer alternative approaches to mitigate buffer lag, allowing developers to capture video with reduced delays. The optimal solution may vary depending on the system and application requirements.

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