video face detection using with ffmpeg and rstp slips

edit

hello! i have a problem. i have a stream from (for now) my webcam to recognize faces. it works only if i do not sleep in the while cycle.

but if i add a delay into the capture, and sleep 30 ms, in the next frame it is not using the current frame ,but is slipping. what is weird is, that if i use /dev/video0 it works, but if i use a stream via ffmpeg, it is happening then.

how could this happen?

  try {
            while(running) {
                capture >> frame;

                if( frame.empty()) {
                    continue;
                }

                Mat frame1 = frame.clone();
                vector<Rect> facesResult = detectAndDraw( frame1, cascade);

                facesMutex.lock();
                faces = facesResult;
                facesMutex.unlock();

  GETTING THIS SLIPPING IF I USE SLEEPING

                //waitKey(250);
                //std::this_thread::sleep_for(std::chrono::milliseconds(1000));

            }
        } catch (std::exception &e) {
            facesMutex.unlock();
            Napi::AsyncWorker::SetError(e.what());
        }

It looks like when the thread sleeps and some frames should be elapsed, but instead from 1..2..3..4..5, i should get the 5th frame, but even though i slept , it going to the 2nd...