需要使用VideoCapture类

读取mp4文件

cv::VideoCapture vcapture("chouyan2.mp4");
    if (vcapture.isOpened()) {
        while (vcapture.grab()) {// 直接从视频文件或者设备捕获中抓取帧
            cv::Mat myFrame;
            //vcapture >> myFrame; // 将视频中的数据流向Mat
            if (vcapture.read(myFrame)) {
                cv::imshow("读取mp4视频文件", myFrame);
                cv::waitKey(40);
            }
        }
    }
    vcapture.release();

读取rtsp流并且按下空格键保存图片

修改文件名即可

cv::VideoCapture vcapture("rtsp://admin:Admin123@192.168.101.234");
    int picNum{ 0 };
    if (vcapture.isOpened()) {
        while (vcapture.grab()) {// 直接从视频文件或者设备捕获中抓取帧
            cv::Mat myFrame;
            //vcapture >> myFrame; // 将视频中的数据流向Mat
            if (vcapture.read(myFrame)) {
                cv::imshow("读取rtsp视频文件", myFrame);
                if (cv::waitKey(20) == 32) {
                    std::string picName = std::to_string(picNum) + ".jpg";
                    cv::imwrite(picName.c_str(), myFrame);
                    picNum++;
                }
            }
        }
    }
    vcapture.release();

编码生成视频文件

cv::VideoCapture vcapture("rtsp://admin:Admin123@192.168.101.234");
    cv::VideoWriter vcwriter("test.mp4", cv::VideoWriter::fourcc('X', '2', '6', '4'), 25
            , cv::Size(vcapture.get(cv::CAP_PROP_FRAME_WIDTH), vcapture.get(cv::CAP_PROP_FRAME_HEIGHT)), true);
    int picNum{ 0 };
    if (vcapture.isOpened()) {
        while (vcapture.grab()) {// 直接从视频文件或者设备捕获中抓取帧
            cv::Mat myFrame;
            //vcapture >> myFrame; // 将视频中的数据流向Mat
            if (vcapture.read(myFrame)) {
                drawrectangle(myFrame);
                cv::imshow("读取rtsp视频文件", myFrame);
                int key = cv::waitKey(20);
                if (key == 32) {
                    std::string picName = std::to_string(picNum) + ".jpg";
                    cv::imwrite(picName.c_str(), myFrame);
                    picNum++;
                    break;
                } else if (key == 'q') {
                    break;
                }
                vcwriter.write(myFrame);
            }
        }
    }
    vcapture.release();
    vcwriter.release();