如何使用 VideoWriter 从 OpenCV 打开 GStreamer 管道

我正在使用 OpenCV VideoCapture 捕获视频帧。捕获工作正常，因为我可以使用这样的框架：

cv::VideoCapture cap("v4l2src device=/dev/video1 ! videoscale ! videorate ! video/x-raw, width=640, height=360, framerate=30/1 ! videoconvert ! appsink");
cv::imshow("feed", frame);

我还想通过网络发送流，这就是我陷入困境的地方。不知怎的，我在 appsrc 管道部分失败了。我想将流编码为 jpeg 并通过 udp 发送。这就是我得到的：

cv::VideoWriter writer
writer.open("appsrc ! videoconvert ! jpegenc ! jpegparse ! rtpjpegpay pt=96 ! udpsink host=192.168.1.25 port=5000", 0, (double)30, cv::Size(640, 360), true);

看起来上面的行没有做任何事情。这writer << frame不执行任何操作。此外，此 gstreamer 命令不显示任何内容：

gst-launch-1.0 udpsrc port=5000 caps = "application/x-rtp, media=(string)video, clock-rate=(int)90000, encoding-name=(string)JPEG, payload=(int)96" ! rtpjpegdepay ! jpegdec ! decodebin ! videoconvert ! autovideosink

我不知道我哪里失败了writer.open部分。如果我像下面这样运行 gstreamer 命令，它们就会工作：

gst-launch-1.0 v4l2src device=/dev/video1 ! videoscale ! videorate ! video/x-raw, width=640, height=360, framerate=30/1 ! jpegenc ! jpegparse ! rtpjpegpay pt=96 ! udpsink host=192.168.1.25 port=5000
gst-launch-1.0 udpsrc port=5000 caps = "application/x-rtp, media=(string)video, clock-rate=(int)90000, encoding-name=(string)JPEG, payload=(int)96" ! rtpjpegdepay ! jpegdec ! decodebin ! videoconvert ! autovideosink

在使用 OpenCV 的 Gstreamer API 之前，我们需要一个使用 Gstreamer 命令行工具的工作管道。

Sender:OP 使用 JPEG 编码，因此该管道将使用相同的编码。

gst-launch-1.0 -v v4l2src \
! video/x-raw,format=YUY2,width=640,height=480 \
! jpegenc \
! rtpjpegpay \
! udpsink host=127.0.0.1 port=5000

接收者：水槽caps for rtpjpegdepay需要匹配srccaps of the rtpjpegpay发送者管道。

gst-launch-1.0 -v udpsrc port=5000 \
! application/x-rtp, media=video, clock-rate=90000, encoding-name=JPEG, payload=26 \
! rtpjpegdepay \
! jpegdec \
! xvimagesink sync=0

现在我们已经有了发送器和接收器的工作管道，我们可以将它们移植到 OpenCV。

Sender:

void sender()
{
    // VideoCapture: Getting frames using 'v4l2src' plugin, format is 'BGR' because
    // the VideoWriter class expects a 3 channel image since we are sending colored images.
    // Both 'YUY2' and 'I420' are single channel images. 
    VideoCapture cap("v4l2src ! video/x-raw,format=BGR,width=640,height=480,framerate=30/1 ! appsink",CAP_GSTREAMER);

    // VideoWriter: 'videoconvert' converts the 'BGR' images into 'YUY2' raw frames to be fed to
    // 'jpegenc' encoder since 'jpegenc' does not accept 'BGR' images. The 'videoconvert' is not
    // in the original pipeline, because in there we are reading frames in 'YUY2' format from 'v4l2src'
    VideoWriter out("appsrc ! videoconvert ! video/x-raw,format=YUY2,width=640,height=480,framerate=30/1 ! jpegenc ! rtpjpegpay ! udpsink host=127.0.0.1 port=5000",CAP_GSTREAMER,0,30,Size(640,480),true);

    if(!cap.isOpened() || !out.isOpened())
    {
        cout<<"VideoCapture or VideoWriter not opened"<<endl;
        exit(-1);
    }

    Mat frame;

    while(true) {

        cap.read(frame);

        if(frame.empty())
            break;

        out.write(frame);

        imshow("Sender", frame);
        if(waitKey(1) == 's')
            break;
    }
    destroyWindow("Sender");
}

接收者：

void receiver()
{    
    // The sink caps for the 'rtpjpegdepay' need to match the src caps of the 'rtpjpegpay' of the sender pipeline
    // Added 'videoconvert' at the end to convert the images into proper format for appsink, without
    // 'videoconvert' the receiver will not read the frames, even though 'videoconvert' is not present
    // in the original working pipeline
    VideoCapture cap("udpsrc port=5000 ! application/x-rtp,media=video,payload=26,clock-rate=90000,encoding-name=JPEG,framerate=30/1 ! rtpjpegdepay ! jpegdec ! videoconvert ! appsink",CAP_GSTREAMER);

    if(!cap.isOpened())
    {
        cout<<"VideoCapture not opened"<<endl;
        exit(-1);
    }

    Mat frame;

    while(true) {

        cap.read(frame);

        if(frame.empty())
            break;

        imshow("Receiver", frame);
        if(waitKey(1) == 'r')
            break;
    }
    destroyWindow("Receiver");
}