try to show a video from camera source... i failed

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtX11Extras/QX11Info>
#include <QVBoxLayout>
#include <QGst/Ui/VideoWidget>

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::MainWindow) {
    ui->setupUi(this);
    widgetVideo = new QGst::Ui::VideoWidget;
    ui->videoVBOX->addWidget(widgetVideo);
    widgetVideo->show();
    populateSourcesCombo();
    connect(ui->videoSourceComboBox, SIGNAL(currentIndexChanged(int)),
            this, SLOT(videoSourceComboBox_currentIndexChanged(int)));
    ui->videoSourceComboBox->setCurrentIndex(1);
}

MainWindow::~MainWindow() {
    delete ui;
    delete dp;
}

void MainWindow::populateSourcesCombo() {
    QGst::ElementFactoryPtr ximagesrc = QGst::ElementFactory::find("ximagesrc");
    QGst::ElementFactoryPtr autovideosrc = QGst::ElementFactory::find("autovideosrc");
    QGst::ElementFactoryPtr v4l2src = QGst::ElementFactory::find("v4l2src");
    if (!ximagesrc)
        qDebug() << "we don't have ximagesrc, disable the choice to use it for screen";
    else
        ui->videoSourceComboBox->addItem("Screen");
    if (!autovideosrc)
        qDebug() << "we don't have autovideosrc, disable the choice to use it";
    else
        ui->videoSourceComboBox->addItem("Camera with autovideosrc");
    if (!v4l2src)
        qDebug() << "we don't have v4l2src, disable the choice to use it";
    else
        ui->videoSourceComboBox->addItem("Camera with v4linux2");
}

void MainWindow::videoSourceComboBox_currentIndexChanged(int index) {
    pipeline = QGst::Pipeline::create("viewer");
    if (!pipeline)
        QMessageBox::warning(this, "Failed", "no pipeline created");
    QGst::BinPtr audioSrcBin = createAudioSrcBin();
    QGst::BinPtr videoSrcBin = createVideoSrcBin();
    if (!videoSrcBin)
        QMessageBox::warning(this, "Failed", "no BIN for video source selected");
    QGst::ElementPtr mux = QGst::ElementFactory::make("oggmux");
    QGst::ElementPtr sink = QGst::ElementFactory::make("qt5videosink");
    if (!sink)
        QMessageBox::warning(this, "Failed", "no qt5videosink");
    if (!audioSrcBin || !videoSrcBin || !mux || !sink) {
        QMessageBox::critical(this, tr("Error"), tr("One or more elements could not be created. "
                              "Verify that you have all the necessary element plugins installed."));
        return; }
    //  sink->setProperty("location", dirToRec);
    //  m_pipeline = QGst::Pipeline::create();
    pipeline->add(audioSrcBin, videoSrcBin, mux, sink);
      //link elements
    QGst::PadPtr audioSinkPad = mux->getRequestPad("audio_%u");
    qDebug() << "audio sink pad = " << audioSinkPad->name();
    QGst::PadPtr audioSrcPad = audioSrcBin->getStaticPad("src");
    qDebug() << "audio source pad = " << audioSrcPad->name();
    if(!audioSrcPad->canLink(audioSinkPad))
        QMessageBox::warning(this, "Failed", "can not link pads for audio");
    else
        audioSrcPad->link(audioSinkPad);
    QGst::PadPtr videoSinkPad = mux->getRequestPad("video_%u");
    qDebug() << "video sink pad = " << videoSinkPad->name();
    QGst::PadPtr videoSrcPad = videoSrcBin->getStaticPad("src");
    qDebug() << "video source pad = " << videoSrcPad->name();
    if(!videoSrcPad->canLink(videoSinkPad))
        QMessageBox::warning(this, "Failed", "can not link pads for video");
    else
        videoSrcPad->link(videoSinkPad);
    
    if(!mux->link(sink))
        QMessageBox::warning(this, "Failed", "can not link pads for video output");
    pipeline->bus()->addSignalWatch(); // connect the bus
    QGlib::connect(pipeline->bus(), "message", this, &MainWindow::onBusMessage);
    ui->record->setText(tr("Stop recording"));
    //link elements
    widgetVideo->setVideoSink(sink);
    pipeline->setState(QGst::StatePlaying);
}

QGst::BinPtr MainWindow::createAudioSrcBin() {
    QGst::BinPtr audioBin;
    try {
        audioBin = QGst::Bin::fromDescription("autoaudiosrc name=\"audiosrc\" ! audioconvert ! "
                                              "audioresample ! audiorate ! speexenc ! queue");
    } catch (const QGlib::Error & error) {
        qCritical() << "Failed to create audio source bin:" << error;
        return QGst::BinPtr();
    }
    QGst::ElementPtr src = audioBin->getElementByName("audiosrc");
    //autoaudiosrc creates the actual source in the READY state
    src->setState(QGst::StateReady);
    return audioBin;
}

QGst::BinPtr MainWindow::createVideoSrcBin() {
    QGst::BinPtr videoBin;
    try {
        switch (ui->videoSourceComboBox->currentIndex()) {
            case 0: //screencast
            videoBin = QGst::Bin::fromDescription("ximagesrc name=\"videosrc\" ! "
                                          "videoconvert ! theoraenc ! queue");
            videoBin->getElementByName("videosrc")->setProperty("screen-num",
                                                                ui->displayNumSpinBox->value());
                break;
            case 1:  //camera
            videoBin = QGst::Bin::fromDescription("autovideosrc name=\"videosrc\" !"
                                                  "videoconvert ! theoraenc ! queue");
            videoBin->getElementByName("videosrc")->setProperty("camera",0);
                break;
            case 2:
                videoBin = QGst::Bin::fromDescription("v4l2src name=\"videosrc\" "
                                                      "        device=\"/dev/video0\" ! "
                                                      "videoconvert ! theoraenc ! queue");
                break; }
    } catch (const QGlib::Error & error) {
        qCritical() << "Failed to create video source bin:" << error;
        return QGst::BinPtr();
    }
    QGst::ElementPtr src = videoBin->getElementByName("videosrc");
    //autoaudiosrc creates the actual source in the READY state
    src->setState(QGst::StateReady);
    return videoBin;
}