问QMediaPlayer命令中的GST (gstreamer)命令

EN

   // Setup GraphicsScene
   mpView = ui->gvCam;
   mpView->setVisible(true);
   mpScene = new QGraphicsScene;
   mpView->setScene(mpScene);
   mpScene->setSceneRect(0, 0, mpView->width(), mpView->height());
   mpView->setSceneRect(QRectF());
   // Setup IP camera
   mpPlayer1 = new QMediaPlayer;
   mpVideoItem1 = new QGraphicsVideoItem;
   mpPlayer1->setVideoOutput(mpVideoItem1);

   //The following line works and I got the live stream.
   mpPlayer1->setMedia(QUrl("rtsp://20.0.2.118:8554/0"));

   //However, I need to use GST type command, like:
   //gst-launch-1.0 rtspsrc location=rtsp://20.0.2.118:8554/0 ! decodebin ! videoscale \
               ! 'video/x-raw, width=480, height=270, format=I420' \
               ! xvimagesink sync=false force-aspect-ratio=false;

   //The above GST command worked if I issued from the terminal and I got the live stream. 
//But, I don't know how to put it as a 'gst pipeline' string as a parameter for  'setMedia' call.

   mpScene->addItem(mpVideoItem1);
   QSizeF qf1(mpView->width(), mpView->height());
   mpVideoItem1->setSize(qf1);
   mpVideoItem1->setAspectRatioMode(Qt::IgnoreAspectRatio);
   mpPlayer1->play();