Transparency of QDeclarativeView containing QML on top of a QWidget playing a video (using either phonon or libvlc)

Question

I am currently developing a video player.

The GUI as the topmost layer is written in QML. It should be transparent to lower layers. It contains control elements, some Lists etc., It's displayed using a QDeclarativeView.

Description

 QDeclarativeView *upperLayer = new QDeclarativeView(this); 
 upperLayer->setSource(QUrl("/home/projects/QtVideo/qml/videoControl.qml"));
 upperLayer->setStyleSheet(QString("background: transparent");
 upperLayer->setResizeMode(QDeclarativeView::SizeRootObjectToView);
 uperLayer->showFullScreen();

The layer underneath is a QWidget: I use the libvlc to display the video content in this widget.

Reason: I am receiving MPEG-TS, which can not be decoded by phonon, afaik. Therefore I need the libvlc to decode the incoming *.ts stream and put the output onto the display.

QWidget *lowerLayer = new QWidget(this);
lowerLayer.setGeometry(QString("background: red"));
QUrl* url = new QUrl("file:///home/projects/QtVideo/video.ts");
libvlc_instance_t*vlcObject;
libvlc_media_t*vlcMedia;
libvlc_media_player_t*vlcPlayer;

vlcPlayer = NULL;
if(vlcObject = libvlc_new(argc, argv)) == NULL)
{
     printf("Not able to initialize";
     exit(1);
}

if(vlcPlayer && libvlc_media_player_is_playing(vlcPlayer))
{
     libvlc_media_player_stop(vlcPlayer);
}

vlcPlayer = libvlc_media_player_new(vlcObject);
vlcMedia = libvlc_media_new_location(vlcObject, url.toString().toUtf8().constData());
libvlc_media_player_set_media(vlcPlayer, vlcMedia);

#if defined(Q_OS_MAC)
     libvlc_media_player_set_nsobject(vlcPlayer, lowerLayer->winId());
#elif defined(Q_OS_UNIX)
     libvlc_media_player_set_x_window(vlcPlayer, lowerLayer->winId());
#elif defined(Q_OS_WIN)
     libvlc_media_player_set_hwnd(vlcPlayer, lowerLayer->winId());
#endif

libvlc_media_player_play(vlc_player);

Both Elements, the QDeclarativeView and the QWidget are embedded in a QMainWindow, lowerLayer created before the upperLayer, upperLayer Transparent to the lowerLayer.

The Problem:

As long as the lowerLayer is displaying static elements such as a picture, or some colored shapes, everything works fine, full transparency and functionality.

As soon as I start displaying a video, such as the described *.ts using the libvlc OR some random video using the Phonon::VideoPlayer, the parts of the upperLayer which are above the video parts of the lowerLayer are displayed in the color of the lowerLayer(default: gray), the parts of the upperLayer which are positioned above parts of the lowerLayer or others which do not contain video elements are displayed in correct behaviour.

Question:

Is there any posibility and if, how, to get the upperLayer transparent, even if there is a video playing?

Answer 1

Are you still fighting with this issue? I, sadly, do not have a satisfying answer for you. The best I can do is point you to reasons why it doesn't work:

http://lists.trolltech.com/qt-interest/2007-02/thread01061-0.html

See Message #4 in the link above.

I have tried many different methods to get transparent painting over a video (specifically Phonon::VideoPlayer) using Qt. The only method I've found so far, is to set the overlaying QWidget as a toolTip doing something like

pWidget->setWindowFlags(Qt::ToolTip)

Depending on what exactly you're wanting to do this may be sufficient, but (in my opinion) it's a hack at best. I'm actively struggling with this issue and if I can find some sort of solution, I'll be sure to post it here.

Best of luck.

Answer 2

you're using direct rendering (by passing the wid of the widget) which draws the video overtop at that geometry:

libvlc_media_player_set_x_window

you need to use offscreen rendering and draw that to your qwidget. this can be done with an opengl context (complicated) or using the callback methods available in libvlc.

if you use the display callback (libvlc_video_display_cb) libvlc will also generate lock/unlock methods also, if you need. in this method libvlc will expect some parameters to be set such as canvas geometry and pixel format.

that said, phonon has a libvlc backend which might help, but may still use direct rendering depending on some factors..