How to implement a video widget in Qt that builds upon GStreamer?

Since Phonon is based on gstreamer, the place to look for details is the Phonon source tree (available here: http://gitorious.org/phonon/import/trees/master). For a video player you are most likely going to need a video display widget, such as the gstreamer/videowidget.h (cpp) that in turn used the X11 renderer (gstreamer/x11renderer.h, cpp). The sink used is the xvimagesink, falling back onto the ximagesink if the first cannot be created.

The basic trick is to overlay the VideoWidget with the video output. The X11 handle needed to do this is retrieved using the QWidget::winId method, which is platform specific (as are the sinks, so no biggie).

Also, if overlay is unavailable, a QWidgetVideoSink is used, which converts the video frames into individual frames for the WidgetRenderer class. This class, in turn, makes the current frame available as a QImage object, ready for any type of processing.

So to answer your question - use either overlays (as X11Renderer) or extract individual QImages from the video stream (as QWidgetVideoSink).

To connect Gstreamer with your QWidget, you need to get the window handle using QWidget::winId() and you pass it to gst_x_overlay_set_xwindow_id();

Rough sample code:

    sink = gst_element_factory_make("xvimagesink", "sink");
    gst_element_set_state(sink, GST_STATE_READY);

    QApplication::syncX();
    gst_x_overlay_set_xwindow_id(GST_X_OVERLAY(sink), widget->winId());

Also, you will want your widget to be backed by a native window which is achieved by setting the Qt::AA_NativeWindows attribute at the application level or the Qt::WA_NativeWindow attribute at the widget level.