Video overlay with Qt 4.8

  • Hello everyone,
    I'm trying to create an overlay on a video, but I find no way to make it work.

    I receive a video from a camera, gstreamer handles it with a pipeline. At the end of the pipeline I use the plugin ximagesink.
    On my Qt application, I use gst-video-overlay-set-window-handle from the plugin videooverlay and give it the WinId of my QgraphicsView.
    My video appears on my application, all is fine.
    I use Qt4.8.6 on a embedded platform.

    My tries
    I try to create an overlay (draw a simple line for example) on this video.
    Here are all my attempts:

    • Subclass a QWidget, with TranslucentBackground, also with PaintOnScreen attributes, redefining the paintEvent function, the line was drawn, but the area of the widget "freeze" the video
    • The same as below, but with redefining the drawForeground function.
    • Add a scene to the QGrahicsView, and use the function addLine, the line was drawn but disappear under the next frame.
    • I do some tests with a QWidget, by setting its stylesheet with alpha channel.

    My thoughts
    I think transparency is not managed with my version of x11, maybe I have not a composition manager (I don't know how to know it).

    If you have any idea about this problem,