Video overlay with Qt 4.8

[PierreO]

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

Context
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,

Greetings,
Pierre-Olivier