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