How to build projectorGeometry field for OpenSourceProjector request ?

[frogeater]

Hi, I am fighting since several days in order to open a windowed projector.
The documentation says that the projectorGeometry is a string for "Size/Position data for a windowed projector, in Qt Base64 encoded format".
I've been searching in QT documentation and found information on QWindow::setGeometry but nothing for a string for "Size/Position data for a windowed projector, in Qt Base64 encoded format".
I've tried several combinations of json data plus Base64 encoding but nothing works.

Is there any description or example somewhere I've not found about "Size/Position data for a windowed projector, in Qt Base64 encoded format" ?