Looking good! I can see some polyline drawn over helsinki, in a white dash-dot pattern, if that was what you expected. I tried some changes, and I succeeded in generating this polyline data from C++ (as a QVariantMap with the geojson data), then adding it on the map layer on the QML side. I will try to do it with a real route, as would be generated by OSRM or another routing engine, to have something more complex than what I have done by hand and see if it breaks at some level. And then make the connections so that changes on the route are updated on the map.