OpenGL under QML, but only in item

Hello, I want to create a OpenGL item that is painted under OpenGL like this one: http://qt-project.org/doc/qt-5/qtquick-scenegraph-openglunderqml-example.html but the item described in the tutorial is painted on the whole window. Is there are way to paint only a specific area of the window (meaning the area of the actual item)? I tried glScissors but it did not work as exspected.