The osg::QTWindow itself is only the OpenSG half of using OpenSG in QT, it has to be accompanied by an OSGQGLWidget, which is the QT half. Usually, in standard QT style, an application would derive its own widget from the OSGQGLWidget and add its data to the widget. The QTWindow needs to know which OSGQGLWidget it is associated with.
The main purpose of the special QGLWidget is to prevent QT from taking over the OpenGL context and let the osg::QTWindow handle that. The standard QT methods for OpenGL handling (makeCurrent, swapBuffers, initializeGL, paintGL, resizeGL) need to be redirected to the QTWindow, see testWindowQT_qt.cpp for an example.
As a convenience class there is a osg::OSGQGLManagedWidget, which is derived from osg::QGLWidget and implements all the necessary functions. Furthermore it has an osg::SimpleSceneManager built in to provide some basic interaction features.
The WindowQT library can also be used as a plugin for the QT Designer. The plugin offers a widget usable in QT Designer or any other application utilizing the QT plugin interface. The widget coming with the plugin is named:
This class is a wrapper for the osg::OSGQGLManagedWidget widget. Wrapping is necessary since QT doesn't fully support namespaces when the QT Designer is used and thus the wrapper class is not within the osg namespace.
In order to use the plugin within QT Designer copy the library to $QTDIR/plugins/designer or symlink it. When starting QT Designer the next time, the plugin widget is filed under the OpenSG tab. You can now use it like any other widget offered by QT Designer.
There are other ways to integrate a QT plugin into designer (and some pitfalls, too). For a full documentation on QT plugins and how to use them in your own application (non Designer usage) refer to QTs plugin documentation at Qt plugins-howto
1.4.3