How to get the 3D coordinates of mouse position in Quick3D?

[hmuelner]

I would like to get the (x,y,z)
coordinates of the 3D point under the mouse cursor
in Quick3D, but I could not find any easy method. I know how to do this in OpenGL by using glReadPixels with GL_DEPTH_COMPONENT and
gluUnProject. The only method I could think of so far (not tested) is to call a C++ slot in the onClicked handler where I use QDeclarativeView3D::viewport() and cast the result to QGLWidget, which I use to construct a QGLPainter to get the worldMatrix() and the projectionMatrix(). But I do not yet know how to get the value of the depth buffer.

Any suggestions or better ideas?