Dear G4 Team,

In G4OpBoundaryProcess the following code is used to get the vector normal to
the surface being crossed by a photon:

theLocalNormal = theNavigator->GetLocalExitNormal(&valid);
...
theGlobalNormal = theNavigator->GetLocalToGlobalTransform().
	                                TransformAxis(theLocalNormal);

The problem appears when a photon exits a solid.  The local normal vector
should be given in the coordinate system of the volume left behind (where
PreStepPoint belongs). Instead, it is given in the coordinate system of the
MOTHER volume, and the tranformation of the volume where PostStep belongs is
applied.  This works only in the case when PostStepPoint belongs in the mother
volume, but fails if two daughter volumes are touching each other and
PostStepPoint belongs in a different daughter volume.

In the last case the local normal vector is given in the coordinate system of
the mother volume, but the transformation of the second daughter volume is
applied.  As a result the local vector is wrong in the global coordinate system.

best regards

Antonis Papanestis

PS I will try and put some more material in the URL, but it may take a couple
of days.