From Wojciech Lewandowski, "----1----

Attached is a fixed version of OverlayNode.cpp. I fixed CustomPolytope::cut( osg::Plane ) method.  Bug was apparent in such scenario:
 
Let P1 be some random frustum polytope
Let P2 be the polytope that was created from P1 bounding box (P2 contains P1 entirely)
 
Then ignoring precision errors: P1.cut( P2 ) == P2.cut( P1 ) == P1.  But this condition was not always met. Cut failed when some of the polytope reference points happened to lie exactly on some intersecting planes in both P1 & P2  (plane distance was = 0).
 
I only use CustomPolytope for my shadowing stuff so I did not test how this affects rest of OverlayNode.cpp.
 
----2----
 
Also attached is a minor precision improvement for osg::Plane intersect method (double version). 
 
----3----
 
I have also one observation regarding osg::Plane - There are two intersect vertices methods (float and double flavour):
 
inline int intersect(const std::vector<Vec3>& vertices) const
inline int intersect(const std::vector<Vec3d>& vertices) const
 
I guess osg::Plane won't compile when someone changes default vec3 typedef to vec3d. Shouldn't the first method be changed to use vec3f explicitly ? Ie:
 
inline int intersect(const std::vector<Vec3f>& vertices) const"

src/osgSim/OverlayNode.cpp

@@ -641,6 +641,9 @@ public:
                     // is first edge point inside plane?
                     if (distance_a>=0.0) newVertices.push_back(vertices[i]);
 
+                    // add point to new face if point exactly on a plane
+                    if (distance_a==0.0) newFace.vertices.push_back(vertices[i]); 
+
                     // does edge intersect plane
                     if (distance_a * distance_b<0.0)
                     {