See discussion following r20930:
BUG: don't ::Copy NULL PolyData

The polydata can be null in a sceneview and should not override
the bulk data that is in the main scene.

A new comment in the Volume and Model node ::Copy methods should
help clarify this for future.

On Thu, Sep 13, 2012 at 5:03 AM, Julien <julien.finet@kitware.com> wrote:

Hi Steve,
I understand that this new Copy behavior is necessary for sceneview copies,
but I'm not sure it is the behavior we would want by default.
I've been thinking lately that there are multiple ways of "copying" a node,
so next time we face a "copy" issue, maybe it will be necessary to add
parameters to the copy function.
For example:
Copy(vtkMRMLNode* anode, unsigned long options)
{
...
if (options & DontOverwriteBulkData)
{
...
}
Let me know what you think,
Thanks,
Julien.

On Thu, Sep 13, 2012 at 8:46 AM, Steve Pieper <pieper@ibility.net> wrote:
Hi Julien -

Yes, I think it's a good idea to be more explicit about the behavior
of the Copy methods.  As a general topic, it would be good to make the
internal structure of scenes more introspectable so users know what is
and isn't inside of a scene view.

Right now I'd like to be sure that the desired behavior is well
captured by the self tests so that we'll have a baseline and know when
things get disrupted.  The SlicerMRB test exercises scene views and
Nicole is going to add more as she works on the atlas testing since
that's a major use case for scene views and hierarchies.

-Steve