TGeoNode from TGeoVolume

ROOT
1

Hello (Andrei),
Thanks for the patience that you have shown with my many queries
recently. I am approaching the end of my present task, and had one
more question.
After constructing a TGeoVolume, adding it to a complete geometry
and closing it, is there a way to obtain the TGeoNode associated with
the volume, or a way to obtain the mother volume? Or is it up to the
user to have knowledge of the path to the volume?

Regards,
Gora

2

Hi Gora,

You need to have in mind that a TGeoNode represents a positioned TGeoVolume. Therefore, for a given volume you might have several TGeoNodes that are not necessary positioned in the same mother volume. Due to this, there is no way to go from a volume to its associated TGeoNode, nor to its mother volume. A TGeoVolume is a logical hierarchy structure that cannot store the information about its position - otherwise it would not be replicable any more.

In the same way, a TGeoNode does not give you full information of the way the associated object is positioned in the geometry, but only with respect to its mother. In other words, having a TGeoNode pointer does not mean : “OK, I can put my hand on this object” - there might be several of them since their container can be replicated in the geometry. The full information is available only when you specify the path to a given node, e.g.: TOP_1/A_2/B_5
You can do this either by enforcing it explicitly:

gGeoManager->cd("myPath");

or by performing the “Where am I ?” query for a given point:

gGeoManager->FindNode(x,y,z);

Only after that you can ask for instance which is the mother of the current volume:

TGeoNode *TGeoManager::GetMother(Int_t nlevel_up=1)

For performance reasons there is no class representing the current “touchable” object - in a real detector geometry you might have up to 1 billion such objects that obviously cannot fit to memory and creating/destroying them on the flight would be a disaster during navigation.

On the other hand, when you navigate a geometry all this information can be retreived. For instance, if you need to know the exact position of the current “touchable” node, after a “where am I?” for instance, you just have to do:

   TGeoHMatrix *current_global_matrix = gGeoManager->GetCurrentMatrix();

or if you want to convert a point from MARS to the local frame directly:

gGeoManager->MasterToLocal(Double_t *master_point, Double_t *local_point);

Sorry for these quite long explanations that in some sense are beyond what you have asked for. The short answer to your question is :yes, it is the task of the user to provide additional information related to the object (e.g. path) if this path is not automatically computed by navigation queries.

Hope this helps,