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Keywords: local global coordinate transform geometry
Forum: Geometry
Re: Question Local/Global coordinate transformation to establish primary vertex? (James Odegaard)
Re: Feedback Re: Local/Global coordinate transformation to establish primary vertex? (Gabriele Cosmo)
Re: Feedback Re: Local/Global coordinate transformation to establish primary vertex? (Jason Detwiler)
Date: 24 Jan, 2007
From: Gabriele Cosmo <Gabriele Cosmo>

Hi Jason,

> I would think that in the majority of cases in which this would be
> useful, the user is going to be requesting some particular volume
> that is placed only once, so only those with lots of copies are going
> to need to do some work by hand (i.e. to find the copy number).

in your last sentence it seems to me you have an idea of what is the
difficulty of the problem. The case you describe above is -much-
simpler than the problem of James. As such, the reply I made to
James (also documented in chapter 4.1.8.2 of the User's Guide for
Application Developers) is appropriate and not 'absurd'.
The navigator provides you all the instruments to transform the points
in your physical volume, also in the case you need to do this outside of
the run, it is just necessary to locate the volume first (by for example
generating 'random' global points in the world and locating the volume
through an alternative navigator).
Such solution applies to -any- kind of geometry setups without
exceptions and, after all, requires minimal effort to the user.

I agree with you that in the simplified case above, the instruments
could be provided by for example transversing the volume tree up to
determine the global transformation. However, such solution is not
technically valid for all possible geometry constructs that Geant4 can
offer (multiple nested-or-not placements, replicas, nested-or-not
parameterisations, divisions, reflections, assemblies, etc...); in order
to avoid confusion and mis-uses, the existing back-pointer to the
mother physical-volume in G4VPhysicalVolume was removed a long
time ago, and it is not possible (or actually, is forbidden!) today to
directly transverse back a physical-volume tree.

At the current stage we do not have a valid solution which is generic
enough to consider all possible geometry setups, other than the one
I described and is mentioned in the documentation. We are considering
to provide in future an alternative solution by extending the cache in
the logical-volume and through that provide the necessary tools for
accessing to such information, but we are not yet there !

Cheers, Gabriele

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