|Message: Re: Local/Global coordinate transformation to establish primary vertex?||Not Logged In (login)|
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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 220.127.116.11 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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