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Forum: Geometry
Re: Question problem with G4Polycone in fresnel lens (Michael Eichler)
Date: 23 Jul, 2013
From: John Apostolakis <John Apostolakis>

Dear Michael,

Parameterised volumes are represent a large number of simple volumes - whose parameters and placements can be 'parameterised' typically using a simple function. 

When each volume described is complicated,  I strongly suggest to use G4PVPlacement instead of G4PVParameterised volumes to describe your volumes.   This avoids creating a Polycone (or other complicated volume) for each ray which attempts to intersect these volumes - which is the behaviour you have seen.

So long as you have only tens up to thousands of volumes, you should see no difference in the required memory.   Even for a very large number of volumes, if you had it (e.g. 100K) it may be worth trying this.  You should at least understand whether this takes too much memory.

I do not have an explanation of the problem which you are observing.  ( And I am not sure that I understand the problem. )

It could be that the problem is due to the code which you are using to create the polycone, or to Geant4 code.  

If the problem persists, could you please try to explain it in a different way.

Best regards,
John
===================================================
John Apostolakis, SFT (SoFTware) Group, PH Department, CERN
Email:  john.apostolakis@cern.ch       Office Tel:  +41-22-767-7239
------------------------------------------------------------------------------------------

On Jul 23, 2013, at 7:34 PM, Michael Eichler wrote:

> 
> *** Discussion title: Geometry
> 
> Dear Experts,
> 
> i am having a problem in building a fresnel lens with Geant4. The fresnel lens is consisting of grooves that are G4Polycones with a
> specific "profile". The replication of the Polycones is done by G4PVParameterised, because the profile shape has to be modified from
> groove to groove.
> 
> If i shoot photons at the lens, it seems, that for every photon all grooves are calculated again.
> 
> The problem is, that for some "adjustments" of the groove shape, the
> photons just pass through the grooves without doing a step there - but i
> can see the whole lens with all grooves in the geometry viewer.
> 
> I have done geometry tests and surface-check=true for all placed volumes
> and the parametrised ones and everything is OK.
> 
> If i output the shape parameters of the grooves in the parametrisation
> function, i can see the grooves (for example number 0-200) being
> calculated for every photon. If i adjust a groove-profile that leads to
> the described problem, i can see in the output, that the calculation of
> the grooves for every photon will not go from 0 to 200 but breaks at a
> certain groove number - lets say at number 15. In the geometry viewer, i
> can now again see all grooves, but only the "inner part" of the lens -
> the first 15 grooves gives the expected behavior for the photos, all
> other grooves (16-200) are not present for the photons.
> 
> I attached 2 pictures of the geometry viewer zooming into the lens and
> showing one groove. The real Profile is marked in red. The groove
> profile with the falling edge does not lead to any problem, but the
> second picture showing a groove profile with a positive pitch at the
> end, so that the grooves touch each other with a face will lead to the
> described problem.
> 
> The fact, that this minimal change in geometry causes the problem makes
> me think that it could be a bug, but i am very new to Geant4 and to C++
> so i dont have a clue.
> 
> Does anyone have an idea about that ? Thank you very much !
> 
> Best regards, Michael Eichler
> 
>   Attachment:
>      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2013/07/23/09.29-57705-groove_working.png
>      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2013/07/23/09.29-93096-groove_notworking.png
> 
> -------------------------------------------------------------
> Visit this GEANT4 at hypernews.slac.stanford.edu message (to reply or unsubscribe) at: 
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1 More: Re: problem with G4Polycone in fresnel lens   (Michael Eichler - 24 Jul, 2013)
(_ News: Re: problem with G4Polycone in fresnel lens   (Michael Eichler - 31 Jul, 2013)
(_ None: Re: problem with G4Polycone in fresnel lens   (Tatiana Nikitina - 31 Jul, 2013)
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