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Forum: Geometry
Re: None Mapping points on a surface to indices (Giuseppe Vacanti)
Re: None Re: Mapping points on a surface to indices (John Apostolakis)
Date: 27 May, 2008
From: Giuseppe Vacanti <Giuseppe Vacanti>

Dear John,

Thanks for your answer.

> Could you please explain somewhat more about what you are seeking to  
> do using this mapping?  It could be that there would be other  
> capabilities of the Geant4 solids that you could utilize.

We have implemented X-ray scattering/reflection as a boundary process.
We followed what is already done for OpticalPhoton, but our process
applies to a standard Gamma.

The scattering of X-ray photons depends on the local surface properties,
which can be measured and/or inferred. These properties can be treated
statistically, in which case you do not care where on a surface you have
interacted, the process applies some average modification to the
standard specular reflection -- for instance, the local normal is
perturbed with some function.

For some surfaces, on the other hand, we have very detailed information,
so that we know the local normal at different places. We would like to
be able to trace these surfaces: when we put such a surface in a test
X-ray beam we can actually aim at a given point, and try to match
simulation with measurements.

Data on a surface can have different origins, but one could be an
interferogram. I would like to map the interferogram coordinates to the
surface coordinates. In 3D modelling/rendering this is called (I think)
texture mapping, and it makes use of NURBS-like mapping between (u,v),
which would be the indices on the interferogram, with (x,y,z) on the
surface.

This is why I have followed the NURBS and BREPS tracks in Geant4. I am
aware that Geant4 has other aims (straight lines in and out of solids),
and we have some budget to do some of this coding ourselves if
necessary. But before we do that, if you know of an existing manner to
do this I am certainly willing to hear about it :)

Thanks again.

Regards, Giuseppe

--
Giuseppe Vacanti - PGP 0x7650713E
cosine Science & Computing - cosine.nl/goto/sciencecomputing
Niels Bohrweg 11, 2333CA Leiden, The Netherlands (KvK 28077080)
Tel: +31-71-5284962

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