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Question Problem with G4ExtrudedSolid 

Forum: Geometry
Date: 01 Feb, 2008
From: Sven Lotze <sven>

Hi everybody,

I am trying to model a multi leaf collimator
in a medical application and the G4ExtrudedSolid
seems to be just the right solid for the individual
leaves.

However, for my geometry the class does not produce
the solid that I would expect. I use the following
polygon:

  // cross section of leaf in xy (polygon)
  std::vector<G4TwoVector> MLCLeaf_section; 
  MLCLeaf_section.push_back( G4TwoVector(-2*mm, 1*mm) );
  MLCLeaf_section.push_back( G4TwoVector(-2*mm, 25*mm) );
  MLCLeaf_section.push_back( G4TwoVector( 1*mm, 25*mm) );
  MLCLeaf_section.push_back( G4TwoVector( 1*mm, -1*mm) );
  MLCLeaf_section.push_back( G4TwoVector( 2*mm, -1*mm) );
  MLCLeaf_section.push_back( G4TwoVector( 2*mm, -25*mm) );
  MLCLeaf_section.push_back( G4TwoVector(-1*mm, -25*mm) );
  MLCLeaf_section.push_back( G4TwoVector(-1*mm, 1*mm) );

and would expect to get a point symmetric shape like this:
http://web.physik.rwth-aachen.de/~lotze/geant4/crosssection.png

However, what I get is
http://web.physik.rwth-aachen.de/~lotze/geant4/vrml.jpg
(the shape on the right, on the left there is an identically
implemented G4ExtrudedSolid but using polygon data from the 
example in the application developers guide, which obviously works).
On the right side of the collimator leaf the identation is
visible in the wire frame, but the surface is somewhat
larger and there are some non-perpendicular angles.

First I thought it might be a visualisation problem, but
RaytracerX gives me the same: 
http://web.physik.rwth-aachen.de/~lotze/geant4/g4RayTracer.viewer-0_0.jpeg
(seen from the other side).

I also tried changing the order of the points in the polygon
but without success. Does anybody have an idea what I am
doing wrong or what is going wrong otherwise? If you want to
take a look at the code:
http://web.physik.rwth-aachen.de/~lotze/geant4/ExtrudedSolidTest.tgz

Thanks for your help!

Sven Lotze

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1 None: Re: Problem with G4ExtrudedSolid   (Ivana Hrivnacova - 04 Feb, 2008)
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