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Forum: Geometry
Re: Question Problem with G4IntersectionSolid (Emanuele Leonardi)
Date: 22 Feb, 2006
From: John Apostolakis <John Apostolakis>

Dear Emanuele,

Thanks for your questions and clear use case. I think that I can help, by providing a couple of clarification comments regarding the intersection solid (and Booleans in general) and their relationship to the G4Polyhedra that they can contain. I also have a couple of questions, as I am not sure if I have missed something.

- The tracking (navigation) will use the 'exact' intersection of the component solids. At each location or ray tracing (computing an intersection with a linear segment) the computation will message the underlying solids in order to provide an answer - with the expectation that the answer will be provided to the best possible precision and certainly better than 10^-9 mm (or 10^-12*distance to volume or diameter of the volume in case either is larger)

- The visualisations uses the G4Polyhedron created for your intersection solid. As the purpose of this is for visualisation (as many Vis systems handle polyhedra much better and 2nd or higher order surfaces) the polyhedron is by construction approximate.

> Some (more-or-less) simple trigonometrics give the following 
> intersection coordinates (all measures are in cm):
> A(12,0)
> B(9.9846,6.6564)
> C(3.9691,6.9459)
> D(7.1554,3.5777)

> As you see the 4 intersection points appear to be at
> A(11.9999,0.0005)   ~OK
> B(9.88633,6.66139)  NO! radius~11.92cm (<>12cm)
> C(3.96911,6.94595)  ~OK
> D(7.0772,3.64595)   NO! radius~7.96cm (<>8cm)

-I see this accuracy as generally good, since it is used for visualisation. Do you have a concern ? Eg as I cannot see your picture, it is difficult to see whether there is an obvious significant problem. Maybe you can attach a jpeg or other picture ?

-One visualisation system (at least) does not such an approximation: the ray tracer. In case there are problems in tracking with a solid these also show up (usually very clearly) in pictures that use the ray tracer. Do you have a chance to look at it with this ? Or are there other difficulties in 'tracking' with this solid ?

I hope these comments help clarify the role of the Polyhedra. Please let us know if there are remaining issues that I did not address.

Best regards, John Apostolakis.

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