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Forum: Geometry
Re: Question problems with BREPS (Nika Valencic)
Date: 13 Mar, 2012
From: Gabriele Cosmo <Gabriele Cosmo>

Hi Nika,

> I have a couple of questions concerning BREP Solids construction.
> 
> Browsing through the forum and User Manuals I saw several warnings that
> BREPS are not fully implemented. I managed to construct a solid using
> G4BREPSolidPolyhedra and it worked. However, when I tried to construct a
> G4BREPSolidCylinder and visualize it I got a cube(?!) (the code is
> attached bellow).
> 
> My first question: Am I doing something wrong? Is the problem only in
> visualization (I use Qt) or is just that this class is not implemented
> completely?

it is a visualization issue, as polyhedrons for most BREPS are not defined
and what you see is just its bounding-box.
You can try using the ray-tracer for visualizing, that would show you the
real surfaces.

> My second question: I want to construct a body that looks like a
> trapezoid, but with two circular edges and two straight but nonparallel
> edges, of thickness z (figure is attached bellow) . So far I
> successfully constructed (approximation) of this body, using
> G4ExtrudedSolid. Now I want to construct the body with actual circular
> edges. I see two options; BREPS or boolean solids. Which method needs
> more CPU time? I really want to try and build this body using BREPS. Can
> this be done? I tried defining all the surfaces that confine this body
> (planes + cylindrical surfaces) but failed. Can you give me some advice
> where can I look for examples of BREP construction and if this is
> possible?

A simple Boolean composition is OK for CPU performance and would not
differ much from a BREPs approach.

> My third question: When I visualize the body made by G4ExtrudedSolid I
> get some lines inside the body (figure attached bellow). I know this is
> just a consequence of visualization, but I was wondering if there is a
> way to get rid of them. I tried with /vis/viewer/set/edge and
> /vis/viewer/set/hiddenEdge commands but the lines are always present.
> Even if I draw this body as solid (/vis/viewer/set/style solid). Can you
> give me some advice how to make these lines invisible?

You should eventually ask this in the visualization forum!

Cheers, Gabriele

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