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Forum: Geometry
Re: Question Making holes in volumes (Marek Szuba)
Date: 05 Nov, 2013
From: Michael H. Kelsey <Michael H. Kelsey>

On Tue, 05 Nov 2013 09:36:52 GMT, Marek Szuba wrote:
> There are a few volumes in my geometry which have got holes in them. So
> far I have had them implemented the GEANT3 way, i.e. by creating
> subvolumes filled with the same material as the surroundings; a quick
> search of these forums shows that at least as far as performance is
> concerned, this is still considered to be the best approach.

Yes. For performance, this is the best approach, but you do have the problem of "shared surfaces", which may not actually be coincident everywhere after they get tesselated. This can lead to the dreaded "trapped particle" errors.

> There are, however, two things I would like to clarify:

> 1. Section 4.1.2.2 of the Geant Application-Developer Guide (which
> discusses solids made by Boolean operations) mentions the following:
> "The constituent solids of a Boolean operation should possibly avoid be
> composed by sharing all or part of their surfaces. This precaution is
> necessary in order to avoid the generation of 'fake' surfaces due to
> precision loss, or errors in the final visualization of the Boolean
> shape.". Given holes implemented through subvolumes share parts of their
> surfaces with respective mother volumes, don't they suffer from the same
> problem? I do occasionally see some visualisation artefacts there;

Yes. In fact, when making holes with Booleans, we recommend that you define the second (subtracted) solid to be _larger_ than the primary solid. That avoids the shared-surface issue, and the "extra" stuff isn't part of the final solid that gets used.

> 2. Is there some way of making holes implemented this way truly
> see-through in Geant visualisation? This is of course purely cosmetic
> because as far as particle transport is concerned it is the material
> that counts - but it would come in handy at times. So far I've only
> tried adjusting transparency of daughter volumes but alas, all it does
> is make them disappear from the mother.

No, you can't really. The problem of tesselation and surface round-off is even greater for visualization than for tracking. Visualization uses a much larger grid (or smaller number of triangles) in order to increase speed, and so the chance of mismatches is greater.

  -- Michael Kelsey

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