GDML Tessellated Object

Geometry, Fields and Transportation
1

We are trying to debug a new option of the GDML workbench that creates a “Minimum Mesh” using Gmsh. It essentially uses an option of Gmsh to create a STL Mesh from a Brep solid and then performs a recombination operation that where possible combines Tri Facets to Quad Facets, so the Tessellated Object ends up a mixture of Tri and Quad Facets.

We did have a problem with some solids where a degenerate Mesh was being created but have addressed this by specifying a MinimumCharactersticLength.

The problem is a bit challenging to debug in that visually the Mesh/Tessellation looks the same in FreeCAD, native Gmsh ( We assume we have have used the same parameters ) and if we load into Geant4
![141CEEAF-7107-449C-B085-B037E1374A55|690x358]
(upload://tTcvlM6BvNpCH8bbgMJYD3xEzDZ.jpeg)

Geant is giving us these errors
-------- WWWW ------- G4Exception-START -------- WWWW -------

*** ExceptionHandler is not defined ***

*** G4Exception : GeomSolids1001

issued by : G4QuadrangularFacet::G4QuadrangularFacet()

Facet has three or more collinear vertices.

P0 = (-19.9911,0.477272,57.8305)
P1 = (-19.4955,0.238636,58.1609)
P2 = (-19,-8e-15,58.4914)
P3 = (-20.65,-8e-15,57.5)

Height in P0-P1-P2 = 2.2045e-15
Height in P1-P2-P3 = 0.240279
Height in P2-P3-P4 = 0.480559
Height in P4-P0-P1 = 0.342227

*** This is just a warning message. ***

-------- WWWW ------- G4Exception-END -------- WWWW -------

-------- WWWW ------- G4Exception-START -------- WWWW -------

*** ExceptionHandler is not defined ***
*** G4Exception : GeomSolids1002
issued by : G4TessellatedSolid::AddFacet()
Attempt to add facet not properly defined.
*** This is just a warning message. ***
-------- WWWW ------- G4Exception-END -------- WWWW -------

FACET TYPE = G4QuadrangularFacet
ABSOLUTE VECTORS =
P[0] = (-19.9911,0.477272,57.8305)
P[1] = (-19.4955,0.238636,58.1609)
P[2] = (-19,-8e-15,58.4914)
P[3] = (-20.65,-8e-15,57.5)

-------- WWWW ------- G4Exception-START -------- WWWW -------

So I am assuming the wrong values are being exported to Geant4, so I added code in FreeCAD at the point of exporting the Quad to create a OCC polygon shape and check its area. I get no errors and area are all > 0

1 Like
2

I should add that with some test shapes we don’t see any errors or problems.

3

We are testing with load_gdml in G01, wondering how hard it would be to change to output vertex name(s) on exceptions.

4

Indeed, the warning message is a bit confusing. A correct message should be like the following:

Height in P0-P1-P2 = 2.2045e-15
Height in P1-P2-P3 = 0.240279
Height in P2-P3-P0 = 0.480559
Height in P3-P0-P1 = 0.342227

It looks that first three vertices are lying on the same line, the minimal height in the triangle P0-P1-P2 is too small, it should be at least 10^-9 mm. The issue can be easily fixed by subdividing the face in two triangles, P0-P1-P3 and P1-P2-P3

5

We are also seeing warning messages like the followiing

FACET TYPE = G4TriangularFacet
ABSOLUTE VECTORS =
P[0] = (-23.4911,-0.477272,73.1695)
P[1] = (-22.9955,-0.238636,72.8391)
P[2] = (-22.5,-8e-15,72.5086)

-------- WWWW ------- G4Exception-START -------- WWWW -------

*** ExceptionHandler is not defined ***
*** G4Exception : GeomSolids1001
issued by : G4TriangularFacet::G4TriangularFacet()
Facet is too small or too narrow.
Triangle area = 3.41991e-15
P0 = (-21.5089,-0.477272,73.1695)
P1 = (-22.5,-8e-15,72.5086)
P2 = (-22.0045,-0.238636,72.8391)
Side1 length (P0->P1) = 1.2833
Side2 length (P1->P2) = 0.641648
Side3 length (P2->P0) = 0.641648
*** This is just a warning message. ***

Although it is just a warning, I assume it means that geant4 will ignore this Facet in its Calculations?

What is the minimum/cutoff area size as far as it is geant4 concerned. Where would I find this documented?

Thanks

6

In reality the check, that resulted in this warning, calculates the minimal height of the triangle, not the face area. The height should be greater than default surface thickness 10^-9 mm. If not, then the face will be marked as improperly defined.

Improperly defined faces are ignored at the construction of G4TessellatedSolid, but this may result in some other warnings, like:

G4TessellatedSolid::AddFacet()
Attempt to add facet not properly defined.
...

This is not documented, you should look in the code of the function that issued the warnings.