In https://github.com/nschloe/meshio/issues/388 ‘[MSH 4.1: nodes have wrong tags in some cases](https://github.com/nschloe/meshio/issues/388#)’, it was reported that sometimes the nodes weren't tagged monotonically. My reading of the specification was that this shouldn't happen:
By default, for non-partitioned, single file meshes, Gmsh will create files
with a
continuous ordering of node and element tags, starting at 1.
Is that right?
A simple two-dimensional GEO file, ordering.geo, attached and listed
SetFactory("OpenCASCADE");
Rectangle(1) = {0, 0, 0, 1, 0.5, 0};
Transfinite Curve {1, 2, 3, 4} = 3 Using Progression 1;
was provided that when run to produce 6-node triangles in the GUI didn't tag
the nodes sequentially in the output MSH 4.1 file whereas it does when run from
the command line with
gmsh -2 -order 2 ordering.geo
The 'Transfinite Curve' doesn't affect the phenomenon, but omitting it here
does increase the number of nodes and elements, so it's handy for keeping the
output easier to inspect at a glance.
Specifically, in the $Nodes block, the one-dimensional entity blocks have
deranged tags for the nodes; e.g., lines 30–33 of ordering-gui.msh, generated
in the GUI, are
1 1 0 3
5
11
12
whereas in ordering-cl.msh, generated from the command line, they are
1 1 0 3
5
6
7
I guess meshio should be rewritten to cope with this, but I thought I'd report
it here in case it was anomalous and unexpected.
It was originally reported for Gmsh 4.2 and I've reproduced it with the latest
git master, 4.4.0-git-bea1e5dde.
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ordering-cl.msh
Description: Mesh model
ordering-gui.msh
Description: Mesh model
ordering.geo
Description: application/vnd.dynageo
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