GMSH IO

[jbrezmorf]

The current implementation use GMSH functions to read and write GMSH files however the internal API is poor as well as the performance.

• Sort elements by entities and allow entity access through the API (if the information is available).
• Use entity based GMSH functions to read and write many nodes and elements at once.
• Test the performance.

[jbrezmorf]

The GmshIO API allows overlapping physical groups/regions for a single entity. Leading to duplicated elements. However, GMSH API and how it is used in GmshIO write fails to write the duplicated entities. Every entity could be added just once.
Not sure if it is intended in GMSH API, however is inconvenient in some corner cases.

TODO:

• verify that one could not add entity elements twice
• possibly fill GMSH issue: In an effort to consistently support GMSH 2 and GMSH 4 file formats as part of the Flow123d ecosystem
  we rewrite a python code for reading and writing the mesh. The new implementation is based on the GMSH API. However we found there an inconsistency
  between the API and capabilities of the GMS 2 format. In GMSH 2 one can have several physical groups for a single entity, resulting to duplicated elements with different physical but the same entity. Although this case is not very common, I arrived to that case in order to represent some result field that could have unique values for some subdomains of the mesh. I can imagine few other scenarios where this is meaningful.

In the GMSH API, the elements are added through addElements and addElements which seems to assume adding the elements of the en
The problem with GMSH API is that duplicated entities are not possible since elements are added

[jbrezmorf]

MSH2 format does not preserve IDs see filled issue

Conclusions from the response:

• MSH4 does support entities used in more physicals, but that case is not compatible with MSH2 that's why MSH2
  do the renumbering by default
• for our use cases we would rather live without entities that are part of more physicals
• for nodes we should do reclasification to the lowes dimension entities as is done through meshing process
• for view data we should use gmsh.view and sort of append support