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[cosama]

Hi,

I just switched to meshio to read/write some of the ply files we have and I noticed that comments are not supported. There are two closed pull requests that tried to add this: #753 #727. The formats that were worked on before are:

• VTK: from title line, split into list items by ;
• MEDIT: lines starting with #
• TETGEN: lines starting with #
• XDMF: <information> tags
• VTU: <!--...-->

Then there is:

• PLY: comment string

I agree with the sentiments brought up in the previous two merge request that this should be a mesh.comments property (a list of a collection of all of the comments).

I am not sure what to do with the ""This file was created by meshio" part, I feel that would need to be filtered on read so that:

mesh.write(<filename>)
mesh.comments == meshio.read(<filename>).comments

I would be happy to contribute some, but I am not sure I can do all the formats, particular the xml ones seem a bit of a stretch, my focus would be mainly on getting ply do work.

Let me know what you think.

As an addition I asked gemini to go through all the different file formats meshio offers and here is what it said:

1. Formats supporting simple line-prefix comments:
   • (These formats allow comments by starting a line with a specific character or keyword.)
   • Abaqus (.inp): lines starting with **
   • Kratos/MDPA (.mdpa): lines starting with // (also supports C-style /* ... */ blocks)
   • Nastran (bulk data, .bdf, .fem, .nas): lines starting with $
   • Netgen (.vol, .vol.gz): lines starting with # (often in definition areas)
   • OBJ (.obj): lines starting with #
   • OFF (.off): lines starting with #
   • PLY (.ply): lines starting with comment (primarily in the header)
   • SU2 (.su2): lines starting with %
   • Tecplot .dat (ASCII): lines starting with # (typically in header/zone info)
   • TetGen (.node/.ele): lines starting with #
   • UGRID (.ugrid): lines starting with # (typically)
   • WKT (TIN) (.wkt): lines starting with -- or # (parser-dependent, as WKT focuses on data grammar)
   • Gmsh (format versions 2.2, 4.0, and 4.1, .msh): lines starting with // or $ can act as comments in many parts (though formal multi-line comments go in $Comments sections).
2. Formats where comments are part of metadata:
   • ANSYS msh (.msh) (Fluent format): Specific Lisp-like comment sections, e.g., (0 "comment text")
   • AVS-UCD (.avs): Descriptive text in initial header lines.
   • CGNS (.cgns): Structured metadata within the HDF5 file (e.g., Descriptor_t nodes).
   • DOLFIN XML (.xml): Standard XML comments
   • Exodus (.e, .exo): Descriptive information in specific variables/attributes within the NetCDF/HDF5 structure.
   • FLAC3D (.f3grid): Binary format; descriptive text would be internal metadata if supported.
   • Gmsh (.msh versions 2.2, 4.0, and 4.1, .msh): Formal multi-line comments enclosed in $Comments and $EndComments sections.
   • H5M (.h5m): Metadata stored as attributes or datasets within the HDF5 file structure.
   • Medit (.mesh, .meshb): ASCII: Descriptive information tied to specific keyword sections. Bbinary: Metadata part of the binary structure.
   • MED/Salome (.med): Structured metadata within the HDF5 file.
   • Neuroglancer precomputed format: Metadata/comments within the JSON info file (often using special _comment fields or similar, as JSON itself has no formal comments).
   • PERMAS (.post, .post.gz, .dato, .dato.gz): Comments often via specific control cards or lines (e.g., starting with $ or * but interpreted by PERMAS's specific parsing rules for those card types).
   • STL (.stl): ASCII: Name/description on the initial solid ... line. Binary: 80-character header string.
   • SVG (.svg): Standard XML comments
   • VTK (.vtk) (Legacy ASCII): Single 256-character header string on the second line.
   • VTU (.vtu): Standard XML comments
   • XDMF (.xdmf, .xmf): Standard XML comments