Add units to simulation results files

[lassoan]

Problem

I'm working on adding simulation result visualization to 3D Slicer and what I am missing is a way to reliably determine the unit system (cgs, mm-g-s, mm-kg-s, ...), units of each quantity (e.g., speed: cm/s or length/time), and other metadata (such as anatomical coordinate system convention - LPS or RAS). Units are not only for labeling but for data transformation: we need to convert mesh point coordinates and all quantities to mm based unit system, as mm is used for all clinical imaging.

Currently, we need to manually specify coordinate system and unit system and use some heuristics based on array names to figure out how to scale them, but it is quite fragile. Could you add field data arrays to specify these information?

Solution

Save unit system, units for each point/cell array, and additional metadata (such as anatomical coordinate system) in the output mesh.

Additional context

It would be nice if this could become a commonly used standard across different solvers.

Code of Conduct

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[lassoan]

Initial response from michelebucelli:

@lassoan In principle it would be easy to add further field data arrays. However, I am not entirely sure where the specific information that you ask for would come from in the current framework. As far as I know, currently, svMultiPhysics is entirely unaware of units and coordinate systems.

I suppose it could be possible to have the user specify in the XML configuration file the coordinate system and units, and then store that information in the output VTU files as well. (Some information, like the mesh units, could be attached to the mesh input files themselves, but other information, such as the time units, would probably not be appropriate to bundle with the mesh.)

This would mean that reference and units would still be manually specified, but at least they would remain attached to the output files. I also like the idea of having a structured way of declaring the intended system of units in the XML file (as opposed to having them manually, and somewhat intermittently, documented via XML comments).

[lassoan]

I suppose it could be possible to have the user specify in the XML configuration file the coordinate system and units, and then store that information in the output VTU files as well

This sounds good.

other information, such as the time units, would probably not be appropriate to bundle with the mesh

It would be good to save time unit in the mesh file, too (wherever the values are stored), because we need to scale them when we import them for visualization. For example, we need to apply a scaling factor to speed array values when we import the simulation output to 3D Slicer, which depends on the length and time unit.