add float16, int64, and uint64

[christian-rauch]

Add additional data type mappings.

See NumPy types:
https://numpy.org/doc/stable/user/basics.types.html
and the same mappings in trimesh:
https://github.com/mikedh/trimesh/blob/a513c0e2cfa6dfbd3121a6dfd8ceb002b9ef55d8/trimesh/exchange/ply.py#L16-L36

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📚 Documentation preview 📚: https://python-plyfile--87.org.readthedocs.build/en/87/

[dranjan]

Hey @christian-rauch, thanks for the interest and the contribution. The reason those types aren't supported by plyfile is that they are not officially part of the PLY format. This project is pretty conservative about extending the PLY format (see https://python-plyfile.readthedocs.io/en/latest/developing.html#contribution-guidelines), and we would only consider it if either the proposed extension is already widespread, or if the author of the format (Greg Turk) approves of it.

This exact proposal has come up before, and it was rejected for these reasons, but I'm always willing to revisit the question given new information. I did what I did then: do a quick survey of other PLY format implementations to see what they do.

• Greg Turk's C implementation still does not support 16-bit floats or 64-bit integers.
• A few other implementations simply depend on or embed Diego Nehab's C implementation RPly, which also doesn't support them.
• Of around 10 other implementations I could find, only 2 (including trimesh) support 64-bit integers, and only trimesh supports 16-bit floats. The most popular libraries do not seem to support these extensions.

Based on that, I'm not especially inclined to accept the extension at this time, but if you have more information or context to share, please do. In particular, if PLY files with these nonstandard property types are becoming more common, that would be good to know, as well as where they are coming from.