OK, thank you for the information !
I'd mistakenly thought h.264 10bit HDR was now "a thing", so I will avoid that and stick with h.265 HDR.
As you both suggest, I'll start looking into the meanings of master-display and colorprim and max-cll and read these docs
http://x265.readthedocs.io/en/defaul...master-display
https://matroska.org/technical/specs/index.html#Colour
and at the info produced by MediaInfo, mkvinfo and ffprobe.
Thank you, now I have a useful place to start, a lot better than aimless wandering