> If I want to render this video on screen, say a SDR screen, or my
> player is just not HDR capable (yet), what transfer do I use? The
> value avcodec gives me has not much meaning in that case (because I
> can't deal with HLG yet), and the original "legacy" value meant to be
> used in these cases was overriden and lost.

Uh? The original "legacy" value has no meaning, bt709, bt2020, and the
SDR part of HLG are all the same.
The libavcodec decoder "prefers" using the alternative transfer
because it is the "real one", and ffmpeg offers means to support it
correctly (zscale).
As long as you don't use any odd transfer (like log316) or the ones
that are typically used for SD content, it will render the SDR
transfer correctly.
I believe this is a safe assumption for a software system (like
ffmpeg), less so for a hardware one (like a broadcaster's decoder),
hence the SEI dance.
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