[ttml2] Clarify that tts:luminanceGain shall not apply unless absolute HDR scheme used

nigelmegitt has just created a new issue for https://github.com/w3c/ttml2:

== Clarify that tts:luminanceGain shall not apply unless absolute HDR scheme used ==
It is not entirely clear that `tts:luminanceGain` must be ignored if the HDR scheme being used does not require absolute luminance values. In practice it would be actively dangerous to apply `tts:luminanceGain` for scene relative HDR schemes, but there is a use case where content providers want to specify `tts:luminanceGain` but have it only apply when, for example, PQ is used, and not apply for HLG, say.

I propose that we add a clarifying normative statement to back up the introductory clause: _, and when that presentation requires an absolute luminance value._ to make this a shall/shall not.

Please view or discuss this issue at https://github.com/w3c/ttml2/issues/796 using your GitHub account

Received on Friday, 25 May 2018 14:44:56 UTC