Consider TTML feature(s) for rw/rh <length> units

[spoeschel]

Currently the rw and rh units of the <length> expression are not covered by a TTML feature, see w3c/ttml2#718.

When this feature(s) has/have been added, it has also to be considered in the list of supported features/extensions.

[palemieux]

The feature #length-root-container-relative has been added to TTML2, and should be prohibited in IMSC 1.1, lacking any requirement.

[nigelmegitt]

I'm a bit surprised by this disposition. When we discussed rw and rh units there seemed to be a strong consensus that they were what most people actually wanted instead of %, in several places anyway, especially tts:fontSize.

I would be in favour of adding it to IMSC1.1 because I think it is likely to resolve the confusion that often arises by computation of % values relative to a parent element that also specifies a % size.

[css-meeting-bot]

The Working Group just discussed Consider TTML feature(s) for rw/rh <length> units imsc#372, and agreed to the following:

• SUMMARY: Feature discussed, different viewpoints currently remain, requirements issue raised.

The full IRC log of that discussion

<nigel> Topic: Consider TTML feature(s) for rw/rh <length> units imsc#372
<nigel> github: https://github.com//issues/372
<nigel> Nigel: My understanding of rw and rh was that they are beneficial especially with fontSize.
<nigel> Pierre: Today you can achieve the same effect using `c`. It's not pretty but you can achieve
<nigel> .. that effect.
<nigel> .. We have discussed those requirements for many months and nobody has suggested we
<nigel> .. add them. I think it is in the "too late" category. I'd be more sympathetic if there were no
<nigel> .. other way to achieve it.
<nigel> Nigel: Why are we too late?
<nigel> Pierre: We are about to go to CR2 and there's a cost for supporting container related dimensions.
<nigel> Nigel: A syntactic cost, given that the semantic is already feasible.
<nigel> Cyril: It's not because it's been there for months that we cannot change it. We have to
<nigel> .. acknowledge we have mostly been working on TTML2 to finish it, so we should give
<nigel> .. ourselves some quality time to look again at the requirements for IMSC 1.1.
<nigel> Nigel: +1 My time has been taken up by TTML2 mainly and I would like that opportunity.
<nigel> Pierre: There's a cost to implementing it.
<nigel> .. So far nobody has made the argument why it is required. Don't get me wrong, I think
<nigel> .. it might be useful, but just nobody has explained why it is required.
<nigel> Nigel: From my perspective the requirement is driven by the errors that I see when QAing
<nigel> .. TTML documents that I see, where people get the % calculations wrong.
<nigel> Pierre: I don't expect anyone to adopt it.
<nigel> Nigel: If it is absent from IMSC 1.1 then hypothetically EBU could not adopt it say in EBU-TT-D.
<nigel> Pierre: If there were a liaison from EBU saying they wanted it then it would make the argument,
<nigel> .. but there is not one.
<nigel> Nigel: I'm making this comment from my own experience, that c units cause difficulties
<nigel> .. and we should move towards rw and rh.
<nigel> Cyril: I don't want to take a closed decision now - I need time to think about this.
<nigel> Pierre: If this is needed then it should be raised as an issue against the requirements.
<nigel> Nigel: Arguably Stefan has raised this issue on the wrong repo.
<nigel> Pierre: Cyril, if Netflix is interested in adding rw and rh I ask that you do it super soon so
<nigel> .. we can get it in and get it done.
<nigel> Cyril: I don't think we need it but I will cross check today or tomorrow.
<nigel> Pierre: Super. Nigel, if EBU has IMSC 1.1 adoption on its roadmap and has strong feelings
<nigel> .. about rw and rh it would be good to know.
<nigel> Nigel: I am not expecting any such statement.
<nigel> Pierre: Or from anyone who plans to use IMSC 1.1 and has substantive input, now is the time.
<nigel> Nigel: I've raised w3c/imsc-vnext-reqs#33 for this.
<nigel> SUMMARY: Feature discussed, different viewpoints currently remain, requirements issue raised.

[palemieux]

See w3c/imsc-vnext-reqs#33