Re: TTML2/TTML1 Backwards compatibility analysis

Thank you Mike for the feedback. See my comments inline.

On Wed, Oct 18, 2017 at 8:52 AM, Michael Dolan <mike@dolan.tv> wrote:

> This is definitely a good exercise – thanks Cyril.  A few comments:
>
>
>    1. The analysis is not clear about whether it covers documents,
>    processors (in general) or presentation processors. The concern is about
>    presentation processors, which is complicated by explicit potential for
>    variations (available fonts, etc.). Some qualifications are needed of the
>    form “all allowed variations being equal”;
>
> In this exercise I was concerned with how a TTML1 document (not containing
any TTML2 feature) would be rendered by a TTML2 presentation processor, in
particular if the rendering would be an acceptable results according to
TTML1. So this includes all variations allowed per TTML1.


>
>    1.
>    2. Our editor came to a different conclusion (there are compatibility
>    issues) with specific examples on a recent call, so we need to resolve
>    that. Perhaps these are captured in the orange highlight (or should I say
>    #FFA500 😊; and
>
> From the 10/12 call, I can see:
- lineHeight. Covered in my document.
- displayAspectRatio, but it is a new TTML2 feature, I'm assuming it was
meant to be pixelAspectRatio. Covered in my document.
Have I missed anything?

I'm concerned about:
- Pierre's words:
"basic things like lineHeight style inheritance."
I did not see the changes in TTML2 that would affect style inheritance. I
may have missed it, so if anyone could point to those differences, it'd be
very useful.

- Glenn's words:
"There are layers that affect every feature - it's not simple than just
talking about
... individual style features."
I'd like to understand what this means. I understand that there are general
aspects that affect every feature (like style resolution) or relationships
between style features (like DAR vs. PAR) but I didn't see "layers that
affect every feature" differently between TTML1 and TTML2.
Glenn, can you clarify? or give an example?



>    1.
>    2. Without consensus of an explicit stated goal of presentation
>    processor backwards compatibility, it doesn’t mean it won’t break before
>    publication as a Rec or be unintentionally vague; But we can’t seem to
>    bring ourselves to make such a statement for some reason despite an
>    agreement in principle 2 meetings ago to do so (and my assignment to
>    propose TTML2 spec language). But maybe this exercise will make everyone
>    more comfortable doing that.
>
> Hopefully that should help the discussion stay focused and avoid
discussing for hours broad statements.

Regards,
Cyril

>
>    1.
>
>
>
> Regards,
>
>                Mike
>
>
>
> *From:* Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
> *Sent:* Tuesday, October 17, 2017 10:00 AM
> *To:* Cyril Concolato <cconcolato@netflix.com>; public-tt@w3.org
> *Subject:* Re: TTML2/TTML1 Backwards compatibility analysis
>
>
>
> Thanks Cyril,
>
>
>
> I think it's very useful to focus on the concrete differences rather than
> the abstract – we may find that we can resolve them.
>
>
>
> I've added this (practical compatibility issues between TTML1 and TTML2)
> to Thursday's agenda.
>
>
>
> The URL in your email had an error in: the correct one should be
> https://docs.google.com/document/d/1Ri7RBBsbIK9SRxA1KsHRejXbYBuL4
> CRRrbmEZRbwZpg/edit?usp=sharing
>
>
>
> In the meantime if everyone interested in this could look at the document
> and comment/edit it etc that would be very helpful.
>
>
>
> Kind regards,
>
>
>
> Nigel
>
>
>
>
>
> *From: *Cyril Concolato <cconcolato@netflix.com>
> *Date: *Saturday, 14 October 2017 at 01:09
> *To: *"public-tt@w3.org" <public-tt@w3.org>
> *Subject: *TTML2/TTML1 Backwards compatibility analysis
> *Resent-From: *<public-tt@w3.org>
> *Resent-Date: *Saturday, 14 October 2017 at 01:11
>
>
>
> Hi all,
>
>
>
> Following yesterday's call, I started an analysis of the possible
> backwards compatibility issues of TTML2 vs TTML1. The results are here:
>
> https://docs.google.com/document/d/1Ri7RBBsbIK9SRxA1KsHRejXbYBuL4
> CRRrbmEZRbwZpg/edit?usp=sharing
> <https://docs..google.com/document/d/1Ri7RBBsbIK9SRxA1KsHRejXbYBuL4CRRrbmEZRbwZpg/edit?usp=sharing>
>
>
>
> This is my analysis, and it might contain errors, oversights. If it is the
> case, feel free to comment on it.
>
>
>
> With the current status, it looks to me that there is no real backwards
> compatibility issue, in the sense that a TTML2 processor would produce a
> result, when processing a TTML1 document, that would be acceptable with
> what the TTML1 spec indicates.
>
>
>
> HTH,
>
> Cyril
>
>
>
>
>
>
>
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