RE: TTML2 backwards compatibility with TTML1

Apologies.  (I keep trying not to send email before my first coffee, but....FYI, I was separately emailing with another colleague about that one and crossed up the issues.)

I am mostly reacting to the recent discussion on this issue https://github.com/w3c/ttml2/issues/331

I missed that it was deprecated in the first place, but why is restoring this even a debate?  I.e. see below about my understanding of a guiding principle. If we in fact all agree with that, then this issue should just be accepted and closed without further discussion.

                Mike

From: Nigel Megitt [mailto:nigel.megitt@bbc.co.uk]
Sent: Wednesday, May 31, 2017 8:26 AM
To: Michael Dolan <mike@dolan.tv>; public-tt@w3.org
Subject: Re: TTML2 backwards compatibility with TTML1

Hi Mike,

The link you provided is an IMSC issue not a TTML2 issue. I'm a bit confused - perhaps there's another example where a TTML2 constraint is being introduced that would mean a TTML2 processor would not process a TTML1 document correctly?

Kind regards,

Nigel


From: Michael Dolan <mike@dolan.tv<mailto:mike@dolan.tv>>
Date: Wednesday, 31 May 2017 at 16:11
To: "public-tt@w3.org<mailto:public-tt@w3.org>" <public-tt@w3.org<mailto:public-tt@w3.org>>
Subject: TTML2 backwards compatibility with TTML1
Resent-From: <public-tt@w3.org<mailto:public-tt@w3.org>>
Resent-Date: Wednesday, 31 May 2017 at 16:12

I thought we had a (perhaps implicit and unspoken) requirement that TTML2 would be backwards compatible with TTML1.  That is, all conformant TTML1 instance documents would also be conformant TTML2 instance documents.

However, we are making decisions that are at odds with the above, e.g. this week's change to constrain origin and extent in issue 239 and its PR: https://github.com/w3c/imsc/pull/240  There are other examples.

I think we need to all be on the same page about the above requirement.

My personal view is that unless something is really broken and unusable/untestable from TTML1, that this requirement should hold.

Perhaps we can discuss this tomorrow before committing any more changes that are at odds with it?

                Mike

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Michael A DOLAN
TBT, Inc;  PO Box 190
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