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Re: New Change Proposal 28 on IMSC 1: Profile refactoring

From: Pierre-Anthony Lemieux <pal@sandflow.com>
Date: Thu, 14 Aug 2014 14:57:32 +0200
Message-ID: <CAF_7JxDBr7g6KfALDsWh6KF3Z7Sxr7SZrmTgk=40JF0wGNzxVw@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: "public-tt@w3.org" <public-tt@w3.org>
Hi Nigel,

Thanks for capturing your thoughts in the change proposal.

I have the following observations:

[*] constraints on document complexity are key to the IMSC Text and
Image profiles: without such constraints, it is not possible to ensure
rendering of a document across implementations.

It is therefore important that the HRM and associated performance
parameters remain part of the existing Text and Image profiles. I also
see no rationale for changing the names of the two profiles.

If there is interest, a set of users may wish to propose a separate,
additional unconstrained profile based on the Text Profile, e.g.
"Unconstrained Text Profile".

[ed.: I personally believe that a distribution profile without
document complexity constraint is much less valuable, based on my
experience in D-Cinema.]

[*] the Change Proposal states "the key constraint in broadcast
environments is often delivery bit rate rather than document
complexity".

While delivery bit rate might have been the ultimate constraint with
teletext and 608, it is not directly applicable to TTML AFAIK. The HRM
proposes a complexity model for TTML documents.

Also, I would think a more practical constraint (in all environments)
is probably the audience reading rate (e.g. 180 words/min). Assuming:

- no glyph is ever repeated across ISDs (worst case)
- the height of characters are 1/24 the height of the root container
(24*40 grid)
- no CJK characters

The default HRM performance parameters allow an average character rate
per second M equal to

M = 1/ (time to render one character)
    = Ren(Gi) / NRGA(Gi)
    = 1.2/(1/24)^2
    = 691 characters per second.

The default values can be overridden by specific applications if needs be.

[*] In general, I think the group needs more specific use cases for
such an additional profiles before undertaking major changes to the
specification.

For instance, is the intent for others to define their own HRM, to
eschew HRMs altogether or merely define different performance
parameters?

Best,

-- Pierre

On Fri, Aug 8, 2014 at 3:04 PM, Nigel Megitt <nigel.megitt@bbc.co.uk> wrote:
> I’ve added a new Change Proposal to resolve issue-307, issue-315, issue-319
> and assist with issue-332:
>
> Change Proposal 28,
> Priority 2 (for LC),
> Product IMSC 1
> Title “Profile refactoring”
> URL: https://www.w3.org/wiki/TTML/changeProposal028
>
> Summary: IMSC 1 currently defines two profiles, a Text and an Image profile,
> both of which include the requirement to meet the complexity constraints
> expressed in the HRM. This proposal is to factor out the complexity
> constraints from the content profiles, while providing a clearly
> identifiable profile that continues to match what implementors of, say,
> CFF-TT, have already built. A third profile not constrained in complexity is
> proposed.
>
> Nigel
>
Received on Thursday, 14 August 2014 12:58:26 UTC

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