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Re: IMSC 1 re: ISSUE-332, ISSUE-342 and ISSUE-339

From: John Birch <John.Birch@screensystems.tv>
Date: Thu, 11 Sep 2014 07:01:29 +0000
To: "'pal@sandflow.com'" <pal@sandflow.com>, "'public-tt@w3.org'" <public-tt@w3.org>
Message-ID: <0981DC6F684DE44FBE8E2602C456E8AB016C0D6B42@SS-IP-EXMB-01.screensystems.tv>
Re: multiRowAlign and linePadding... I am happy to move directly to:

"their rendering recommended rather than

Re overflow...
Your assumption regarding the ability of an author to (pre)determine necessary region size is unfounded in practise, since it requires an author to have prior knowledge of all fonts and rendering techniques (including kerning and leading) that might be used in all potential devices that render from IMSC text. I am certain from experience that this is impractical and unlikely. Consequently a more likely outcome (if ocerflow remains hidden) is that authors will over size regions which would have potential impact on achieving identical justification or vertical alignment between those devices using different fonts / rendering.

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P Before printing, think about the environment----- Original Message -----
From: Pierre-Anthony Lemieux [mailto:pal@sandflow.com]
Sent: Thursday, September 11, 2014 12:04 AM
To: public-tt@w3.org <public-tt@w3.org>
Subject: IMSC 1 re: ISSUE-332, ISSUE-342 and ISSUE-339

Good morning/evening,

In preparation for our call tomorrow (and meeting next week), below is
my initial take on resolutions to selected outstanding IMSC 1 issues.

Looking forward to the discussion.


-- Pierre

ISSUE-332 #cellResolution support

Background: IMSC 1 currently forbids specifying ttp:cellResolution.

Editor's input: AFAIK, specifying ttp:cellResolution only affects the
calculation of the value of the 'c' metric. Thus no additional HRM
and/or layout engine feature are required.

Proposed resolution: Permit any value of ttp:cellResolution to be specified.

ISSUE-342 Add ebutts:multiRowAlign and ebutts:linePadding attributes
to the IMSC Text Profile

Background: These two features are intended to enable timed-text
styling consistent with European practice (see [1]). These features
require changes to the IMSC 1 layout engine. They features can however
be ignored by a processor and still yields a reasonable presentation.
No changes to the IMSC 1 HRM are necessary.

Editor's input: I am not aware of implementations supporting these features.

Proposed resolution: Add ebutts:multiRowAlign and ebutts:linePadding
to the Text Profile, referencing the EBU document directly. If
interoperability cannot be demonstrated within the CR timeframe, then
the features can be removed or their rendering recommended rather than

[1] Annex C and D at https://tech.ebu.ch/docs/tech/tech3380.pdf

ISSUE-339 Allow the use of #overflow

Background: Per TTML1, tts:overflow is set to 'visible' means "region
composition and layout must be performed as if the region's width and
height were unconstrained". IMSC 1 currently forbids
tts:overflow='visible'. The idea to make sure that the region extent
is deterministic so that (a) the document complexity can be
constrained by the HRM (which uses region extent as one of its
criterion) and (b) authors can have confidence that the document will
be rendered as authored.

Editor's input: Use cases where tts:overflow='visible' provides a
benefit to authors are unclear in my mind: if text overflows during
authoring, the author can simply increase the region's extents, and,
in any event, tts:overflow='visible' does not guarantee that the text
will be shown, e.g. the overflow could overlap/be hidden by other

Proposed resolution: Retain current restriction on #overflow.

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Received on Thursday, 11 September 2014 07:02:02 UTC

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