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RE: ISSUE-188 (render-complexity): Bounding SDP-US rendering complexity [Simple Delivery Profile for Closed Captions]

From: Michael A Dolan <mdolan@newtbt.com>
Date: Thu, 27 Sep 2012 13:15:12 -0700
To: "'Timed Text Working Group'" <public-tt@w3.org>
Message-ID: <067101cd9cec$cf74b6c0$6e5e2440$@newtbt.com>
John-

Although CFF-TT does not explicitly address incremental additions to the region, that does not mean the model does not apply.  As drafted, it just takes the time events as a full re-rendering.  A simplification, yes; but it is incorrect to say that incremental flow ("paint-on") is not supported.  The model is therefore a constraint on the complexity of the Intermediate Synchronic Documents, not the authored document. And it is definitely not a constraints the decoder - it can do whatever it wants for efficiency.

I've started a discussion in DECE about the interest in making the model more complex to explicitly deal with incremental additions. My guess is that it will not be worth the effort.  And, decoders can always implement whatever efficiencies that they want.

There is a question that regions scroll at all.  If they do, the behavior in TTML 1.0 needs a good deal of work, and the same rendering model would apply as for paint-on described above.  If not, that would be irrelevant to the CFF-TT (or any) rendering model.  Hence the new issue 189.

Regards,

	Mike

-----Original Message-----
From: John Birch [mailto:John.Birch@screensystems.tv] 
Sent: Thursday, September 27, 2012 1:23 AM
To: Timed Text Working Group
Subject: RE: ISSUE-188 (render-complexity): Bounding SDP-US rendering complexity [Simple Delivery Profile for Closed Captions]

On a quick inspection, the CFF-TT rendering model does not appear to support Paint on or Roll on (cumulative) subtitles, as every Subtitle Event causes a clear of the subtitle plane root container?
Certainly a Paint on / Roll On effect could be emulated by resending the previous caption content already 'assumed' to be displayed (although note what is currently 'on screen' does depend on when the caption stream was acquired)... but such a repetitious approach would be markedly inefficient!
Is this not a fundamental limitation for using the CFF model in SDP-US?

Regards,
John Birch

John Birch | Screen Systems | Strategic Partnerships Manager Main Line : +44 1473 831700 | Ext : 270 | Direct Dial : +44 1473 834532 Mobile : +44 7919 558380 | Fax : +44 1473 830078 John.Birch@screensystems.tv | www.screensystems.tv | http://twitter.com/ScreenSubtitles

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P Before printing, think about the environment-----Original Message-----
From: Timed Text Working Group Issue Tracker [mailto:sysbot+tracker@w3.org]
Sent: 26 September 2012 21:16
To: public-tt@w3.org
Subject: ISSUE-188 (render-complexity): Bounding SDP-US rendering complexity [Simple Delivery Profile for Closed Captions]

ISSUE-188 (render-complexity): Bounding SDP-US rendering complexity [Simple Delivery Profile for Closed Captions]

http://www.w3.org/AudioVideo/TT/tracker/issues/188

Raised by: Pierre-Anthony Lemieux
On product: Simple Delivery Profile for Closed Captions

Bounding SDP-US rendering complexity
====================================

What
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SDP-US is a profile of TTML that specifies constraints such as supported TTML features and number of regions active at any given time. It does not however impose bounds on key aspects of rendering complexity, such as character and background drawing rates. Without such bounds, a valid SDP-US document might not successfully play on all implementations or, equivalently, determining the processing requirements of an implementation is not possible.

CFF-TT is a profile of TTML developed by the DECE consortium (http://uvvu.com) for internet delivery of subtitles and captions. Consumer devices implementing CFF-TT are expected to be widely deployed. The CFF-TT specification is publicly available at http://uvvu.com/docs/public/tspec/CFFMediaFormat-1.0.4.pdf.

As with SDP-US, CFF-TT specifies supported TTML features -- largely a superset of the features supported by SDP-US. To further simplify implementation and improve interoperability, CFF-TT also imposes bounds on rendering complexity through the use of an hypothetical rendering model.

SDP-US should consider adopting, a subset of or in its entirety, the rendering complexity bounds (and rendering model) defined by CFF-TT.

Why
---

Such adoption would futher:
        - simplify implementations and improve interoperability by bounding rendering (and thus document) complexity
        - encourage adoption of SDP-US and TTML by ensuring that SDP-US content can be played on any CFF-compliant CE device

How
---

Adopting the CFF-TT hypothetical renderer and bounds on document complexity could be achieved in a number of ways, including:

(a) mapping the CFF-TT rendering model to the existing (XSL-based) TTML rendering model
(b) referencing the relevant sections of the CFF-TT specification defining the CFF-TT rendering model
(c) importing the CFF-TT rendering model into the SDP-US specification




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Received on Thursday, 27 September 2012 20:15:49 UTC

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