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

From: John Birch <John.Birch@screensystems.tv>
Date: Thu, 27 Sep 2012 08:22:52 +0000
To: Timed Text Working Group <public-tt@w3.org>
Message-ID: <0981DC6F684DE44FBE8E2602C456E8AB0101DE2A@SS-IP-EXMB-01.screensystems.tv>
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?

John Birch

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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]


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

Bounding SDP-US rendering complexity


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.


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


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 08:23:18 UTC

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