[Webkit-unassigned] [Bug 110479] [Meta] Implement support for TTML

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=110479





--- Comment #23 from Glenn Adams <glenn at skynav.com>  2013-02-25 11:27:51 PST ---
(In reply to comment #21)
> TTML's spec says: "It is intended to be used for the purpose of transcoding or exchanging timed text information among legacy distribution content formats presently in use for subtitling and captioning functions."

Just so there's no misunderstanding, TTML as published is definitely intended to be used as a distribution format. Indeed, it was previously named DFXP (Distribution Format Exchange Profile) to distinguish it from a more ambitious, but never fully specified AFXP (Authoring Format Exchange Profile).

The paragraph just following the one you cite above states:

"In addition to being used for interchange among legacy distribution content formats, TTML content may be used directly as a distribution format..."

The System Model Diagram [1], makes it clear that DFXP (TTML) is intended for direct distribution.

[1] http://www.w3.org/TR/2010/REC-ttaf1-dfxp-20101118/#model-graphic

Historically, the TTWG started defining a more ambitious authoring format (AFXP), but eventually chose to focus on those features that gave priority to direct distribution, a process which eventually gave birth to DFXP, later relabeled as TTML.

It is certainly true,  however, that TTML and WebVTT have different characteristics and efficiencies for use in direct distribution. For example, a full streaming model for TTML was never formalized; but only suggested in Annex L [2]. Some of the more recently defined profiles of TTML, e.g., SDP-US, have added constraints that facilitate streaming use cases.

[2] http://www.w3.org/TR/2010/REC-ttaf1-dfxp-20101118/#streaming

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