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RE: [media] Moving forward with captions / subtitles

From: Geoff Freed <geoff_freed@wgbh.org>
Date: Sat, 13 Feb 2010 18:17:39 -0500
To: Eric Carlson <eric.carlson@apple.com>, Philip Jägenstedt <philipj@opera.com>
CC: Silvia Pfeiffer <silviapfeiffer1@gmail.com>, HTML Accessibility Task Force <public-html-a11y@w3.org>
Message-ID: <B3526F4AC3C3C64388BF661A8B2112A75EE20ABB82@EXCHCCR.wgbh.org>

My comments in-line at the end.

Geoff/WGBH

________________________________________
From: public-html-a11y-request@w3.org [public-html-a11y-request@w3.org] On Behalf Of Eric Carlson [eric.carlson@apple.com]
Sent: Saturday, February 13, 2010 11:42 AM
To: Philip Jägenstedt
Cc: Silvia Pfeiffer; HTML Accessibility Task Force
Subject: Re: [media] Moving forward with captions / subtitles

<SNIP>

Yes, but is it expected that the UA should do something with the attribute, like make context menus based on it? Or should it be part of the track selection algorithm? (Where "track selection algorithm" does not exist yet, but is what will select which tracks are enabled by default based on... language and such?)

  I think the selection of alternates is an important point. Some media container formats (eg. QuickTime and MPEG-4)  allow an author to mark tracks as begin part of an "alternate group". This instructs the media engine to enable only one track in the group based on a condition on the user's machine when the file is opened for playback. For example, a movie can have subtitle tracks and chapter tracks in multiple languages, but only one of each is rendered when the movie plays.

  We need to support this use case with external "tracks", and we need to define the selection algorithm when a file has both internal and external tracks.

  We also need to define a mechanism to mark tracks as being part of an alternate group. Is an attribute on <source> enough?

    <tracks>
        <source type="text/srt" src="en-captions.srt" lang="en" role="caption">
        <source type="text/srt" src="zh-captions.srt" lang="zh" role="caption">

        <source type="text/srt" src="en-chapters.srt" lang="en" role="chapters">
        <source type="text/srt" src="zh-chapters.srt" lang="zh" role="chapters">
    </tracks>

Or should we have a grouping element like Silvia had in her early proposal?

    <tracks>
        <track role="caption">
            <source type="text/srt" src="en-captions.srt" lang="en">
            <source type="text/srt" src="zh-captions.srt" lang="zh">
        </track>

        <track role="chapters">
            <source type="text/srt" src="en-chapters.srt" lang="en">
            <source type="text/srt" src="zh-chapters.srt" lang="zh">
        </track>
    </tracks>

  I hesitate to define yet another element, but I think the markup in a complex case like Silvia's Elephants Dream sample, http://annodex.net/~silvia/itext/elephant_no_skin_v2.html, is clearer because of it. On the other hand, will complex cases like this be common enough that we need it?

GF:  I prefer the latter example.  There will definitely be cases like the one Silvia proposed, and those that are even more complex.  Movies that contain caption tracks, foreign-language subtitle tracks and description tracks are not common now, but there is a growing demand for them.  Add to these tracks the other features the group is discussing, like karoke, chapters, etc., and it becomes obvious that we need to have a way to describe them as thoroughly as possible.  In this sense, grouping them with an attribute like role ="" makes good sense to me.
Received on Saturday, 13 February 2010 23:18:33 GMT

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