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Re: [whatwg] How to expose caption tracks without TextTrackCues

From: Elliott Sprehn <esprehn@chromium.org>
Date: Mon, 13 Oct 2014 13:45:45 -0400
Message-ID: <CAO9Q3i+MVYS7=YVeXcsbWn+T6FmcPH6Cz8E6yWdz2BOSNPe1hA@mail.gmail.com>
To: Bob Lund <B.Lund@cablelabs.com>
Cc: Silvia Pfeiffer <silviapfeiffer1@gmail.com>, WHAT Working Group Mailing List <whatwg@whatwg.org>
What does "UA rendered" mean? How does the UA render it? Can the UA just
convert the format into WebVTT instead?

On Mon, Oct 13, 2014 at 11:15 AM, Bob Lund <B.Lund@cablelabs.com> wrote:

>
>
> On 10/12/14, 3:45 AM, "Silvia Pfeiffer" <silviapfeiffer1@gmail.com> wrote:
>
> >Hi all,
> >
> >In the Inband Text Tracks Community Group we've recently had a
> >discussion about a proposal by HbbTV. I'd like to bring it up here to
> >get some opinions on how to resolve the issue.
> >
> >(The discussion thread is at
> >http://lists.w3.org/Archives/Public/public-inbandtracks/2014Sep/0008.html
> >, but let me summarize it here, because it's a bit spread out.)
> >
> >The proposed use case is as follows:
> >* there are MPEG-2 files that have an audio, a video and several caption
> >tracks
> >* the caption tracks are not in WebVTT format but in formats that
> >existing Digital TV receivers are already capable of decoding and
> >displaying (e.g. CEA708, DVB-T, DVB-S, TTML)
> >* there is no intention to standardize a TextTrackCue format for those
> >other formats (statements are: there are too many formats to deal
> >with, a set-top-box won't need access to cues)
> >
> >The request was to expose such caption tracks as textTracks:
> >interface HTMLMediaElement : HTMLElement {
> >...
> >  readonly attribute TextTrackList textTracks;
> >...
> >}
> >
> >Then, the TextTrack interface would list them as a kind="captions",
> >but without any cues, since they're not exposed. This then allows
> >turning the caption tracks on/off via JavaScript. However, for
> >JavaScript it is indistinguishable from a text track that has no
> >captions. So the suggestion was to introduce a new kind="UARendered".
>
> A clarification - the suggestion was for a new TextTrack.mode value of
> ³UARendered² and for this type of TextTrack the only valid modes would be
> ³UARendered² and ³disabled². The ³hidden² and ³showing² modes would not be
> allowed since no Cues are generated. @kind would continue to denote the
> type of TextTrack.
>
> Bob
> >
> >
> >My suggestion was to instead treat such tracks as burnt-in video
> >tracks (by combination with the main video track):
> >interface HTMLMediaElement : HTMLElement {
> >...
> >
> >readonly attribute VideoTrackList videoTracks;
> >...
> >}
> >
> >Using the VideoTrack interface it would list them as a kind="captions"
> >and would thus also be able to be activated by JavaScript. The
> >downside would that if you have N video tracks and m caption tracks in
> >the media file, you'd have to expose NxM videoTracks in the interface.
> >
> >
> >So, given this, should we introduce a kind="UARendered" or expose such
> >tracks a videoTracks or is there another solution that we're
> >overlooking?
> >
> >Silvia.
>
>
Received on Monday, 13 October 2014 17:46:55 UTC

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