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

From: Bob Lund <B.Lund@CableLabs.com>
Date: Mon, 13 Oct 2014 15:15:00 +0000
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>, "WHAT Working Group Mailing List" <whatwg@whatwg.org>
Message-ID: <D0614769.47603%b.lund@cablelabs.com>

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
>, 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
>* 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.

>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
Received on Monday, 13 October 2014 15:15:37 UTC

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