Re: Resolving TextTrackCue issues

On Wed, Sep 4, 2013 at 9:19 AM, Eric Carlson <> wrote:
>> Do you expose the existance of these in-band captions somehow to JS?
>> I'm concerned that if the browser renders captions automatically on
>> top of video without the JS developer being able to find out about it,
>> how would the JS developer know that there are captions and to avoid
>> rendering another lot themselves - or rendering something else in the
>> space of the captions?
>   On versions of the OS where it is possible for WebKit to "take over"
> rendering of in-band captions from the media engine, they behave just like
> out-of-band tracks: in-band tracks are part of the video.textTracks and the
> cues are part of track.cues/activeCues (when appropriate).

So for the JS dev they are exposed as instances of the old
TextTrackCue interface? The VTTCue interface sufficiently satisfies
this use case then? What is the content in .text ?

>   On versions of the OS where the system frameworks do not have the
> necessary API to override cue rendering, in-band tracks are part of
> video.tracks so they can be enabled/disabled by script but cues are rendered
> by the media engine.

In this case, I assume only the existence of the track, but not of the
cues is exposed to JS? I.e. track.cues/activeCues is empty? Or are you
listing fully-abstract TextTrackCue instances here to at least provide
starttime/endtime to the JS devs?

And an orthogonal question: you've probably seen the Cablelabs spec
for exposing MPEG-2 in-band text tracks of different types to HTML[1].
Seeing as both Cablelabs and the HTML spec are trying to accommodate
using in-band text tracks in HTML/JS, what do you suggest is the best
way forward to specify this?

* Program Map Table: would you suggest to use VTTCue with
@kind=metadata, GenericCue, or a new PMTCue interface?

* CEA708 track: assuming we don't want to introduce CEA708Cue, how
would that best be supported?

Thanks for helping us work through this!



Received on Wednesday, 4 September 2013 00:17:15 UTC