Re: MPEG-2 TS Closed Caption mapping

On 5/18/14, 8:31 PM, "Silvia Pfeiffer" <silviapfeiffer1@gmail.com> wrote:

>On Wed, May 14, 2014 at 11:26 PM, Bob Lund <B.Lund@cablelabs.com> wrote:
>>
>>
>>> -----Original Message-----
>>> From: Silvia Pfeiffer [mailto:silviapfeiffer1@gmail.com]
>>> Sent: Wednesday, May 14, 2014 3:19 AM
>>> To: Ladd, Pat
>>> Cc: Clift, Graham; Bob Lund; public-inbandtracks@w3.org
>>> Subject: Re: MPEG-2 TS Closed Caption mapping
>>>
>>> On Tue, May 13, 2014 at 7:13 AM, Ladd, Pat
>>> <Pat_Ladd2@cable.comcast.com> wrote:
>>> > Graham;
>>> >
>>> >     I took a different meaning from your question.  In
>>> 
>>>http://rawgit.com/silviapfeiffer/HTMLSourcingInbandTracks/master/index.h
>>> tml section 3, the table in step 3 indicates that when a Caption
>>>Service
>>> Descriptor is signaled in a PMT a TextTrack is created with kind
>>>"captions" and
>>> in the case of captions digitized in the video stream, i.e. CEA708,
>>>the id is the
>>> PID of the video elementary stream carrying the captions.
>>>
>>> Correct.
>>>
>>> >  What I gather from that is setting the captions TextTrack mode to
>>>showing
>>> will cause the captions in the video stream to be rendered.
>>>
>>> Incorrect. They can only be rendered by the browser if the browser
>>> understands the caption format. For example, WebM has captions in
>>> WebVTT format - they will be exposed as a TextTrack with VTTCue
>>>captions
>>> and thus follow the WebVTT rendering spec. If your captions come from a
>>> MPEG4 file in a CEA708 track, a TextTrack can be created, but the
>>>captions
>>> cannot be exposed in a renderable form, since there is no TextTrackCue
>>> format for CEA708.
>>
>> If the captions are to be rendered ONLY by the UA then it would seem
>>there is no need to define a cue format. It would be implementation
>>dependent how the UA formats caption data for rendering.
>
>Are you thinking of a situation where browsers will render caption
>tracks, but not expose them to the JS API?

Yes.

> In this situation, how does
>the Web developer know that there is a caption track visible?

A caption text track will be created. The app can check the mode of that
track. If Œshowingı then the UA is rendering the captions. If hidden, then
the UA is not rendering.

> This is
>as "bad" as burnt-in captions

Not sure what you mean by Œas bad ası.

> and would better be exposed to the
>browser API in that way: as a video track with burnt-in captions (a
>VideoTrack with VideoTrack.kind="captions"). There would be no need to
>expose any text track for this.

AFAICT, there is no kind for burned in captions, which might be
main-captions?

>
>Silvia.

Received on Monday, 19 May 2014 14:49:07 UTC