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 �showing� 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 �as bad as�.

> 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