Re: MPEG-2 TS Closed Caption mapping

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? In this situation, how does
the Web developer know that there is a caption track visible? This is
as "bad" as burnt-in captions 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.

Silvia.

Received on Monday, 19 May 2014 02:32:08 UTC