Re: MPEG-2 TS Closed Caption mapping

On Wed, May 28, 2014 at 12:06 AM, Bob Lund <B.Lund@cablelabs.com> wrote:
>
>
>> -----Original Message-----
>> From: Silvia Pfeiffer [mailto:silviapfeiffer1@gmail.com]
>> Sent: Monday, May 26, 2014 7:44 AM
>> To: Bob Lund
>> Cc: Ladd, Pat; Clift, Graham; public-inbandtracks@w3.org
>> Subject: Re: MPEG-2 TS Closed Caption mapping
>>
>> On Tue, May 20, 2014 at 12:48 AM, Bob Lund <B.Lund@cablelabs.com>
>> wrote:
>> >
>> >
>> > 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/in
>> d
>> >>>>ex.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 Œshowing¹ then the UA is rendering the captions. If
>> > hidden, then the UA is not rendering.
>
> Yes
>
>>
>> So you *are* exposing the track to JS - but without cues? You are expecting
>> the browser to react to JS changing the track @mode.
>
> Yes
>
>>
>> You are expecting it to render CEA708 cues when this empty track is made
>> 'showing'. But you are not expecting to write a spec for browsers for how to
>> do the rendering? Are you expecting them to implement a
>> CEA708 spec?
>
> Yes
>
>  >How is that going to fit into the browser rendering loop and the
>> browser coodinate system? You're going to need a spec.
>
> Why isn't this an UA  implementation issue? The reference implementation we did created WebVTT cues from the raw 708 captions data.

Because if you leave it to the browsers to decide how to support
CEA708, every browser will come up with a different solution.

Silvia.

Received on Tuesday, 27 May 2014 23:40:15 UTC