[Bug 26927] [InbandTracks] MPEG-2 TS Mapping

https://www.w3.org/Bugs/Public/show_bug.cgi?id=26927

--- Comment #2 from Bob Lund <b.lund@cablelabs.com> ---
(In reply to Bob Lund from comment #1)
> (In reply to Cyril Concolato from comment #0)
> > The MPEG-2 TS section has some problems:
> > 
> > That sentence:
> > "The order in which elementary streams are listed in the "Program Map Table"
> > (PMT) of a MPEG-2 TS is maintained when sourcing multiple MPEG-2 tracks into
> > HTML."
> > should be rewritten using normative statements
> 
> No, per agreement with [1]
> 
> > and should indicate what
> > happens when PMT changes occur, as follows:
> > "UA shall expose elementary streams as HTML Tracks in the order of the
> > "Program Map Table" (PMT) of a MPEG-2 TS. UA shall trigger addtrack or
> > removetrack events when PMT changes are detected."
> 
> Agreed.
> 
> > 
> > That other sentence:
> > "A user agent recognises and supports data from a MPEG-2 TS resource as
> > being equivalent to a HTML track based on the value of the 'stream_id' field
> > of an elementary stream as given in a Transport or Program Stream header and
> > which maps to a "stream type":"
> > It refers to 'stream_id' or 'stream type'. It is unclear if those are the
> > MPEG-2 TS 'PID', 'stream_type', 'PES stream_id' ... or if they are new terms
> > introduced in this text. I think we should also clearly indicate that
> > Program Streams are out-of-scope.
> 
> This wording was confusing and will be changed to "A user agent recognizes
> and supports data in an MPEG-2 TS elementary stream identified by the
> 'elementary_PID' field in the Program Map Table as being equivalent to an
> HTML track based on the value of the 'stream_type' field associated with
> that 'elementary_PID'" 
> > 
> > Note also that the stream_type 0x02 is mapped twice: as a TextTrack and as a
> > VideoTrack.
> 
> Are you referring to the caption service in an stream_type 0x02? That is
> only mapping the caption in the video stream, if present, to a text track.
> 
> > 
> > The overall idea is also not very clear:
> > - Does a UA have to expose all tracks from a TS? all tracks that have
> > characteristics described in the table? I agree it may be desirable from an
> > application point of view but it may be too resource consuming. Maybe we
> > should think of a mechanism to register tracks for which the application
> > would like data to be exposed, a bit like addSourceBuffer
> 
> Bug 26893 was submitted to address this. TextTracks are created with mode =
> "disabled". No UA resources, beyond creating the track are consumed until
> the app changes the mode.
> 
> > - Why would a UA expose data as a VideoTrack if it does not support it for
> > rendering, e.g. ISO/IEC 14496-2 ? It should rather expose it as a TextTrack.
> > So I think if we should if the data is supported, then it shall be exposed
> > as VideoTrack otherwise it may be exposed as a TextTrack.
> 
> It is unclear what the specific problem is.
> 
> 
> [1] http://lists.w3.org/Archives/Public/public-html-admin/2014Jun/0050.html

Fixed with PR #39 https://github.com/w3c/HTMLSourcingInbandTracks/pull/39

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Received on Wednesday, 5 November 2014 15:54:33 UTC