Re: Proposal for initial CG focus

Hi Bob,

Thanks for starting this discussion.

I've got some comments inline.


On Fri, Nov 1, 2013 at 7:10 AM, Bob Lund <B.Lund@cablelabs.com> wrote:
> Hi,
>
> I'd like to start the discussion about the requirements and scope for the
> CG work.
>
> Currently, AFAIK there is no specification that fully defines how a UA
> should expose in-band tracks to Web applications.  Unspecified details are:
>
> * Which in-band tracks of a media resource the UA will expose.By expose I
> mean make available as VideoTrack, AudioTrack or TextTrack objects.

If the browser doesn't recognize a track as a Video or Audio track, it
has no choice but to expose it as a Text track, if at all. So, I think
we're really only talking about text tracks here, right?


> * How metadata in the media resource about the in-band tracks is made
> available to Web applications. If the UA recognizes the in-band track,
> some of the metadata associated with the track will be made available by
> the "kind", "language" and "inbandMetadataTrackDispatchType"  attributes.
> This works fine when the UA fully recognizes the track and when the
> metadata maps completely to the predefined valid attribute values. But,
> this needn't be the case. For example, the UA may recognize an MPEG-2 TS
> audio track but not recognize metadata designating it as a Descriptive
> Video track.

Do you have example files for which this applies? I think such
examples would be the best place to start.


> For more deterministic handling of inband tracks, the UA could be required
> to:
>
> * Expose all in-band tracks, in some form.

If they are audio or video tracks that the browser does not recognize
as such, it will be pretty useless to expose them, since a JS app is
likely not in a better place to decode them than the browser.


> * Make media resource metadata  associated with the inband tracks
> available to Web applications. [1] does this through a TextTrack.

Yes, I think that's the correct approach.


> * Provide data so that the Web application can correlate the metadata data
> with the appropriate Track object.

What does that mean?


> [1] is a specification written by CableLabs about how a UA would meet
> these requirements in the case of MPEG-2 transport stream media resources.
> This spec was written before some of the recent additions to HTML5, e.g.
> inbandTrackDispatchType. The users of [1] would like to see a W3C spec
> that addresses the same problem, takes into account the current HTML5.1
> work and addresses some details that [1] missed, e.g. more precise
> definition of what constitutes a metadata TextTrack, how in-band data gets
> segmented into TextTrackCues, how Cue start and end times are derived from
> the in-band track.
>
> [2] is an informal draft describing how the technique in [1] can be
> applied to WebM, Ogg and MPEG4 media resources. This CG could address
> these media container formats as well.

I was under the impression that this CG is working on re-writing an
updated version of [2] that would also include some of the salient
points of [1].


> So, at a minimum, I propose:
>
> * That the CG start by discussing the requirements outlined above, with
> the goal of creating a spec at least for MPEG-2 TS (recognizing that some
> of this may already be covered by HTML5/1.

That's what "updating [2]" means, right?


> * WebM and MP4 are widely supported in browsers so it might make sense to
> cover these formats.

MP4 is not covered by [1] or [2], correct?

I think mapping WebM would be trivial because it has been built to
match the HTML5 spec.


> If there is sufficient interest, we could take on other inband track
> formats as well.

I can help make sure the mapping to HTML5 is accurate. However, I have
no experience with the format of inband tracks in MPEG-2 or MP4.
Hopefully we have some experts of these formats on the list who have
example files and format specifications that we can reference to make
the right proposals.

Regards,
Silvia.


> Comments?
>
> [1] http://www.cablelabs.com/specifications/CL-SP-HTML5-MAP-I02-120510.pdf
> [2] http://html5.cablelabs.com/tracks/media-container-mapping.html
>
> Thanks,
> Bob
>
>
>

Received on Wednesday, 6 November 2013 08:37:42 UTC