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Re: Survey ready on Media Multitrack API proposal

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Wed, 3 Mar 2010 21:45:28 +1100
Message-ID: <2c0e02831003030245t7ff2b0e6v1ce5d424dadcfaf4@mail.gmail.com>
To: "Michael(tm) Smith" <mike@w3.org>, Philip Jägenstedt <philipj@opera.com>, Eric Carlson <eric.carlson@apple.com>
Cc: HTML Accessibility Task Force <public-html-a11y@w3.org>
On Wed, Mar 3, 2010 at 5:35 PM, Michael(tm) Smith <mike@w3.org> wrote:
> A survey is ready on the "Media Multitrack API" draft change
> proposal.
>
>  http://www.w3.org/2002/09/wbs/44061/multitrack-api/
>
> The survey asks you to evaluate the proposal, which is for a
> JavaScript API for extracting basic information about tracks
> contained inside a media (audio or video) resource, and indicate
> whether you support submitting it to the HTML WG as a decision
> from the a11y TF; it also provides you with an opportunity to
> suggest changes to the proposal, and, if you indicate that you
> don't support submitting the proposal to the group, asks that you
> provide a comment giving your reasons.
>
> Only members of the HTML Accessibility Task Force can complete
> this survey. It is only a straw poll of the members of this task
> force. However, note that the results are publicly visible:
>
>  http://www.w3.org/2002/09/wbs/44061/multitrack-api/results
>
> --
> Michael(tm) Smith
> http://people.w3.org/mike


Hmm, it seems there is more discussion to be had about this proposal.

I'd like to address Philip's concerns, so we can move forward.

> The MediaTrack interface:
> * Drop role unless it is actually exposed in exactly that way in existing media formats, or describe how it can be derived from the
> information in existing media formats. Describe what value it should have when the information is not available (null or the empty
> string?)

There will be a need to map from existing media formats to this role,
indeed. IIUC (and Eric or David can correct me), both QuickTime and
MPEG-4 expose certain information that relates to the roles that we
are describing. However, it is a text field and values have not been
pre-defined, so conventions will need to be used to map to our defined
roles.

To be sure, Ogg does not currently require such a field, though the
Skeleton header allows for such fields. The Xiph community is in the
process of introducing such fields - also as a result from what we are
finding here as necessary information to be attributed and exposed on
a per track basis.


> * Type cannot be a mime type as the tracks of media files are not described with MIME types internally. For example, what is the
> MIME type of a MP3 stream in a RIFF WAVE container? Should the MIME type actually be the type of the container, with the tracks
> type in the codecs parameter?

Mime types are actually both defined on full containers as well as
codecs alone. That practice started with RTP/RTSP streaming when
codecs needed to be identified rather than file types. As such, almost
all codecs now have a mime type associated with them (e.g.
http://www.rfc-editor.org/rfc/rfc3016.txt,
http://tools.ietf.org/html/draft-ietf-avt-rtp-vorbis-09,
http://tools.ietf.org/html/draft-barbato-avt-rtp-theora-01), so it
should indeed be possible to extract/associate a mime type with a
track.


> * Drop media as it belongs in the HTML markup, this information is not available from tracks in the media resource.

I am not overly fussed about this one. Though I think some of the
media information could be possible to extract about a track, e.g.
whether it is appropriate for mobile. But seeing as we haven't really
formalised yet what media queries really mean for media resources, it
is a bit void to add this yet.


> Also, what about track groups? If the suggestion for referencing external tracks is going to be <trackgroup><track>, shouldn't the
> groups of internal tracks be exposed in some way too?

This is a good point. We could add a field that signifies for each
track which track group it belongs to. Then, by looking at all tracks,
we can identify the group. This is completely exposing the way it is
done in MPEG.

Hopefully we can get consent on how to resolve these issues, then make
these changes to the proposal and still have it go forward to the HTML
WG.

Best Regards,
Silvia.
Received on Wednesday, 3 March 2010 10:46:21 GMT

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