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Re: Track kinds

From: Mark Watson <watsonm@netflix.com>
Date: Fri, 6 May 2011 16:16:55 -0700
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
CC: HTML Accessibility Task Force <public-html-a11y@w3.org>
Message-ID: <D7BD4FF2-D233-44D1-9385-5C8CF72A3A49@netflix.com>

On May 6, 2011, at 2:18 PM, Silvia Pfeiffer wrote:

On Sat, May 7, 2011 at 5:22 AM, Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>> wrote:

I also have a procedural question: do we consider that we have received the
liaison from 3GPP (mentioned on the above page) ? Are we going to answer it
?

There should be an email by a 3GPP member with all the relevant
requests to the WG list. I haven't seen one such.

I doubt the 3GPP staff will post to the list. They will send the liaison to the chair of the group, possibly via W3C staff members, and expect them to distribute it to the list. I imagine it is stuck somewhere in that path.

Unless that arrives,
there is nothing to reply to.

We can see that it has been sent from their minutes and we can see the text of the document in their archives, so we can at least prepare for its arrival ;-)


One question they ask is whether we will define a URN to identify the
space of kind values defined by W3C. One advantage of doing that is that
these kinds are then immediately supported in 3GPP and MPEG adaptive
streaming manifests, which means that there *is* a media container
supporting those kinds (perhaps addressing one of the editor's concerns
about new kind values).

I don't think HTML needs these values as anything else but short
strings that we now have.

Agreed.


If 3GPP and MPEG need them as URNs

No, they need a URN that refers to the codepoint space containing these values. But I notice W3C doesn't appear to have a top-level URN space. It could use urn:fdc:w3.org:2011....

The 3GPP and DASH specs allow you to tag a track with any number of "Roles" - so it can be tagged with roles from this W3C space (if anyone defines a URN) as well as equivalent roles from other spaces if necessary for some application.

- and they probably have a swag
more that they have proposed and will use

Not really - they'd prefer W3C to define some container-format-independent ones, as I understand it.

- it makes a lot more sense
to me for them to define these themselves.

They seem to think the opposite ;-) In particular it's been stated at MPEG that the W3C HTML a11y group has more a11y expertise than the MPEG group.

And there is nothing container-format-specific about this concept of track kinds. The abstract kinds and their definitions need to be defined somewhere with responsibility for all containers or for none, not in the groups focussed on particular containers. The containers (and HTML) just need to define how those kinds are labeled in their syntax.

Some of the kinds that
media containers will expose may just end up creating getLabel() text
rather than be exposed in getKind(). Ogg already seems to have a few
of those.

Agreed, but I think this is a symptom of having media container formats lead the definition of kinds: they define kinds which are either not very well-defined or not universally applicable and so we decide not to expose these over HTML because we want the HTML interface to be clean and well-defined. The best way to achieve that is to define these things in a container-independent place and ask the containers to align.


Finally, we discussed the "commentary" kind here at Netflix and in the end
we are happy to have it dealt with simply as "alternative". I do think
though that in principle there could be other (UI-related) reasons for
exposing a new track kind than triggering default behavior or application of
user preferences. This is certainly the case for accessibility use-cases
where the UI to enable/disable a particular track could usefully be tailored
to the intended users of that track (for example, enabling/disabling tracks
intended for the blind or those with low vision should ideally not involve
complex visual UI elements).

OK. I think we'll cross that bridge when we get to it.

Cheers,
Silvia.


...Mark


On May 3, 2011, at 10:12 PM, Silvia Pfeiffer wrote:

I understand the problem of additive/alternative tracks, too, and have
tried to approach it with markup before. However, I think this is
making something that is supposedly simple much too difficult. The
ultimate choice of active tracks has got to be left to the user. For
this reason, I think @kind (or getKind()) should only ever expose what
content is available in the track, but there should not be an
automatic choice made by the browser. It's up to the user to
activate/deactivate the correct tracks.

Before we dive into anything more complex, we should get some
experience with an implementation of multitrack and the roles. I don't
think we will have much to go by for making a decision beforehand.

Cheers,
Silvia.


On Wed, May 4, 2011 at 2:02 PM, Mark Watson <watsonm@netflix.com<mailto:watsonm@netflix.com>> wrote:

So, if we are looking for a generic approach, where a track can have
multiple "roles", then I think the correct logic is indeed to pick the
fewest number of tracks which fulfill the intersection of the desired roles
and the available roles such that no role is fulfilled more than once. You
need a priority list of roles to drop from the desired list if that isn't
possible (which would mean some badly authored content, but has to be dealt
with). It may be a mouthful, but I think it would be reasonably
straightforward to implement.

However, I'm still not sure a generic approach is necessary. A "simpler"
approach is to say every track has a single role. But for some applications
(like audio descriptions) there are two distinct role values defined - an
additive one and an alternative one. The problem is addressed at a semantic
level - i.e. people implement support for audio descriptions - and they know
what these are and how to handle them - rather than trying for a generic
descriptor matching algorithm.

Regarding Repetitive Stimulus Safe, I guess that since most content is
unfortunately not labeled one way or the other the default assumption has to
be up to the user themselves. i.e. that user preferences associated with
this aspect should support required, preferred and don't care. In a really
generic approach every role may have a status from { require, prefer, don't
care, prefer not, require not }.

Again, this suggests that a generic approach might be over-ambitious - who
says some new role doesn't come along next week with a sixth user-preference
status of "required unless role Y present" or similar ... I think maybe the
UA needs to understand what these things are and act appropriately.

...Mark





On May 3, 2011, at 11:55 AM, David Singer wrote:


On May 2, 2011, at 16:55 , Mark Watson wrote:


I think it's evidence that there is something to be solved.

I'd prefer a solution where adding a track to an existing presentation
didn't require me to change the properties of existing tracks, though, since
there is an error waiting to happen in that case.


Yes.  This idea made some sense when it was the tracks in a multiplex (e.g.
MP4 file), perhaps makes sense when all the tracks are annotated in the
markup (e.g. in HTML5 or DASH MPD) but makes much less sense when some
tracks are in a multiplex and some are added in the markup - a track added
in the markup might need the annotations in a multiplex changed, ugh.

So, thinking out loud here.

Assume the user has a set of roles that they would kinda like to experience.
 The default is 'main, supplementary', I think, or something like.

Now, we have a set of tracks, each of which satisfies some roles.  Let's
ignore tracks we have discarded because they are the wrong mime type, codec,
language, etc., and focus just on this selection mechanism.  What is the
right simple way to get the set of tracks?

It's easy to 'go overboard' and treat this as a very general problem of
finding the minimal set of tracks that will span a set of design roles.  I
don't think anyone will author *for the same language*

track - main

track - captions

track - main +  captions

so an algorithm designed to pick only (3) instead of (1 + 2) for the
main+captions desiring user is probably overkill.

'enable the tracks whose roles are a subset of the desired roles, and
disable the rest' may be too simple, unless tracks are ordered from the
most-labelled to the least-labelled.

So, audio-description replacing the main audio:

track - main description

track - main

Audio description adding to the main audio

track - main

track - description

The same works for all the adaptations that might require re-authoring or
might be achievable with an additional track (captions, burned in or
separate, for example).


Where this fails is when the 'base content' is good enough for both the
plain user and the user who desires more roles.  The obvious case here (Mark
will laugh) is repetitive-stimulus-safeness;  we have to assume unlabelled
content is unsafe, but much content is naturally safe and can be labelled as
such.

David Singer

Multimedia and Software Standards, Apple Inc.
Received on Friday, 6 May 2011 23:17:25 GMT

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