[whatwg] Video, Closed Captions, and Audio Description Tracks
Dave Singer
singer at apple.com
Mon Oct 8 12:05:33 PDT 2007
At 9:45 +1200 8/10/07, Chris Double wrote:
>The video element description states that Theora, Voribis and Ogg
>container should be supported. How should closed captions and audio
>description tracks for accessibility be supported using video and
>these formats?
>
>I was pointed to a page outlining some previous discussion on the issue:
>
>http://wiki.whatwg.org/wiki/Video_accessibility
>
>Is there a way of identifying which track is the closed caption track,
>which is the alternate audio track, etc? How are other implementors of
>the video element handling this issue?
>
>Is CMML for the closed captions viable? Or a speex track for the
>alternate audio? Or using Ogg Skeleton in some way to get information
>about the other tracks?
There was also a thread I started in June, which I can't find on the
archives; my initial email is below.
We suggested two ways to achieve captioning (a) by selection of
element, at the HTML level ('if you need captions, use this
resource') and (b) styling of elements at the HTML level ('this video
can be asked to display captions').
Choice (a) means that it is possible, for example, to prepare
alternative versions with 'burned in' accessibility (e.g. captions),
and then explicit support for them is not needed in the format.
Choice (b) is more economical in media resources, and recognizes that
'true captioning' is sometimes better (e.g. it might be delivered out
on analog video as line 21 data).
The previous thread faded away, but with the W3C meeting approaching,
I'd like to get a sense of how we make progress in this area. Should
we (Apple) edit this into the Wiki, should we (Apple or WhatWG) carry
the proposal to the W3C, and if so, which group? And so on.
Thanks for re-raising this!
* * * * *
Date: Fri, 08 Jun 2007 16:22:00 -0700
From: Dave Singer <singer at apple.com>
Subject: [whatwg] accessibility management for timed media elements, proposal
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Hi
we promised to get back to the whatwg with a proposal for a way to
handle accessibility for timed media, and here it is. sorry it took
a while...
* * * * *
To allow the UA to select among alternative sources for media
elements based on users' accessibility preferences, we propose to:
1) Expose accessibility preferences to users
2) Allow the UA to evaluate the suitability of content for specific
accessibility needs via CSS media queries
Details:
1) Expose accessibility preferences to users
Proposal: user settings that correspond to a accessibility needs. For
each need, the user can choose among the following three dispositions:
* favor (want): I prefer media that is adapted for this kind of
accessibility.
* disfavor (don't want): I prefer media that is not adapted for
this kind of accessibility.
* disinterest (don't care): I have no preference regarding this
kind of accessibility.
The initial set of user preferences for consideration in the
selection of alternative media resources correspond to the following
accessibility options:
captions (corresponds to SMIL systemCaptions)
descriptive audio (corresponds to SMIL systemAudioDesc)
high contrast video
high contrast audio (audio with minimal background noise, music
etc., so speech is maximally intelligible)
This list is not intended to be exhaustive; additional accessibility
options and corresponding preferences may be considered for inclusion
in the future.
Herein we describe only those user preferences that are useful in the
process of evaluating multiple alternative media resources for
suitability. Note that these proposed preferences are not intended to
exclude or supplant user preferences that may be offered by the UA to
provide accessibility options according to the W3C accessibility
guidelines, such as a global volume control
<http://www.w3.org/TR/WAI-USERAGENT/uaag10-chktable.html>.
2) Allow the UA to evaluate the suitability of content for specific
accessibility needs via CSS media queries
Note that the current specification of <video> and <audio> includes a
mechanism for selection among multiple alternate resources
<http://www.whatwg.org/specs/web-apps/current-work/#location>. The
scope of our proposal here is to extend that mechanism to cover
accessibility options.
Proposal: the media attribute of the <source> element as described in
the current working draft of Web Applications 1.0 takes a CSS media
query as its value <http://www.w3.org/TR/css3-mediaqueries/>, which
the UA will evaluate in the process of selecting an appropriate media
resource for presentation. To extend the set of media features that
can be queried to include accessibility preferences, we define a new
media feature for each supported accessibility preference:
captions
descriptive-audio
high-contrast-video
high-contrast-audio
For each of these media features the following values are defined:
* The user prefers media adapted for this kind of accessibility (": want").
* The user prefers media that is not adapted for this kind of
accessibility (": dont-want").
* The user has expressed no preference regarding this kind of
accessibility (": either").
For each media feature that corresponds to accessibility preferences,
an expression evaluates to FALSE if and only if the user has an
explicit preference (want or don't want), and the media feature has a
value of want or dont-want that doesn't correspond. For all other
combinations (user disinterest or a value of "either"), then the
expression evaluates to TRUE.
Example. If the user has asked for
captions: want
high contrast video: don't want
and the video element has
<video ... >
<source media="all and (captions: dont-want)" ... />
<source media="all and (captions: either)" ... />
</video>
The second source will be selected for presentation; the second would
also be selected if the media attribute were completely omitted.
Once a candidate source has been selected, the UA must attempt to
apply the user's accessibility preferences to its presentation, so
that adaptable content is presented appropriately.
--
David Singer
Apple/QuickTime
--
David Singer
Apple/QuickTime
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