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RE: [media] progress on multitrack api - issue-152

From: Sean Hayes <Sean.Hayes@microsoft.com>
Date: Tue, 19 Apr 2011 08:45:39 +0000
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
CC: HTML Accessibility Task Force <public-html-a11y@w3.org>, "Ian Hickson (ian@hixie.ch)" <ian@hixie.ch>, Paul Cotton <Paul.Cotton@microsoft.com>, "Sam Ruby (rubys@intertwingly.net)" <rubys@intertwingly.net>, "Maciej Stachowiak (mjs@apple.com)" <mjs@apple.com>
Message-ID: <8DEFC0D8B72E054E97DC307774FE4B91442CF2BE@DB3EX14MBXC313.europe.corp.microsoft.com>
It is possibly a broader issue, but it directly addresses the use case that I was targeting with my CP, which is the creation of an independent caption overlay which can span multiple videos.  

The specification is not clear (to me at least) that this is the required behavior; although the Editor and at least one implementor have said that this is what they expect, so I don't think it's a big change but I do want it included in order to be confident that withdrawing my CP will drop this ability.

-----Original Message-----
From: Silvia Pfeiffer [mailto:silviapfeiffer1@gmail.com] 
Sent: 19 April 2011 02:29
To: Sean Hayes
Cc: HTML Accessibility Task Force; Ian Hickson (ian@hixie.ch); Paul Cotton; Sam Ruby (rubys@intertwingly.net); Maciej Stachowiak (mjs@apple.com)
Subject: Re: [media] progress on multitrack api - issue-152

Are you in particular referring to a change of the following sentence
in the video element section 4.8.6:
http://www.whatwg.org/specs/web-apps/current-work/multipage/video.html#video or

"A video element is used for playing videos or movies."

Would you prefer this to read something like:

"A video element is used for playing videos, movies, or sound files."

Further a change of:

"The video element is a media element whose media data is ostensibly
video data, possibly with associated audio data."

to something like:

"The video element is a media element whose media data is ostensibly
video data, possibly with associated audio data. This includes
audio-only resources that may be displayed with a representative image
in @poster or with captions or subtitles for visual representation."

Maybe something of this sort can be achieved?

I would, however, suggest to take this up as a separate bug in the
bugtracker, since it doesn't effectively have any relation to
multitrack - it's a broader issue on video elements in general.


On Tue, Apr 19, 2011 at 1:32 AM, Sean Hayes <Sean.Hayes@microsoft.com> wrote:
> If the clarifications I proposed last week are included (i.e. to indicate expressly that the <video> element operates on audio only data, and has a display rectangle to renders captions into, even if there is no video data supplied), then I would formally withdraw my CP in favor of Proposal 4 as amended.
> -----Original Message-----
> From: public-html-a11y-request@w3.org [mailto:public-html-a11y-request@w3.org] On Behalf Of Silvia Pfeiffer
> Sent: 18 April 2011 12:49
> To: HTML Accessibility Task Force
> Subject: Re: [media] progress on multitrack api - issue-152
> Note that I have certainly missed issues, so do speak up if you've
> noticed anything.
> Cheers,
> Silvia.
> On Mon, Apr 18, 2011 at 12:05 AM, Silvia Pfeiffer
> <silviapfeiffer1@gmail.com> wrote:
>> Hi all,
>> In the last media subgroup meeting we further discussed the different
>> change proposals that we have for issue-152.
>> A summary of all the submitted change proposals is at
>> http://www.w3.org/WAI/PF/HTML/wiki/Media_Multitrack_Change_Proposals_Summary
>> .
>> We discussed that Proposal 4 can, with a few changes, provide for the
>> requirements of in-band and externally composed multitrack resources.
>> Proposal 4 introduces an interface for in-band audio and video tracks,
>> and a Controller object to maintain the shared state between the
>> individual media elements that together make up a composed multitrack
>> resource.
>> This email serves two purposes:
>> Firstly it asks others on the accessibility task force whether there
>> are any objections to going with proposal 4 (Philip?, Geoff?). The
>> people present at the meeting agreed that they would be prepared to
>> withdraw their change proposals in favor of proposal 4. This include
>> all proposals numbered 1 to 3 on the summary page.
>> Secondly it summarizes the remaining issues that we would like
>> addressed for proposal 4.
>> The remaining issues are:
>> (1) videoTracks should be MultipleTrackList, too:
>> The current HTMLMediaElement has the following IDL to expose in-band
>> media tracks:
>>  readonly attribute MultipleTrackList audioTracks;
>>  readonly attribute ExclusiveTrackList videoTracks;
>> The objection is to the use of ExclusiveTrackList on videoTracks. It
>> should be allowed to have multiple in-band video tracks activated at
>> the same time. In particular it seems that MP4 files have a means of
>> specifying how multiple video tracks should be displayed on screen and
>> Safari is already able to display such.
>> In contrast, proposal 4 requires that only one in-band video track can
>> be active and displayed into the video viewport at one time. If more
>> than one video track is to be displayed, it needs to be specified with
>> a media fragment URI in a separate video element and connected through
>> a controller.
>> Some questions here are: what do other browsers want to do with
>> multiple in-band video tracks? Does it make sense to restrict the
>> display to a single video track? Or should it be left to the browser
>> what to do - in which case a MultipleTrackList approach to videoTracks
>> would be sensible? If MultipleTrackList is sensible for audio and
>> video, maybe it could further be harmonized with TextTrack.
>> (2) interface on TrackList:
>> The current interface of TrackList is:
>>  readonly attribute unsigned long length;
>>  DOMString getName(in unsigned long index);
>>  DOMString getLanguage(in unsigned long index);
>>           attribute Function onchange;
>> The proposal is that in addition to exposing name and language
>> attributes - in analogy to TextTrack it should also expose a label and
>> a kind.
>> The label is necessary to include the track into menus for track
>> activation/deactivation.
>> The kind is necessary to classify the track correctly in menus, e.g.
>> as sign language, audio description, or even a transparent caption
>> track.
>> (3) looping should be possible on combined multitrack:
>> In proposal 4 the loop attribute on individual media elements is
>> disabled on multitrack created through a controller, because it is not
>> clear what looping means for the individual element.
>> However, looping on a multitrack resource with in-band tracks is well
>> defined and goes over the complete resource.
>> In analogy, it makes sense to interpret loop on a combined multitrack
>> resource in the same way. Thus, the controller should also have a
>> muted attribute which is activated when a single loop attribute on a
>> slave media element is activated and the effect should be to loop over
>> the combined resource, i.e. when the duration of the controller is
>> reached, all slave media elements' currentTime-s are reset to
>> initialPlaybackPosition.
>> (4) autoplay should be possible on combined multitrack:
>> Similar to looping, autoplay could also be defined on a combined
>> multitrack resource as the union of all the autoplay settings of all
>> the slaves: if one of them is on autoplay, the whole combined resource
>> is.
>> (5) more events should be available for combined multitrack:
>> The following events should be available in the controller:
>> * onloadedmetadata: is raised when all slave media elements have
>> reached at minimum a readyState of HAVE_METADATA
>> * onloadeddata: is raised when all slave media elements have reached
>> at minimum a readyState of HAVE_CURRENT_DATA
>> * canplaythrough: is raised when all slave media elements have reached
>> at minimum a readyState of HAVE_FUTURE_DATA
>> * onended: is raised when all  slave media elements are in ended state
>> or said differently: these events are raised when the last slave in a
>> group reaches that state.
>> These are convenience events that will for example help write combined
>> transport bars. It is easier to attach just a single event handler to
>> the controller than to attach one to each individual slave and make
>> sure they all fire. Also, they help to maintain the logic of when a
>> combined resource is loaded. Since these are very commonly used
>> events, their introduction makes sense.
>> Alternatively or in addition, readyState could be added to the controller.
>> (6) controls on slaves control the combined multitrack:
>> Proposal 4 does not provide any information on what happens with media
>> elements when the @controls attribute is specified. Do the controls
>> stay in sync with the controls of the other elements? Do they in fact
>> represent combined state? Do they represent the state of the slave
>> resource? What happens when the user interacts with them? Is the
>> information on the interaction - in particular seeking, muting, volume
>> change, play/pause change, rate change - handed on to the controller
>> and do the others follow?
>> Hopefully we can move forward on all of these issues before the 22nd
>> April deadline for issue-152.
>> Best Regards,
>> Silvia.
Received on Tuesday, 19 April 2011 08:46:13 UTC

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