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Re: Feedback on MediaController

From: Mark Watson <watsonm@netflix.com>
Date: Wed, 20 Apr 2011 19:27:44 -0700
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
CC: Ian Hickson <ian@hixie.ch>, HTML Accessibility Task Force <public-html-a11y@w3.org>
Message-ID: <94A0E0CE-B890-4CB9-B319-F8A741C7557C@netflix.com>


Sent from my iPhone

On Apr 20, 2011, at 7:01 PM, "Silvia Pfeiffer" <silviapfeiffer1@gmail.com> wrote:

> On Thu, Apr 21, 2011 at 9:01 AM, Ian Hickson <ian@hixie.ch> wrote:
>> On Mon, 18 Apr 2011, Silvia Pfeiffer wrote:
>>> 
>>> (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.
>> 
>> Name and label are the same.
> 
> Name was supposed to be what now is ID, so I'm happy with the changes.
> 
> 
>>> The kind is necessary to classify the track correctly in menus, e.g.
>>> as sign language, audio description, or even a transparent caption
>>> track.
>> 
>> I'm fine with exposing kind; is there any documentation on what video
>> formats expose for this?
> 
>> On Wed, 20 Apr 2011, Silvia Pfeiffer wrote:
>>> 
>>> I have thus far come up with the following:
>>> 
>>> video:
>>> * sign language video (in different sign languages)
>>> * captions (as in: burnt-in video that may just be overlays)
>>> * different camera angle
>>> 
>>> audio:
>>> * audio descriptions
>>> * language dub
>> 
>> We should derive these from the kinds that are exposed in media formats,
>> it doesn't make sense for us to come up with them.
> 
> http://wiki.xiph.org/Ogg_Skeleton_4 has a specification for what it is
> in Ogg now.
> This includes the roles as specified here:
> http://wiki.xiph.org/SkeletonHeaders#Role
> 
> I don't know if MPEG has anything like it.

At least the MPEG DASH group is looking to W3C HTML Accessibility Task Force to define kinds for accessibility.

The other ones: dubbed audio and commentary are kind-of obvious if you considers movies as a use-case.

...Mark
> 
> WebM has a bunch of metadata on tracks e.g. TrackType, but not much
> semantics IIUC.
> http://www.webmproject.org/code/specs/container/#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.
>> 
>> It's not especially well-defined, since there's no concept of "ending"
>> with the controller, given how streaming is handled.
>> 
>> But more importantly, what are the use cases?
>> 
>> The use case for looping a single track is things like this:
>> 
>>  http://www.google.com/green/
>> 
>> ...but I don't see why you would use a MediaController to do that kind of
>> thing. It's not like you'd want the multiple videos there in sync, they're
>> just background.
>> 
>> I'm also skeptical of introducing loop at the MediaController level even
>> in the simple case of finite resources, because it's not clear how to make
>> it work with looping subresources. Say you had two resources, both set to
>> loop, one of which was 5s and one 3s, and that you then further say that
>> the whole thing should loop. What should happen? We don't want to define
>> MediaController looping in a way that precludes that from being possible,
>> IMHO, at least not unless we have a strong use case.
>> 
>> 
>>> 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.
>> 
>> Why would an attribute on any one of the <video>s affect the
>> MediaController as a whole? Why would they jump back to
>> initialPlaybackTime? I don't think this makes sense.
> 
> To me looping on individual slaves of a multitrack resource
> individually doesn't make sense. That would mean that this element is
> independent of the others. Either they all loop or none does.
> 
> 
>>> (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.
>> 
>> Actually currently autoplay is the only behaviour; MediaControllers start
>> off playing and just wait for any autoplaying resources to be ready. If
>> none of the resources are autoplaying the controller just advances without
>> anything playing. This is probably suboptimal.
>> 
>> I guess we could say that if none of the resources have autoplay enabled
>> it doesn't play, but how would you handle dynamic changes to the set of
>> slaved media elements?
> 
> When attributes on slaves are changed, the state of the group changes.
> Autoplay changed during playback has no effect anyway. Autoplay is
> only relevant right after the first load.
> 
> I also think that having autoplay on individual elements while not on
> others decouples their timeline and is wrong. That should not be
> possible. They either slave to the same timeline or they don't.
> 
> 
>>> (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
>> 
>> These are supported now.
> 
> Cool!
> 
> 
>>> * onended: is raised when all  slave media elements are in ended state
>> 
>> This isn't supported; a media controller can't be "ended" currently.
> 
> Hmm, I guess my arguments were really a bit messed up between IDL
> attribute and event. I really care about the event, not so much about
> the IDL attribute.
> 
> (Also, I didn't know that @foo was only for content attributes -
> thanks for clarifying!)
> 
>>>> But what's the use case?
>>> 
>>> If I reach the end, I want to present something different, such as a
>>> post-roll add or an overlay with links to other videos that are related.
>>> It is much easier to wait on a onended event on the combined resource
>>> than having to register an event handler with each slave and then try
>>> and combine the result.
>> 
>> I'm confused. Are we talking about the event or the attributed? You seem
>> to be arguing for the attribute but giving use cases for the event.
>> 
>> If you want to display an overlay at the end of a video, it seems like
>> you'd want to do that as soon as the video ended, you wouldn't want to
>> wait until the end of the entire resource, no? So you'd want onended on
>> the <video> element, not the controller.
> 
> No, I'd actually want to wait until all elements are ended before
> showing an overlay, in particular if an audio description or a sign
> language video or a commentary continues. The grouped resource is not
> ended before all of the slaves are ended and I don't want random
> advertising in my face before it's all over. That would be really
> disturbing.
> 
> 
> 
>>> (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.
>> 
>> The user interface section covers this already.
> 
> I'm happy with the @controls after all your feedback, thanks.
> 
> 
> Cheers,
> Silvia.
> 
> 
Received on Thursday, 21 April 2011 02:28:51 UTC

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