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Re: Feedback on MediaController

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Thu, 21 Apr 2011 11:58:01 +1000
Message-ID: <BANLkTinJXqnX3Dti0wtdqEQhOffwbSf8og@mail.gmail.com>
To: Ian Hickson <ian@hixie.ch>
Cc: HTML Accessibility Task Force <public-html-a11y@w3.org>
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:

I don't know if MPEG has anything like it.

WebM has a bunch of metadata on tracks e.g. TrackType, but not much
semantics IIUC.

>> (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.


>> * 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

>> (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.

Received on Thursday, 21 April 2011 01:58:53 UTC

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