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Re: [media] issue-152: documents for further discussion

From: Ian Hickson <ian@hixie.ch>
Date: Wed, 20 Apr 2011 00:51:00 +0000 (UTC)
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>, Philip J├Ągenstedt <philipj@opera.com>
cc: "public-html-a11y@w3.org" <public-html-a11y@w3.org>
Message-ID: <Pine.LNX.4.64.1104150008230.19153@ps20323.dreamhostps.com>
On Tue, 12 Apr 2011, Silvia Pfeiffer wrote:
> > > the TrackList only includes name and language attributes - in 
> > > analogy to TextTrack it should probably rather include (name, label, 
> > > language, kind)
> >
> > I'm fine with exposing more data, but I don't know what data in-band 
> > tracks typically have. What do in-band tracks in popular video formats 
> > expose? Is there any documentation on this?
> There is a discussion on the main list about metadata right now and I 
> have posted a link there about what the W3C Media Annotations WG 's 
> analysis of media formats found as typically used metadata on audio and 
> video. If you want to understand what is generally available, that is a 
> good starting point, see http://www.w3.org/TR/mediaont-10/ .

Woah, that's a lot of data. I guess a better approach for this will be to 
look at use cases and figure out what needs exposing.

> I would, however, regard these two attributes that we discussed here as 
> a separate issue, because if somebody wants to create custom controls 
> and e.g. provide all the alternative video descriptions in one menu, 
> they would want all the text descriptions and audio descriptions listed 
> - similarly if they want all the alternative captions in one menu, they 
> would want all the text track captions as well as all the videos that 
> are created from bitmaps as overlay captions as well as all the 
> alternative video tracks with burnt-in captions. So, providing a label 
> (for use in the menu) and a kind (for classification) is very useful. 
> These can all be mapped from fields from within video formats.

I assume you're talking primarily about "kind" here. "name" and "label" 
are the same thing (actually I've renamed "name" to "label" to improve 
consistency with other parts of the platform).

Looking at the metadata list cited above, I don't see anything in either 
ogg, mp4, or webm that maps to "kind", so I don't see much point exposing 
that on the audio/video track lists, though I agree that in principle it 
would be a good idea.

Realistically though, for in-band tracks it's more likely that that data 
will be provided to the script out-of-band so that it can construct the UI 
before the movie loads, and for out-of-band tracks the information can be 
made available in the markup (e.g. using data-* attributes). For UA-driven 
menus, the title is probably sufficient for most purposes, and that can 
already be made available.

> > Note that for media-element-level data, you can already use data-* 
> > attributes to get anything you want, so the out-of-band case is 
> > already fully handled as far as I can tell.
> Interesting. In the audio description case, would a label, kind, and 
> language be added to the menu of the related video element?

For scripted UIs, that's up to the script.

For UA UIs, it depends if we are talking about multiple video tracks or 
multiple audio tracks. Multiple video tracks aren't handled, because 
there's no sane way to have the UA turn the video tracks on and off. For 
the audio case, I don't really see much reason to expose more than a 
title. A kind could be used but it's going to be used so rarely that in 
practice the UA will want to handle the case of only having a title 
anyway, and once you support that, it's not clear what a kind would really 
do to make things better.

It's something we can always provide in the future though, if it turns out 
to be more common than one would guess from looking at content today.

> > | a group should be able to loop over the full multitrack rather than a
> > | single slave
> >
> > Not sure what this means.
> We discussed the looping behaviour. To make it symmetrical with in-band 
> multitrack resources, it would make sense to be able to loop over 
> composed multitrack resources, too. The expected looping behaviour is 
> that a loop on the composed resource loops over the composite as a 
> whole. So, the question is then how to turn such looping on.
> The proposal is that when one media element in the group has a @loop 
> attribute, that would turn the looping on the composite resource on. 
> This means that when the loop is set and the end of the composite 
> resource is reached (its duration), the currentTime would be reset to 
> its beginning and playback of the composite resource would start again. 
> Looping on individual elements is turned off and only the composite 
> resource can loop.

What's the use case?

> > | some attributes of HTMLMediaElement are missing in the MediaController
> > | that might make sense to collect state from the slaves: error,
> >
> > Errors only occur as part of loading, which is a per-media-element 
> > issue, so I don't really know what it would mean for the controller to 
> > have it.
> The MediaController is generally regarded as the state keeper for the 
> composite resource.

It is? That's certainly not how it's defined. It's just a central 
controller, it doesn't keep any of the state for the resources.

> So, what happens when a single slave goes into error state. Does the 
> full composite resource go into error state? Or does it ignore the slave 
> - turn it off, and continue?

Media elements don't really have an error state. They have a networkState 
and a readyState, which affect the MediaController, but the 'error' 
attribute is just for exposing the last error for events, it's not part of 
the state machine.

> > | readyState
> >
> > I could expose a readyState that returns the lowest value of all the 
> > readyState values of the slaved media elements, would that be useful? 
> > It would be helpful to see a sample script that would make use of 
> > this; I don't really understand why someone would care about doing 
> > this at the controller level rather than the individual track level.
> I think it makes sense, in particular when script is waiting for all 
> elements to go to HAVE_METADATA state, which is often the case when you 
> are trying to do something on the media resource, but have to wait until 
> it's actually available.
> An example JS would be where you are running your own controls for the 
> combined resource and want to determine the combined duration and volume 
> for visual display, e.g.
>       video.controller.addEventListener("loadedmetadata", init, false);
>       function init(evt) {
>         duration.innerHTML = video.controller.duration.toFixed(2);
>         vol.innerHTML      = video.controller.volume.toFixed(2);
>       }
> So, I think a combined readyState makes sense in the way you described.

That example doesn't use readyState at all. Is there a use case for 
readyState specifically?

> > | (this one is particularly important for onmetadatavailable events)
> >
> > The events are independent of the attributes. What events would you 
> > want on a MediaController, and why? Again, sample code would really 
> > help clarify the use cases you have in mind.
> Maybe a onmetadatavailable event is more useful than a readyState then?

I've updated the spec to fire a number of events on MediaController, 
including 'metadataavailable' and 'playing'/'waiting'.

> I am not aware of many scripts that use the readyState values directly 
> for anything, even on the media elements themselves.

One example of readyState usage on a media element would be a 'waiting' 
event handler that checks whether readyState is HAVE_CURRENT_DATA or 
HAVE_METADATA and uses that information to decide whether to display a 
poster frame overlay or not.

> > | TimeRanges played
> >
> > Would this return the union or the intersection of the slaves'?
> That would probably be the union, because those parts of the timeline 
> are what the user has viewed, so he/she would expect them to be marked 
> in manually created controls.

Ok, added.

> > | ended
> >
> > Since tracks can vary in length, this doesn't make much sense at the 
> > media controller level. You can tell if you're at the end by looking 
> > at currentTime and duration, but with infinite streams and no 
> > buffering the underlying slaves might keep moving things along 
> > (actively playing) with currentTime and duration both equal to zero 
> > the whole time. So I'm not sure how to really expose 'ended' on the 
> > media controller.
> "ended" on the individual elements (in the absence of loop) returns true when
> Either:
>     The current playback position is the end of the media resource, and
>     The direction of playback is forwards.
> Or:
>     The current playback position is the earliest possible position, and
>     The direction of playback is backwards.

No, 'ended' only fires when going forwards.

> So, in analogy, for the composed resource: it would return the union of 
> the ended result on all individual elements, namely "ended" only when 
> all of them are in ended state.

But what's the use case?

> > | and autoplay.
> >
> > How would this work? Autoplay doesn't really make sense as an IDL
> > attribute, it's the content attribute that matters. And we already have
> > that set up to work with media controllers.
> As with @loop, it would be possible to say that when one media element
> in the union has @autoplay set, then the combined resource is in
> autoplay state.

I don't understand the use case for exposing this as an IDL attribute on 
the controller.

> One more question turned up today: is there any means in which we could 
> possibly create @controls (with track menu and all) for the combined 
> resource? Maybe they could be the same controls on all the elements that 
> have a @controls active, but would actually be driven by the 
> controller's state rather than the element's? Maybe the first video 
> element that has a @controls attribute would get the full controller's 
> state represented in the controls? Could there be any way to make 
> @controls work?

The UA is responsible for this, but the spec requires that the UI 
displayed for a control that has a controller control the controller.

On Tue, 12 Apr 2011, Philip J├Ągenstedt wrote:
> Since HTMLMediaElement.played seems almost useless I haven't implemented 
> it in Opera. I still have hopes that it will be removed from the spec, 
> but failing that let's not copy it around unless we have a good use case 
> for it.

The use case Silvia suggests seems reasonable (marking on the timeline 
what has been played), why is it not good?

On Tue, 12 Apr 2011, Silvia Pfeiffer wrote:
> Gapless playing also works when all media data is loaded and you reset 
> current time when reaching ended.
> To me the feature of looping is to restart from the beginning, whether 
> gap or not. Browsers make a best effort. And I don't really see that 
> being different for multitrack, in particular since it works for in-band 
> multitrack. The in-band behavior is also the one to mirror, so I don't 
> think that's trying to be particularly clever.

We had looping in the spec before, but I took it out due to implementation 
burden concerns.

Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'
Received on Wednesday, 20 April 2011 00:51:24 UTC

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