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Re: [media] Moving forward with captions / subtitles

From: Philip Jägenstedt <philipj@opera.com>
Date: Tue, 16 Feb 2010 16:19:19 +0800
To: "Silvia Pfeiffer" <silviapfeiffer1@gmail.com>
Cc: "Geoff Freed" <geoff_freed@wgbh.org>, "Eric Carlson" <eric.carlson@apple.com>, "HTML Accessibility Task Force" <public-html-a11y@w3.org>
Message-ID: <op.u77n2hbcatwj1d@philip-pc>
On Tue, 16 Feb 2010 15:46:00 +0800, Silvia Pfeiffer  
<silviapfeiffer1@gmail.com> wrote:

> On Tue, Feb 16, 2010 at 6:37 PM, Philip Jägenstedt <philipj@opera.com>  
> wrote:
>> On Tue, 16 Feb 2010 04:36:09 +0800, Geoff Freed <geoff_freed@wgbh.org>
>> wrote:
>>
>>> GF:  I prefer <trackgroup><track> as well--  grouping tracks by role  
>>> makes
>>> the most sense to me. But I'm still confused about one thing after  
>>> reading
>>> today's thread.  From this markup, it looks to me like  
>>> <trackgroup><track>
>>> also would permit multiple tracks of the same role to appear  
>>> simultaneously.
>>>  True?  Playing simultaneous tracks of the same role is still what I'd
>>> prefer (in addition to playing simultaneous tracks of differing roles,  
>>> of
>>> course).
>>
>> My idea is that <trackgroup> be used to group mutually exclusive tracks,
>> independently of their roles. I struggle to come up with an example  
>> when you
>> would want it, but if you wrap each <track> in its own <trackgroup> then
>> *all* tracks can be enabled simultaneously. It is of course up to the  
>> author
>> to make groups that make sense. Power users could override this using  
>> user
>> JavaScript or other browser extensions if they really want to.
>
> I'd actually prefer the opposite functionality - and that would also
> be much more like what is in a media resource:
>
> <track>s in a list without <trackgroup> can be activated in parallel -
> they are like non-grouped MP4 tracks.
>
> <track>s inside a <trackgroup> are mutually exclusive - only one of
> them can be activated at any point in time.
>
> IIUC, that's how grouping works in MP4 and QuickTime and thus applying
> this same principle here seems to make sense to me. Thus, if you
> didn't want tracks to be active together, you'd pack them in a
> trackgroup. Much easier than having to package each single <track> in
> a <trackgroup> to enable them to be active in parallel.

If I understand you, the only difference is the semantics when  
<trackgroup> is omitted. We can make either behavior the default. What it  
comes down to is what authors actually expect and which case is more  
common. I don't know anything about MPEG-4, but I do know that for any  
file with multiple text tracks I have opened in any media player (software  
or hardware), tracks have been mutually exclusive.

We could sidestep the problem by making <trackgroup> mandatory, although I  
think it would be unfortunate if all authors have to pay the price for the  
very uncommon (in my guesstimate) practice of having multiple text tracks  
active in parallel.

(Also, I don't want to come up with a UI for enabling multiple tracks when  
there is no grouping, as it would have to be something strange like a list  
of checkboxes in a context menu.)

-- 
Philip Jägenstedt
Core Developer
Opera Software
Received on Tuesday, 16 February 2010 08:20:00 GMT

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