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Re: a11y TF CfC on resolution to support "Media Text Associations" change proposal for HTML issue 9

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Tue, 6 Apr 2010 15:08:56 +1000
Message-Id: <35C20E92-771A-40A2-9188-BF7E301C2B7E@gmail.com>
To: Philip Jägenstedt <philipj@opera.com>
Cc: Sean Hayes <Sean.Hayes@microsoft.com>, "public-html-a11y@w3.org" <public-html-a11y@w3.org>
What do you mean by "rendering above the video but below the  
controls"? Do you mean "overlaying" them on the video in the same spot  
as the controls?

I would think the way that the demo in Firefox works that I screencast  
and posted to YouTube (link posted earlier) is most intuitive.

Regards,
Silvia.

Sent from my iPhone

On 06/04/2010, at 1:11 PM, Philip Jägenstedt <philipj@opera.com> wrote:

> Long thread, 2 comments:
>
> I think external captions should always be fitted to the entire  
> <video> element, regardless of whether or not letterboxing would  
> apply. If the captions can only be seen correctly at the size of the  
> video, then just don't set width or height on the <video> element to  
> have the size calculated automatically (no letterboxing can occur).  
> I don't think we need to introduce any new attributes to handle this.
>
> I think that browsers should render captions above the video but  
> under the controls, so that controls can never be obscured. I'm not  
> sure which spec (if any) needs to say this.
>
> Philip
>
> On Mon, 05 Apr 2010 09:26:56 +0800, Silvia Pfeiffer <silviapfeiffer1@gmail.com 
> > wrote:
>
>> Hi Sean,
>>
>> On Mon, Apr 5, 2010 at 2:49 AM, Sean Hayes  
>> <Sean.Hayes@microsoft.com> wrote:
>>> For 1) I would suggest we define the semantics of SRT, and any  
>>> other format that has no formal timing model, in terms of TTML,  
>>> (not that it has to be implemented that way of course). This will  
>>> fix the interval issue and probably a whole bunch of other stuff  
>>> too.
>>
>> SP:
>> The text associations proposal has the ability to include other
>> time-aligned text formats. Things such as SmilText, SSA/ASS or LRC or
>> any other format could be used. You are assuming that every external
>> time-aligned text format that is not TTML has no formal timing model.
>> Also, you are assuming that every external time-aligned text format
>> can be mapped without loss to TTML. I do not believe either of these
>> are true. Thus, I don't think what you are proposing will work.
>>
>> Thus, what is proposed is a basic set of rules towards which we
>> interpret the important parts of an external time-aligned text  
>> format.
>> Right now, the only rule that we have is that start and end time of a
>> timed text segment are interpreted as a semi-open interval:
>> [start,end) . Since this is also how it works in TTML, there is no
>> problem in interpreting TTML timed text segments.
>>
>> As for SRT - there will eventually be an RFC that specifies its basic
>> rules, but everything that has described it this far also states the
>> same rule, so while it currently doesn't have a formally specified
>> timing model, it is de-facto specified.
>>
>> So, I agree partially with what you are saying: namely that the  
>> timing
>> model for timed text segments as they are to be interpreted in HTML5
>> should be fixed to being an open interval of [start,end).
>>
>> I do, however, disagree with the proposal to map everything to TTML,
>> since that may lead to lossy representation. Rather, I believe that
>> every timed text segments needs to be mapped to HTML with a start and
>> end time specification. Thus, anything but the start/end time can
>> already be interpreted by HTML, while the start/end time provides the
>> mapping to the timeline of the media resource.
>>
>> In this way, text formats are only exposed to any loss created by
>> mapping from FORMAT X -> HTML, rather than a potential double loss by
>> mapping from FORMAT X -> TTML -> HTML.
>>
>>
>>
>>> For 2 ". Maybe a CSS attribute such as "letterbox: include/ 
>>> exclude"?."  Making this an author choice is a good idea. But I  
>>> wouldn't want to punt it to CSS. CSS controls the extent of the  
>>> div, but this is slightly different.
>>> Maybe we can have an additional attribute on track:
>>> @extent with values {media, container}
>>>
>>> Extent="media" means put the origin of the root rendering area for  
>>> that track at the top left pixel in the video frame, and absent  
>>> any information to the contrary in the TTML, makes the extent of  
>>> it extend to the bottom right pixel in the video frame. The video  
>>> frame may contain black bars, but these are not the same as bars  
>>> applied by the UA
>>>
>>> Extent="container" means make the root rendering area coincide  
>>> exactly with the layout div (as you had it before), this would  
>>> cause the TTML to render over any black bars or padding applied by  
>>> the UA.
>>
>> SP:
>> I believe it's ultimately a styling issue, but I would also be
>> hesitant to have to wait for a CSS change.
>>
>> So, that sounds like a good proposal to me. It would only apply to
>> video, I assume, since the @extent=media on an audio element is
>> non-existant space.
>>
>> Also, I believe it would imply that @extent=container relates to the
>> calculated width x height of the media resource. We need to pay
>> attention here to the height of any default @controls that may be
>> present. These controls are overlayed onto the bottom part of the
>> video in all implementations and disappear if not used, so we need to
>> make sure to state something that stops the captions from colliding
>> with the controls.
>>
>> Maybe we can make the container extent for video only be the video
>> height without the controls height but displayed above the controls.
>> The captions will then move up when the controls appear and back down
>> when they disappear. I've just uploaded a demo with Firefox that  
>> shows
>> the problem, see http://www.youtube.com/watch?v=Ojeh7ffhAk4 .
>>
>>
>>
>>> 3) DAB has a number of possible text associations, including full  
>>> web pages, but it seems they haven't thought of captions or  
>>> subtitles yet.
>>>
>>> Provided we allow that <audio> can have a rendering area, then we  
>>> can just give it the same default rendering as the <video>  
>>> element, which is I believe 300x150px.if authors want they can get  
>>> rid of it by making it 0 in either dimension using CSS, they won't  
>>> be able to apply captions in that case; but perhaps they will have  
>>> a transcript.
>>
>> SP:
>> HTML5 doesn't define a rendering area for <audio> by default, since
>> <audio> is often used for background music on a Web page and thus is
>> not rendered at all. It will thus have to be the other way around: if
>> you want captions for your audio file, you have to give it a width  
>> and
>> height in CSS, which then defines the container extent.
>>
>> We could, however, propose that if the audio resource has an enabled
>> text track, the audio element receives a default rendering area
>> similar to the <video> element. Though I would propose the area for
>> audio to be smaller - maybe 100x150px?
>>
>>
>> Regards,
>> Silvia.
>>
>
>
> -- 
> Philip Jägenstedt
> Core Developer
> Opera Software
Received on Tuesday, 6 April 2010 05:09:37 GMT

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