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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: Sun, 4 Apr 2010 01:24:07 +1100
Message-ID: <g2z2c0e02831004030724sb9bc5910v9bd4b2e3b19b3ea4@mail.gmail.com>
To: Sean Hayes <Sean.Hayes@microsoft.com>
Cc: "public-html-a11y@w3.org" <public-html-a11y@w3.org>
Hi Sean,

On Sun, Apr 4, 2010 at 12:20 AM, Sean Hayes <Sean.Hayes@microsoft.com> wrote:
> #6 <..> The semi open interval semantics is intrinsic to TTML semantics, so I'm not sure it's necessary to state that here, as it's not clear what interval you are talking about.  Is this something to do with media fragment URI's?

Since the Media Text Associations proposal is not only referring to
TTML, but also to SRT (and may in future refer to other text elements,
too), there is a need to specify how to interpret the start and end
time of the text intervals. Maybe the term "text interval" is not a
good choice - maybe you prefer "text fragment"? Or have an even better
proposal for how to identify the individual text segments that have a
start and end time?

> " This was also addressed in the text that you refer to in [#1]: The default rendering area is a <div>-like area on top of the video or above the audio controls. We can tighten up this specification if you prefer."
> Yes I think this is necessary. The UA may have added bars if the aspect ratio of the div is different to the video, but generally captions will be authored with the assumption they will appear with respect to the "safe region" of video,  If this region is altered it will throw off measures in the TTML which might be used to place text with respect to elements in the video. If the captions are authored to appear on black bars that are in the actual video, then that's OK and up to the media author. That would still count as active video pixels, what I'm looking for is something that excludes padding added by the UA.

If we assume that TTML is authored towards a particular display style
of the video, then that has to be specified somehow. If it is authored
with the assumption of there not being black bars, but the black bars
are - when the video is rendered - part of the video display area,
then it's not displaying as expected.

Reading the TTML spec, there is reference to an externally defined
"root container region", for which the extent can be changed in DFXP
(i.e. width and height), but not the root coordinate. It might make
sense to do this through CSS somehow - then it would also be
applicable to SRT and other formats. Maybe a CSS attribute such as
"letterbox: include/exclude"? Not sure how to solve this, actually.

> "I am not sure how to pick the default height for the default rendering area for audio elements, though. Maybe there is a default from TV that could be re-used."
> Digital radio might be a better place to look, as TV pretty much always assumes a picture, even if it's a still.

I have listened to radio on a TV, but you are of course right. I've
tried finding out how text works on DAB and mostly just came across a
one-line scrolling text capability (as is being used in car radios).
That probably won't be sufficient for reading captions, so maybe
something more like 3-4 lines or whatever is used on TV might be more
appropriate. Again, keen on other people's opinions here.

Received on Saturday, 3 April 2010 14:24:59 UTC

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