[whatwg] Introduction of media accessibility features

Jonas Sicking jonas at sicking.cc
Tue Apr 13 00:28:54 PDT 2010

On Sun, Apr 11, 2010 at 11:39 PM, Silvia Pfeiffer
<silviapfeiffer1 at gmail.com> wrote:
> On Mon, Apr 12, 2010 at 4:00 PM, Philip Jägenstedt <philipj at opera.com> wrote:
>> On Mon, 12 Apr 2010 08:47:33 +0800, Silvia Pfeiffer
>> <silviapfeiffer1 at gmail.com> wrote:
>>> On Mon, Apr 12, 2010 at 7:59 AM, Jonas Sicking <jonas at sicking.cc> wrote:
>>>> On Sun, Apr 11, 2010 at 5:30 AM, Silvia Pfeiffer
>>>> <silviapfeiffer1 at gmail.com> wrote:
>>>> f>> Is it expected that all of TTML will be required? The proposal
>>>> suggests
>>>>>> 'starting with the simplest profile', being the transformation profile.
>>>>>> Does
>>>>>> this mean only the transformation profile is needed to provide subtitle
>>>>>> features equivalent to SRT?
>>>>> That is also something that still has to be discussed further. Initial
>>>>> feedback from browser vendors was that the full TTML spec is too
>>>>> complicated and too much to support from the start. Thus, the
>>>>> implementation path with the TTML profiles is being suggested.
>>>>> However, it is as yet unclear if there should be a native parsing
>>>>> implementation of TTML implemented in browsers or simply a mapping of
>>>>> TTML markup to HTML/CSS/JavaScript. My gut feeling is that the latter
>>>>> would be easier, in particular since such a mapping has been started
>>>>> already with Philippe's implementation, see
>>>>> http://www.w3.org/2009/02/ThisIsCoffee.html . The mapping would need
>>>>> to be documented.
>>>> Personally I'm concerned that if we start heading down the TTML path,
>>>> browsers are ultimately going to end up forced to implement the whole
>>>> thing. Useful parts as well as parts less so. We see this time and
>>>> again where if we implement part of a spec we end up forced to
>>>> implement the whole thing.
>>>> Things like test suites, blogging advocates, authoring tools, etc
>>>> often means that for marketing reasons we're forced to implement much
>>>> more than we'd like. And much more than is useful. This is why spec
>>>> writing is a big responseibility, every feature has a large cost and
>>>> means that implementors will be working on implementing that feature
>>>> instead of something else.
>>> Understood. But what is actually the cost of implementing all of TTML?
>>> The features in TTML all map onto existing Web technology, so all it
>>> takes is a bit more parsing code over time. And if we chose not to
>>> implement TTML, we will have to eventually support some other format
>>> that provides formatting and positioning capabilities, seeing how the
>>> legal landscape has evolved for traditional media (e.g. TV, set-top
>>> box technology). Since TTML was originally developed to be the
>>> exchange format for all such formats, it should have a sensible set of
>>> features for this space. So, I personally think it's not a bad choice
>>> for the purpose. Which other format did you have in mind to replace
>>> it?
>> For the record, I am also not enthusiastic about TTML, specifically the
>> styling mechanism which even makes creative use of XML namespaces. An
>> example [1] for those that haven't seen it before:
>> <region xml:id="r1">
>>  <style tts:extent="306px 114px"/>
>>  <style tts:backgroundColor="red"/>
>>  <style tts:color="white"/>
>>  <style tts:displayAlign="after"/>
>>  <style tts:padding="3px 40px"/>
>> </region>
>> ...
>> <p region="r1" tts:backgroundColor="purple" tts:textAlign="center">
>>  Twinkle, twinkle, little bat!<br/>
>>  How <span tts:backgroundColor="green">I wonder</span> where you're at!
>> </p>
>> While I don't have any suggestions about what to use instead, I'd much
>> prefer something which just uses CSS with the same syntax we're all used to.
>> [1]
>> http://www.w3.org/TR/ttaf1-dfxp/#style-attribute-backgroundColor-example-1
> I have looked at alternative formats the provide styling and
> positioning functionality. There is, for example, ASS/SAA
> http://www.matroska.org/technical/specs/subtitles/ssa.html . We could
> decide to support something like that instead, but it would be
> essentially the same work as for TTML: define a mapping to Web
> technologies (HTML, CSS, JavaScript) and implement that mapping, as I
> don't think anyone would implement SSA natively either.
> I am myself not excited by the way that TTML turned out and would have
> wished for a more Web-friendly format to have come out of the W3C, but
> that's what it is now. Also, I am really missing hyperlinking
> functionality in it, but believe it is possible to extend it in the
> future.
> We could, of course, decide to develop a totally new specification,
> but then we are left on our own to push that into the world. At least
> TTML already has support by several existing vendors in the caption
> space (see e.g.
> http://www.adobe.com/accessibility/products/flash/captioning_tools.html,
> http://www.ooyala.com/www3/support_closedcaptions,
> http://broadcastengineering.com/automation/ninsight-unveils-dfxp-subtitling-mxf-ayoto-0901/).
> I just think of all the options available TTML is the least
> problematic way to go. But if somebody has a better idea, I'm more
> than open to it! (This is me personally saying it - it's not a
> representative opinion of the W3C HTML5 accessibility task force).

Henri Sivonen brought up an interesting point on the HTML WG list.
Bringing it up here as I don't know if people are following both

Will implementations want to do the rendering of the subtitles off the
main thread? I believe many browsers are, or are planning to, render
the actual video graphics using a separate thread. If that is correct,
do we want to support rendering of the subtitles on a separate thread

Or is it enough to do the rendering on the main thread, but composit
using a separate thread?

If rendering is expected to happen on a separate thread, then CSS is
possibly not the right solution as most CSS engines are
main-thread-only today.

/ Jonas

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