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Subtitle/caption styling + TTML + CSS

From: Pierre-Anthony Lemieux <pal@sandflow.com>
Date: Sun, 11 Dec 2016 21:04:58 -0800
Message-ID: <CAF_7JxCgfZyLZh0Wqd9q6zceoyr5UoMJ8jsdHj3pRP=z3ZMA+Q@mail.gmail.com>
To: "L. David Baron" <dbaron@dbaron.org>
Cc: Thierry MICHEL <tmichel@w3.org>, Bert Bos <bert@w3.org>, Chris Lilley <chris@w3.org>, www-style list <www-style@w3.org>, Nigel Megitt <nigel.megitt@bbc.co.uk>
Hi David et al.,

> I don't think TTML1 was designed in a
> way that would fit well in browser implementations.

I have been working on an open source JavaScript library [1] for
rendering TTML1 documents to HTML5 fragments.

[1] https://github.com/sandflow/imscJS

TTML is based on XSL, which is based on CSS, and so the mapping has
been straightforward. [ed.: I am not sure what you mean by "does not
fit well", perhaps you can elaborate.]

The most significant challenge has been supporting two features
(linePadding and multiRowAlign [2]), which are not supported in CSS,
but have been identified as essential to captioning in Europe by the
TTWG (and EBU).

[2] https://www.w3.org/TR/ttml-imsc1/#linepadding

I would think that features that are important to
subtitling/captioning should be considered for CSS, regardless of the
ultimate timed text format (TTML, WebVTT, etc...)

Best,

-- Pierre

On Wed, Nov 23, 2016 at 3:33 PM, L. David Baron <dbaron@dbaron.org> wrote:
> On Friday 2016-11-18 17:48 +0100, Thierry MICHEL wrote:
>> CSS colleagues,
>>
>> The Timed Text Working Group (TTWG) published yesterday an ordinary Working
>> Draft of Timed Text Markup Language 2 (TTML2)
>> W3C Working Draft 17 November 2016
>> https://www.w3.org/TR/2016/WD-ttml2-20161117/
>>
>> FYI, this publication is not the last publication before requesting
>> transition to Candidate Recommendation. The TTWG plans to publish a final WD
>> soon. We will let you know.
>>
>> Meanwhile, the TTWG invites you to review this TTML2 WD.
>>
>> The horizontal review should focus only on the new features
>> introduced in TTML2.
>> Please refer to the section for changes between Timed Text Markup Language
>> (TTML) Version 1 (TTML1) and Version 2 (TTML2).
>> https://www.w3.org/TR/2016/WD-ttml2-20161117/#changes-from-ttml1-vocabulary
>>
>> Please send your comments to  TTWG Public mailing list <public-tt@w3.org>.
>
> So it's worth noting that the styling section of the draft:
>   https://w3c.github.io/ttml2/spec/ttml2.html#styling
> has considerable new additions relative to TTML1.  This section
> contains a vocabulary that is rather similar to many CSS properties,
> but also contains significant divergence.
>
> In particular, TTML1 had in
> https://www.w3.org/TR/ttaf1-dfxp/#styling-attribute-vocabulary
> the following styling attributes that appear to match CSS at first
> glance, at least in semantics:
>   backgroundColor
>   color
>   direction
>   display (only auto vs. none)
>   extent (a shorthand for width and height)
>   fontFamily
>   fontSize
>   fontStyle
>   fontWeight
>   lineHeight
>   opacity
>   overflow
>   padding
>   textAlign
>   textDecoration (but with extra values)
>   unicodeBidi
>   visibility
>   wrapOption (like text-wrap in css-text-4)
>   writingMode (but using old values)
>   zIndex
> and the following styling attributes that do not match CSS:
>   displayAlign
>   origin (a bit like x and y in SVG)
>   showBackground
>   textOutline
>
> TTML2 introduces the following new properties that appear to have
> similar CSS properties at first glance:
>   backgroundClip (with different names for the values)
>   backgroundExtent (equivalent to background-size)
>   backgroundImage
>   backgroundOrigin (with different names for the values)
>   backgroundPosition
>   backgroundRepeat
>   border (with border-radius included in the property)
>   bpd (equivalent to block-size in css-logical-properties)
>   fontKerning (though without CSS's initial value, which is auto!)
>   ipd (equivalent to inline-size in css-logical-properties)
>   letterSpacing
>   ruby (this is done using the display property in CSS)
>   rubyAlign (with additional auto, end, and withBase values)
>   rubyPosition (with before/after names instead of over/under)
>   textCombine (equivalent to CSS text-combine-horizontal)
>   textEmphasis
>   textOrientation (but retaining the sidewaysLeft and sidewaysRight
>     values that CSS removed)
>   textShadow
> and the following that appear not to have corresponding CSS properties:
>   disparity
>   fontSelectionStrategy
>   fontShear
>   fontVariant (this is a property name used in CSS, but with a
>     different meaning!)
>   position (this is a property name used in CSS, but with a
>     different value, "center", although one that has been proposed
>     to be added to the CSS property)
>   rubyOffset
>   rubyOverflow
>   rubyOverhang
>   rubyOverhangClass
>   rubyReserve
>
>
> My opinion on this is that this seems like a lot of divergence from
> CSS.  It's divergence in naming (using different names for the same
> thing and the same names for different things), divergence in value
> spaces, and given that everything is redefined in the TTML spec
> (although often non-normatively "based on" CSS specs), almost
> certainly massive divergence in semantics.
>
> I think TTML and CSS have largely been implemented in separate
> implementations (which means that TTML has largely not been
> implemented in browsers), and I don't think TTML1 was designed in a
> way that would fit well in browser implementations.  That's why
> browsers implemented WebVTT instead.  I think continuing to diverge
> from CSS to this degree simply makes TTML implementation in browsers
> even less likely than it already was (which was already unlikely).
>
> On the flip side, I don't think fixing that divergence is
> particularly valuable (at least to browsers) since the communities
> are already separate, and I think the chance of getting substantial
> TTML implementation in browsers is low even without additional
> divergence.
>
> -David
>
> --
> 𝄞   L. David Baron                         http://dbaron.org/   𝄂
> 𝄢   Mozilla                          https://www.mozilla.org/   𝄂
>              Before I built a wall I'd ask to know
>              What I was walling in or walling out,
>              And to whom I was like to give offense.
>                - Robert Frost, Mending Wall (1914)
Received on Monday, 12 December 2016 05:05:49 UTC

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