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Re: TTML writingMode vs CSS Writing Modes Level 3 writing-mode

From: Glenn Adams <glenn@skynav.com>
Date: Sat, 5 Nov 2016 09:43:53 -0600
Message-ID: <CACQ=j+fkLMw7bCBH3WaBnVZPPpvKXzcnsMeuO8GoSg5pVHi8cg@mail.gmail.com>
To: Pierre-Anthony Lemieux <pal@sandflow.com>
Cc: "public-tt@w3.org" <public-tt@w3.org>
On Sat, Nov 5, 2016 at 12:19 AM, Pierre-Anthony Lemieux <pal@sandflow.com>
wrote:

> Hi Glenn,
>
> Thanks! So is writingMode="rltb" in TTML equivalent to {writing-mode:
> "horizontal-tb", direction: "rtl"} in CSS? In other words,
> writingMode="rltb" sets the default to default paragraph embedding
> level to "RTL" just as {direction: "rtl"} does?
>

Yes. But keep in mind that WM applies at a higher context (region) than
direction (p/span). So it can be viewed as a special case of semantic
inheritance.


>
> Best,
>
> -- Pierre
>
> On Fri, Nov 4, 2016 at 11:00 PM, Glenn Adams <glenn@skynav.com> wrote:
> >
> >
> > On Fri, Nov 4, 2016 at 11:13 PM, Pierre-Anthony Lemieux <
> pal@sandflow.com>
> > wrote:
> >>
> >> Hi all,
> >>
> >> TTML allows both 'lrtb' and 'rltb' as values for writingMode, and
> >> references Section 7.29.7 at XSL.
> >>
> >> CSS Writing Modes Level 3 [1] states that both 'lr-tb' and 'rl-tb' (as
> >> defined in SVG) are both replaced by 'horizontal-tb'.
> >>
> >> Why would 'lr-tb' and 'rl-tb' be redundant?
> >
> >
> > They aren't (in XSL-FO or TTML), since they have the added effect of
> > indicating the default paragraph embedding level (of LTR or RTL) for all
> > content targeted to a region. Keep in mind that XSL-FO defined writing
> mode
> > before CSS3 WM took it up for consideration. Apparently, CSS3 WM prefers
> to
> > use the direction property and/or character properties exclusively for
> this
> > purpose.
> >
> > In TTML, a paragraph's default embedding level can be influenced by the
> > computed value of tts:writingMode, the computed value of tts:direction,
> and
> > the character content of the paragraph.
> >
> >>
> >>
> >> Thanks,
> >>
> >> -- Pierre
> >>
> >
>
Received on Saturday, 5 November 2016 15:44:47 UTC

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