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

From: Pierre-Anthony Lemieux <pal@sandflow.com>
Date: Tue, 8 Nov 2016 22:27:07 -0800
Message-ID: <CAF_7JxA4siU_evDu7ZQMbj0iwg4c1hj7n5nvoMQrBct2BeXwsQ@mail.gmail.com>
To: Glenn Adams <glenn@skynav.com>
Cc: "public-tt@w3.org" <public-tt@w3.org>
> so, yes, we need to document this special behavior regarding how the initial value of direction is determined

Ok. I have filed https://github.com/w3c/ttml2/issues/221 .

On Tue, Nov 8, 2016 at 5:58 PM, Glenn Adams <glenn@skynav.com> wrote:
> as I said above:
>
>> if region specifies writingMode but not direction, then direction on
>> region is initialized to the WM's inline direction
>
>
> so, yes, we need to document this special behavior regarding how the initial
> value of direction is determined
>
> On Tue, Nov 8, 2016 at 6:29 PM, Pierre-Anthony Lemieux <pal@sandflow.com>
> wrote:
>>
>>   while if both writingMode and direction are specified on region, then
>> the
>> > value of direction will override the WM's inline direction;
>>
>> Style inheritance propagates direction="ltr" (the initial value) on
>> all elements (unless explicitly specified as "rtl" somewhere), thereby
>> overriding any right-to-left direction specified using writingMode on
>> region.
>>
>> It sounds like the style resolution process should have an exception
>> for 'direction', which mimics XSL (7.29.1 Second Bullet) and, whenever
>> writingMode is specified, also sets the value of 'direction' to the
>> horizontal direction implied by writingMode.
>>
>> Best,
>>
>> -- Pierre
>>
>> On Sat, Nov 5, 2016 at 2:03 PM, Glenn Adams <glenn@skynav.com> wrote:
>> > yes; i.e.,
>> >
>> > if region specifies writingMode but not direction, then direction on
>> > region
>> > is initialized to the WM's inline direction; while if both writingMode
>> > and
>> > direction are specified on region, then the value of direction will
>> > override
>> > the WM's inline direction;
>> >
>> > we should probably note this in TTML
>> >
>> >
>> > On Sat, Nov 5, 2016 at 2:09 PM, Pierre-Anthony Lemieux
>> > <pal@sandflow.com>
>> > wrote:
>> >>
>> >> Hi Glenn,
>> >>
>> >> Ok. To determine precedence between tts:writingMode and tts:direction,
>> >> the following prose from XSL 7.29.1 applies, right?
>> >>
>> >> """To insure consistency with the "writing-mode" property, the
>> >> "direction" property is initialized to the value that sets the same
>> >> inline-progression-direction as is set by the "writing-mode" property
>> >> whenever that "writing-mode" property sets that direction. If the
>> >> "direction" property is explicitly specified on the same formatting
>> >> object the value of the "direction" property will override the
>> >> inline-progression-direction set by the "writing-mode"."""
>> >>
>> >> Best,
>> >>
>> >> -- Pierre
>> >>
>> >> On Sat, Nov 5, 2016 at 8:43 AM, Glenn Adams <glenn@skynav.com> wrote:
>> >> >
>> >> >
>> >> > 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 Wednesday, 9 November 2016 06:28:01 UTC

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