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Re: ISSUE-366 (condition vs xml:id): xml:id uniqueness needs to be broken for some uses of condition [TTML2]

From: Glenn Adams <glenn@skynav.com>
Date: Fri, 16 Jan 2015 08:31:11 -0700
Message-ID: <CACQ=j+ezHc=33riCHi+6nm6A=KeTx4qpnf9S9Si67K7EwjbZag@mail.gmail.com>
To: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Cc: Nigel Megitt <nigel.megitt@bbc.co.uk>, Public TTWG List <public-tt@w3.org>
On Fri, Jan 16, 2015 at 8:01 AM, Silvia Pfeiffer <silviapfeiffer1@gmail.com>
wrote:

>
> On 16 Jan 2015 15:40, "Nigel Megitt" <nigel.megitt@bbc.co.uk> wrote:
> >
> > From: Glenn Adams <glenn@skynav.com> Date: Friday, 16 January 2015 14:33
> >
> >> We could also upgrade @region to IDREFS, like @style, in which case, if
> more than one of the referenced regions is semantically active, i.e.,
> condition is not false on multiple referenced regions, we could define
> behavior as one of:
> >> use the first (and only first) referenced region that is semantically
> active;
> >
> > Seems like a good idea, as a minimum.
> >>
> >> select the content into all referenced regions that are semantically
> active;
> >
> > I don't know a use case for this.
>
> FWIW: on the web, ID is semantically supposed to point to 1 or 0 elements,
> not to more. I'd highly recommend for the first option to avoid confusions.
>
The distinction is between IDREF and IDREFS. The former points exactly one
element (not zero). The latter points at one or more elements.

We already have a few attributes typed as IDREFS, such as @animate, @style,
and @ttm:agent.

The idea being discussed here is upgrading @region from IDREF to IDREFS so
that it may reference multiple region elements, some (or all but one) of
which may be semantically excluded by @condition. The only question in my
mind was whether the interpretation of @region with more than one non-null,
i.e., more than one non-conditionally-excluded, region references should
mean (1) use first non-null region, or (2) use all non-null regions (i.e.,
map the same content to the multiple non-null regions).

I have a preference for the latter interpretation for the sake of symmetry.


> Cheers,
> Silvia.
>
> >
> >
> >>
> >> On Fri, Jan 16, 2015 at 7:18 AM, Glenn Adams <glenn@skynav.com> wrote:
> >>>
> >>>
> >>>
> >>> On Fri, Jan 16, 2015 at 2:52 AM, Timed Text Working Group Issue
> Tracker <sysbot+tracker@w3.org> wrote:
> >>>>
> >>>> ISSUE-366 (condition vs xml:id): xml:id uniqueness needs to be broken
> for some uses of condition [TTML2]
> >>>>
> >>>> http://www.w3.org/AudioVideo/TT/tracker/issues/366
> >>>>
> >>>> Raised by: Nigel Megitt
> >>>> On product: TTML2
> >>>>
> >>>> Consider the use case in which an author wishes to permit the viewer
> of a TTML2 document to select from one of a number of style choices, either
> depending on a parameter or a media query, for example choices that vary
> tts:fontSize and tts:extent to accommodate 'normal size font', 'large size
> font' and 'small size font' options.
> >>>>
> >>>>
> >>>> The condition attribute can only be used to omit an element from
> semantic processing, not to change its behaviour. One might imagine that
> the following is a way to proceed:
> >>>>
> >>>> ...
> >>>> <layout>
> >>>> <region xml:id="r1" condition="parameter(text_size)=='large size
> font'" tts:extent="95vw 30vh" .../>
> >>>> <region xml:id="r1" condition="parameter(text_size)=='normal size
> font'" tts:extent="80vw 20vh" .../>
> >>>> <region xml:id="r1" condition="parameter(text_size)=='small size
> font'" tts:extent="60vw 15vh" .../>
> >>>> </layout>
> >>>> <styling>
> >>>> <style xml:id="sFontSize" condition="parameter(text_size)=='large
> size font'" tts:fontSize="15vh"/>
> >>>> <style xml:id="sFontSize" condition="parameter(text_size)=='normal
> size font'" tts:fontSize="10vh"/>
> >>>> <style xml:id="sFontSize" condition="parameter(text_size)=='small
> size font'" tts:fontSize="7.5vh"/>
> >>>> <style xml:id="sDefaultFont" style="sFontSize"
> tts:fontFamily="myFontFamily"/>
> >>>> </styling>
> >>>>
> >>>> ...
> >>>>
> >>>> <body>
> >>>> <div region="r1" style="sDefaultFont">
> >>>> ...
> >>>> </div>
> >>>> </body>
> >>>
> >>>
> >>> Condition is not intended to be used like #ifdef | #ifndef, i.e., as a
> syntactic inclusion/exclusion system, but as semantic inclusion/exclusion
> only. There are perfectly good alternatives to the above that do not
> require duplicating ids, such as:
> >>>
> >>> <styling>
> >>> <style xml:id="s0" tts:fontFamily="myFontFamily"/>
> >>> <style xml:id="s1" condition="parameter(text_size)=='large size font'"
> tts:fontSize="15vh" tts:extent="95vw 30vh"/>
> >>> <style xml:id="s2" condition="parameter(text_size)=='normal size
> font'" tts:fontSize="10vh" tts:extent="80vw 20vh"/>
> >>> <style xml:id="s3" condition="parameter(text_size)=='small size font'"
> tts:fontSize="7.5vh" tts:extent="60vw 15vh"/>
> >>> </styling>
> >>>
> >>> <layout>
> >>> <region xml:id="r1" style="s0 s1 s2 s3"/>
> >>> </layout>
> >>>
> >>>>
> >>>>
> >>>> However this construct, which requires use of xml:id for style and
> region reference, breaks xml:id uniqueness rules, resulting in invalid
> documents. What options are there for achieving this use case? I can see:
> >>>>
> >>>> a) repeating all the content in the document with different style and
> region references and specifying condition only on the content,
> >>>>
> >>>> b) basing everything on the initial element and making that
> conditional (since nothing needs to refer to initial by xml:id), and
> specifying all regions inline - unfortunately this may be very verbose in
> terms of repeating regions on many content elements, but it could work for
> cases where there are only a few regions and they can be associated with
> body or div elements.
> >>>>
> >>>> Neither of these two options is particularly attractive - a) is
> highly repetitious and offers no advantage over the provision of multiple
> documents with any associated costs for asset management and distribution
> there. b) is limited in basing style on initial so it is a 'one chance'
> condition, and it is potentially repetitious in region definition.
> >>>>
> >>>>
> >>>> By the way, there are at least three audience groups for which this
> use case exists: 1) Those who have reading difficulties with normal size
> text; 2) users of different devices, where it has been established that
> text needs to be rendered at different sizes on large screen televisions
> from smartphones for example; 3) those who just want to be able to
> customise the display.
> >>>>
> >>>>
> >>>> It would be great if the condition construct could be used to allow
> some predefined viewing options to be authored into the document, i.e. in a
> controlled way by the document author. I can't see how this can be achieved
> at present though.
> >>>>
> >>>>
> >>>> What solution choices are there? Perhaps the easiest is to redefine
> the condition construct so that it also includes an 'if then else if'
> syntax in which attributes can be defined, so you might end up with, for
> example:
> >>>>
> >>>> <region xml:id="r1" condition="if parameter(text_size)=='large size
> font' then (tts:extent='95vw 30vh' elseif parameter(text_size)='normal size
> font' then (tts:extent='80vw 20vh') elseif parameter(text_size)='small size
> font' then (tts:extent='60vw 15vh') else (tts:extent='80vw 20vh')"/>
> >>>>
> >>>> Then xml:id rules are not broken and region r1 can be referenced
> safely with the attribute evaluation only being conditional. I'd advocate
> retaining the ability to specify a condition that can be used to exclude
> the entire element from semantic processing, as now.
> >>>>
> >>>>
> >>>> Another solution to this problem might be to define some
> preprocessing using XPath to select specific elements and/or attributes and
> set values on the basis of the same condition functions that have already
> been specified, i.e. parameter, media, supports. Something like:
> >>>>
> >>>> <tt:tt [parameters etc]>
> >>>> <preprocess>
> >>>>    <rule condition="parameter(text_size)=='large size text'"
> path="//region[@xml:id='r1']">
> >>>>       <attributes tts:extent="95vw 30vh">
> >>>>    </rule>
> >>>> </preprocess>
> >>>> <head>
> >>>>    <layout><region xml:id="r1"/></layout>
> >>>> ...
> >>>>
> >>>> It would be an error for a path attribute to refer to anywhere except
> <head> or <body> or their descendants.
> >>>>
> >>>> This option would also have the incidental effect that it would
> provide similar functionality to declarative styling. All the rules would
> be executed in document order prior to processing the <head>. Preprocessing
> could of course also be performed externally to the document before
> processing, if a 'user style' is desirable (as is the case for any XML
> document) .
> >>>>
> >>>>
> >>>>
> >>>
> >>
>
Received on Friday, 16 January 2015 15:31:58 UTC

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