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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 08:56:47 -0700
Message-ID: <CACQ=j+cSZZ=3Lfh2Pn=8XtTRsPaUSkEZpbCt7NoTpo7TgSUJZA@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: Timed Text Working Group <public-tt@w3.org>
On Fri, Jan 16, 2015 at 8:33 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>

>  From: Glenn Adams <glenn@skynav.com>
>  Date: Friday, 16 January 2015 15:24
> To: Nigel Megitt <nigel.megitt@bbc.co.uk>
> Cc: Timed Text Working Group <public-tt@w3.org>
> Subject: Re: ISSUE-366 (condition vs xml:id): xml:id uniqueness needs to
> be broken for some uses of condition [TTML2]
> On Fri, Jan 16, 2015 at 7:38 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>
> wrote:
>>  Ah, thanks Glenn, that solution didn't occur to me. It would be worth
>> clarifying  in the spec that semantic exclusion applies also to references
>> to excluded elements, or equivalently that referenced elements that are
>> semantically excluded evaluate to 'null' and can be pruned. At present
>> there's nothing in the spec that defines the behaviour in the scenario in
>> which an element references another that has been semantically excluded.
>  I can add some informative text on this, but these normative semantics
> currently fall out naturally in step 2 of [1].
>  [1]
> https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html#semantics-style-resolution-processing-sss
>  I don't see how it does naturally fall out – perhaps it needs some
> normative wording such as changing "for each style element *SREF*" to "for
> each semantically included style element *SREF*". I'd argue that this is
> not a tautology since we have a new concept of style elements that exist
> but are semantically excluded.

that's reasonable

>      For example either as a change to the condition section or to places
>> where references might be made, e.g. §10.2.1 style.
>>  Presumably the same arises when a src attribute is a fragment
>> identifier that points to a semantically excluded data element.
>  Could you elaborate this scenario? I can see how a <source> child of
> <data> would not resolve to a resource if @condition were false, but not
> sure what you have in mind about @src attribute, since one can only have
> one @src attribute and it only takes one URI.
>  Take the example from data, but modify it to exclude the data via a
> condition, leaving with a source element whose src URI points to nothing:
>     <head>
>   <resources>
>     <data xml:id="sharedImageData" type="image/png" length="119" condition="false">
>     </data>
>     <image xml:id="sharedImage">
>       <source src="#sharedImageData"/> <!-- This points to a semantically excluded resource now -->
>     </image>
>   </resources>
> </head>
> ...
> <body xmlns:tts="http://www.w3.org/ns/ttml#styling">
>   <div tts:backgroundImage="#sharedImage"/>
>   <div tts:backgroundImage="#sharedImage"/>
> </body>
OK. It is clear I need to better define what I intended by "exclude an
element from semantic processing". What I intended is that the element
still exists and can be referenced, by that its meaning is null, e.g., a
data element that is conditionally excluded doesn't contain any data (or
resolve to a resource), a style element that is conditionally excluded
doesn't contain any styles, etc.

>>  From: Glenn Adams <glenn@skynav.com> Date: Friday, 16 January 2015 14:18
>> 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 15:57:35 UTC

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