Re: ISSUE-331 (forcedDisplay region background note): An advisory note on the use of backgrounds on regions in combination with forcedDisplay [TTML IMSC 1.0]

On Fri, Aug 1, 2014 at 9:19 AM, Nigel Megitt <nigel.megitt@bbc.co.uk> wrote:

>  On 01/08/2014 16:01, "Glenn Adams" <glenn@skynav.com> wrote:
>
>
> On Fri, Aug 1, 2014 at 7:51 AM, Timed Text Working Group Issue Tracker <
> sysbot+tracker@w3.org> wrote:
>
>> ISSUE-331 (forcedDisplay region background note): An advisory note on the
>> use of backgrounds on regions in combination with forcedDisplay [TTML IMSC
>> 1.0]
>>
>> http://www.w3.org/AudioVideo/TT/tracker/issues/331
>>
>> Raised by: Nigel Megitt
>> On product: TTML IMSC 1.0
>>
>> This issue is created to fulfil Action-314.
>>
>> Rationale:
>>
>> Since forcedDisplay affects the computed value of the tts:visibility
>> property
>
>
>  Actually it doesn't change the computed value of tts:visibility (or
> shouldn't). It qualifies how the computed value is used, e.g., by doing
> something like:
>
>  if (computedValue('tts:visibility') == 'visible') {
>   if (!displayForcedOnlyMode || (computedValue('itts:forcedDisplay') !=
> 'false')) {
>     renderContent()
>   }
> }
>
>
>
>  That would be one way to do it, but the current spec does seem to state
> that the tts:visibility computed value should change.
>

Then it needs to be changed.


>
>  Aren't they completely equivalent though?
>

No, since computed values are referenced elsewhere, e.g., inheritance.


>  I don't see what the difference would be between your algorithm and:
>
>  if (!displayForcedOnlyMode || (computedValue('itts:forcedDisplay')
> !='false')) {
>    setComputedValue('tts:visibility', 'visible')
> }
>
>  if (computedValue('tts:visibility')=='visible') {
>    renderContent()
> }
>

The problem is that computed value is referenced elsewhere, and this logic
shouldn't impact it.


>
>      it has no effect on whether or not the hidden/visible content is
>> temporally active. This means that if non-forced content is assigned to a
>> region with a background colour then the background will be shown whenever
>> the content is active even if it is hidden. This is likely to be unexpected
>> behaviour for some readers of the specification, who may imagine that by
>> using a setting of tts:showBackground of "whenActive" they can prevent this
>> effect.
>>
>> (incidentally they'd be correct in thinking this if forcedDisplay were
>> changed to do what its name suggests and affect tts:display, which arguably
>> would be more useful functionality)
>>
>> Proposal:
>>
>> Include a non-normative note such as the following:
>> <--
>> NOTE
>>
>> If the forcedDisplay attribute is used for content in combination with
>> regions that have a non-transparent computed background color then authors
>> should be aware that those regions' backgrounds will be drawn whenever the
>> selected content is active, even if the computed tts:visibility of that
>> content is "none".
>
>
>  Note that the two legal values of tts:visibility are 'visible' and
> 'hidden'. The values 'none' and 'false' and 'true' are not legal. [IMSC ED
> currently refers to an illegal value 'true'.
>
>
>  Yes, sorry, my mistake – as Pierre also pointed out I meant "hidden" in
> place of "none".
>
>
>  BTW, I'd still like to see the name changed to itts:forced in order to
> avoid the display vs visibility confusion.
>
>
>
>  I'd like to see it changed to itts:forcedVisibility to make it even
> clearer, if we're going to change the name at all.
>
>
>      One strategy for avoiding this scenario would be to assign content
>> elements only to regions that have the same value of forcedDisplay.
>> -->
>>
>>
>>
>>
>>
>