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 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()
  }
}


> 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'.

BTW, I'd still like to see the name changed to itts:forced in order to
avoid the display vs visibility confusion.


> One strategy for avoiding this scenario would be to assign content
> elements only to regions that have the same value of forcedDisplay.
> -->
>
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