ISSUE-314: Temporally active is not defined for regions

This impacts IMSC 1.0 - see Issue-313.

i don't recall us ever discussing this distinction that you point out from SMIL; the primary focus when we added showBackground was to provide a mechanism that ensured background was visible on a temporally active region when no content was associated with it;

in particularly, i think we did not consider or expect presentation of a showBackground=true region when it was not temporally active; note that even though regions can be independently timed, they are mostly not timed, i.e., when not explicitly timed, they effectively follow the document's temporal extent, so the point is moot (i.e., such regions are always temporally active);

i think my preference at this point is to explicitly ignore the SMIL semantics that would cause any form of region rendering for a temporally inactive region; i say this because doing otherwise would produce a semantic exception to the current rendering model, and I don't see a particularly strong use case to drive such an exception

I do not understand the differentiation that showBackground provides unless it is exactly as I've described from the SMIL reference. Otherwise "always" and "whenActive" would be synonymous.

The distinction you describe is a useful one, but different - it would be "whenActive" vs "whenContainsActiveContent" or something similar. I'm uncomfortable bending the current interpretation of TTML 1 to mean something other than it says, and still have the opinion that the correct resolution is an erratum to the process for constructing ISDs. The alternative distinction is something that could be added to TTML 2 if anyone wants to do so.

There is in fact a usage for this: some implementations display permanently a background rectangle over video, on which subtitles/captions are rendered when present. This gives a cue that subtitles are enabled and avoids the repeated appearance and disappearance of a background rectangle.

Issue-330 on IMSC 1 is now dependent on this issue.

I have reviewed the issue regarding showBackground, and determined that:

(1) the showBackground property has no affect on whether a region is temporally active or not; it is only temporally active during its active time interval (as determined by its begin, dur, end attributes and those same attributes on ancestor elements in conjunction with the root temporal context interval);
(2) the value name "whenActive" actually means "when temporally active and contains content" and does not mean "when temporally active";

The phrase "temporally active region" is not synonymous with "presented region", since a temporally active region that contains no content will not produce any rendering unless showBackground is "always".

Thanks Glenn. This was not obvious to me, so I think it would be worth adding a definition of 'temporally active' or 'temporally active region' to TTML 1 SE as an erratum, and to TTML 2 also. The erratum may include both an addition to the definitions section and maybe §9.3.2 also, depending on how you think the information is best indicated.

Definitions of temporally active and temporally active region as well as text clarifying meaning of whenActive has been added in https://dvcs.w3.org/hg/ttml/rev/1142499d1b93

[nigel]: The fix has a typo - the term Temporally Active Region is mislabelled as 'Temporally Active'.

Fixed typo https://dvcs.w3.org/hg/ttml/rev/288e14ff2047