[ttml2] Clarify if the first ISD must/may be constructed when empty (#1232)

nigelmegitt has just created a new issue for https://github.com/w3c/ttml2:

== Clarify if the first ISD must/may be constructed when empty ==
This issue arises because there's discussion in MPEG regarding 14496-30 about defining the correct semantics of processing sequences of documents each in a wrapper, where the draft text explains it in terms of sequences of ISDs. @cconcolato @mikedo 

The specification for [[resolve timing]](https://www.w3.org/TR/ttml2/#procedure-resolve-timing) step 2 is:
> divide the active time duration of the current document instance into an ordered sequence of time coordinates `{T0, T1, T2, ...}` where, at each time coordinate `Ti`, some element becomes temporally active or inactive.

The question I have is: in the case that the first element that becomes temporally active does so at a time `T1` where `T1 > 0`, does this imply or require that there is a first empty ISD that begins at time 0 and ends at `T1`?

Taking it literally, no element becomes temporally active or inactive at time 0, so arguably no ISD should be generated. However, the draft text I've had sight of implies an expectation that there is such an "empty document", and it is possible that implementers would like to be able to rely on this behaviour.

I'd like to discover what our expectation and understanding is here, and if there is implementation experience to guide us, so that we can state this more clearly in the specification, and feed this information to others who may depend on it.

Please view or discuss this issue at https://github.com/w3c/ttml2/issues/1232 using your GitHub account


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Received on Friday, 14 May 2021 09:21:12 UTC