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[ttml2] begin times being relative to the root temporal extent can be wrong

From: Nigel Megitt via GitHub <sysbot+gh@w3.org>
Date: Sun, 24 Dec 2017 23:12:43 +0000
To: public-tt@w3.org
Message-ID: <issues.opened-284386088-1514157162-sysbot+gh@w3.org>
nigelmegitt has just created a new issue for https://github.com/w3c/ttml2:

== begin times being relative to the root temporal extent can be wrong ==
The term "root temporal extent" is used in several places in TTML2 as a reference point for the beginning of the document. However the root temporal extent itself is defined as:

> The temporal extent (interval) defined by the temporal beginning and ending of a document instance in relationship with some external application or presentation context.

This seems problematic to me. I think it would be clearer to relate unstated begin times, of e.g. `<body>` elements and `isd:isd` documents, to the applicable origin for the document's timebase. This frees up the root temporal extent also to begin at a time related to that same origin, and makes more sense especially in the context of ยง12.4.1 Media Timing Semantics, which goes into further detail about one consequence of the above definition, that the root temporal extent may be truncated at the beginning by the external application or presentation context. I would not expect the times in `isd:isd` documents for example to require offsetting to accommodate such a truncation - rather, they would remain the same relative to a fixed origin.

Please view or discuss this issue at https://github.com/w3c/ttml2/issues/512 using your GitHub account
Received on Sunday, 24 December 2017 23:12:46 UTC

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