ISSUE-270 (nigel): Appendix N assumption that root temporal extent corresponds with the beginning of a related media object [TTML 1.0]

ISSUE-270 (nigel): Appendix N assumption that root temporal extent corresponds with the beginning of a related media object [TTML 1.0]

http://www.w3.org/AudioVideo/TT/tracker/issues/270

Raised by: Nigel Megitt
On product: TTML 1.0

>From Appendix N.2:

"The above formalisms assumes that the Root Temporal Extent corresponds with the beginning of a related media object. If this assumption doesn't hold, then an additional offset that accounts for the difference may be introduced when computing media time M."

In a streaming environment it is very likely, especially for live-created subtitles, that a set of TTML samples will be produced each with a different Root Temporal Extent that starts at a different point relative to a single Related Media Object (which may itself be packaged up into an independent set of samples). However I would not in general account for this with an offset and assume (or hope) that every sample's begin and end times are zero-based; rather I'd expect the begin of the TT to be coincident with the point within the Related Media Object at which that sample begins. As a side-effect this should reduce the complexity of the transformations needed to accumulate the [short] samples together to create a longer one.

For this reason I propose that we reverse this assumption.

Received on Monday, 12 August 2013 12:47:12 UTC