Make begin times relative to the document temporal coordinate space (#512)

[nigelmegitt]

Closes #512 and #550 and partly addresses #378. Resolves the ambiguity of whether the root temporal extent begins at the same time as the document's temporal coordinate space and moves the media timing informative section to appendix I.

Clarifies that body element and isd:isd begin and end times are relative to that coordinate space, rather than the root temporal extent, whose begin time is not necessarily the origin of the document's temporal coordinate space.

[palemieux]

Built at https://rawgit.com/w3c/ttml2/issue-0512-begin-times-relative-to-doc-time-build/index.html

[palemieux]

Couple of questions:

The root temporal extent, i.e., the time interval over which a document instance is active, has an implicit duration that is equal to the implicit duration of the body element of the document, if the body element is present, or zero, if the body element is absent.

Does this mean that the implicit duration of a document with no body is 0 even if some regions have tts:showBackground="always"? That does not seem right.

then the begin point is interpreted as the later of the beginning point of the document temporal coordinate space and the beginning point of the root temporal extent.

How does one compute the the beginning point of the root temporal extent?

[nigelmegitt]

Does this mean that the implicit duration of a document with no body is 0 even if some regions have tts:showBackground="always"? That does not seem right.

@palemieux It may do, however that is not a change introduced in this pull request - you might want to open a separate issue about that. It looks like this text doesn't take into account the rehoming of body under region.

How does one compute the the beginning point of the root temporal extent?

The only help the spec gives you (in the current ED or in this pull request) is "in relationship with some external application or presentation context" so I think that's defined out of band. The reason I added the "later of" construction is because a document with no begin attributes in practice begins whenever the presentation processor chooses to begin playing it, which is either at the document temporal origin or some time later, but by definition cannot be earlier.

[nigelmegitt]

I realised this pull request also addresses #550, and updated the issue to match.

[nigelmegitt]

Brought in one of the changes needed also to address #378, i.e. moving the informative section on media timing.

[skynavga]

Prior comment withdrawn.

[skynavga]

After a careful review, I have determined that, as currently formed, this PR does not break existing timing semantics. However, the new term introduced, [document temporal coordinate space] is defined by referring to ttp:timeBase, which returns to the former term, so there is a circular reference here leaving the reader to wonder what the new term really means. Also, the begin time of body is now changed to be the "later" of the beginning time of the [document temporal coordinate space] and the root temporal extent, but we do not know how these are related so as to resolve the meaning of "later". Notwithstanding these concerns, I am going to approve this, since, in my estimation it marginally improves the tools we may use to more precisely nail down these semantics in the future.