[ttml2] Add parameters to define the temporal extent

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

== Add parameters to define the temporal extent ==
Split out from #323 - add parameters analogous to the `clipBegin` and `clipEnd` attributes of the `<audio>` element but applicable to the TTML document, whose effect is to define the period of time in the document's timebase for which the document defines some behaviour.

Initial value of the `clipBegin` parameter would be 0s, which is the current behaviour.
There would be no initial value of the `clipEnd` parameter: if present it would specify a clipping behaviour, or if absent the document contents would define the end time.

The behaviour is as follows:
* The computed value of `clipBegin` is the specified value if present or `0s` if absent.
* When generating ISDs any ISDs ending before the computed value of `clipBegin` shall be pruned.
* Any remaining ISDs beginning before  the computed value of `clipBegin` shall have their `/isd:isd/@begin` attribute set to `clipBegin`.
* If `clipEnd` is specified, any remaining ISDs beginning after `clipEnd` shall be pruned.
* If `clipEnd` is specified, any remaining ISDs ending after `clipEnd` shall have their `/isd:isd/@end` attribute set to `clipEnd`.
  * For the purposes of comparison any ISD with an unspecified `end` time or an end equivalent to "indefinite" is considered to end after any specified `clipEnd` time.

Incidentally, it is possible to construct a TTML document with an ISD that has an indefinite end time but it is not possible to express this in an `isd:isd` document, because the `end` attribute on the root element is required and `<timeExpression>` does not permit a value equivalent to the SMIL keyword "indefinite" - I will raise that as a separate issue.

The rationale for _not_ using `begin` and `end` attributes for this on the `tt:tt` element is that the `begin` attribute would create an offset for the descendant elements' timing, and the `tt:tt` element is not formally considered to be a time container.


Please view or discuss this issue at https://github.com/w3c/ttml2/issues/483 using your GitHub account

Received on Tuesday, 7 November 2017 20:04:49 UTC