ittp:activeArea clarification

[palemieux]

From https://lists.w3.org/Archives/Public/public-tt/2017May/0018.html

It is understood that the purpose of the activeArea attribute is to
specify an area where nothing disturbs presentation of an element with
this attribute. However, it is unclear for us how to do it because
behavioral model of this attribute is not defined. The draft notes the
case in which video cropping involves cropping of captions. But
considering a case that the other object(s) are trying to overlap the
object with this attribute, a relation between the attribute and z-index
of CSS should come to an issue.

[palemieux]

The behavioral model of this attribute is left unspecified on purpose. This attribute is similar to the AFD metadata commonly used in broadcast applications. AFD metadata does not dictate video decoder and display behavior. Instead, the latter can use the AFD metadata to choose a presentation mode, based on display shape and user preferences.

See discussion at #191

[andreastai]

I agree. The strategies how to make use of this piece of extra information was left unspecified on purpose.

[nigelmegitt]

I also agree; maybe we need to be clearer in an informative note about why this is the case?

[palemieux]

Informative note added by #231

[palemieux]

Closed per https://lists.w3.org/Archives/Public/public-tt/2017Jun/0111.html