[imsc] backgroundImage specification needs to require the image to be displayed

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

== backgroundImage specification needs to require the image to be 
displayed ==
§8.3 describes the syntax of a `div` element that has a 
`smpte:backgroundImage` attribute but does not require that the image 
is processed as defined in SMPTE-TT.

At least a comment like that on TTML2's `tts:backgroundImage` 
attribute would be useful:

> **Note:**
>
> The semantics of the style property represented by this attribute 
are based upon that defined by [XSL 1.1], § 7.8.3.

This matches what's in SMPTE-TT by the way.

I think a cleaner thing to do would be to add a new section before 
§8.3 (making the current §8.3 §8.4 etc) to describe 
`smpte:backgroundImage`as being a mechanism for displaying an image in
 a region with the duration specified by element element on which it 
is specified, as defined by [SMPTE-TT] and including the note above. 

This section should normatively define that the referenced image must 
have numerically equal size (in its pixels) as the extent of the 
region (in TTML pixels) regardless of pixel aspect ratio, and that 
conformant presentation processors shall scale and display that image 
coincident with the TTML region into which the content element is 
flowed.

Then the section defining constraints on `div` becomes simpler and 
only relates to constraints on `div`.

Please view or discuss this issue at 
https://github.com/w3c/imsc/issues/97 using your GitHub account

Received on Wednesday, 25 November 2015 17:43:39 UTC