Re: Timed tracks

<chair hat off>

On May 8, 2010, at 5:26 AM, Sean Hayes wrote:

>
> It is difficult in the W3C to keep a bunch of related and  
> interdependent specs progressing in different WGs at different  
> speeds in synch; but let’s be clear TTML *is* defined in terms of  
> CSS by reference. So yes, implementing TTML using a CSS engine *is*  
> a good long term solution if you have one that is consistent with  
> CSS3 and will remain so, however TTML is defined the way it is so  
> that a full CSS implementation is not a requirement while retaining  
> presentational interoperability, and so that it is based on stable  
> references; since doing otherwise would limit its usefulness in  
> other contexts.  Defining a new format in the W3C would simply re- 
> encounter all the same logistical problems that TTML has had to  
> negotiate.

The problem is not primarily the style properties, it's the basic  
layout model. Let me give some examples.

1) Look at this algorithm, which explicitly defines TTML layout in  
terms of mapping to XSL-FO:
http://www.w3.org/TR/ttaf1-dfxp/#semantics-region-layout-step-2

Here is just one step from that process:

"3. map the tt element to an fo:root element, populated initially with  
an fo:layout-master-set element that contains a valid fo:simple-page- 
master that, in turn, contains an fo:region-body child, where the root  
container extent expressed on the tt element is mapped to page-width  
and page-height attributes on the fo:simple-page-master element;"

Can you explain how this is defined in terms of CSS by reference? I  
looked up the definitions of "fo:root", "fo-layout-master-set",  
"fo:simple-page-master" and "fo:region-body" in the XSL spec, and I  
did not see any references to CSS. Rather, I saw some direct  
definitions of layout constraints.

The rest of the steps are similarly defined directly as a mapping to  
xsl:fo.


2) This section defines TTML line layout as equivalent to XSL line  
layout:
http://www.w3.org/TR/ttaf1-dfxp/#semantics-line-layout

Can you explain how this is defined in terms of CSS by reference? This  
is the section of XSL 1.1 that most directly references line layout: <http://www.w3.org/TR/2006/REC-xsl11-20061205/#area-linebuild 
 >. I don't see CSS references here, rather, XSL defines its own line  
layout algorithm.


Regards,
Maciej