SDP-US - list of potential incompatibilties when translating EBU-TT into SDP-US

Dear Sean, W3C TTML WG,

As promised by Andreas Tai (chair of the EBU Group that created EBU-TT), please find here a list of potential incompatibilities we think would hinder translating EBU-TT into SDP-US. The lines prefixed with '>>' detail our understanding, giving a work-around where possible.

I believe the last two are the most important ones.
	

4. Structural Constraints
R0003 A document must not use a div element inside a div element.

 >> All nested divs have to be placed in the outermost div. If nested divs have a style reference this has to mapped to a different styling pattern.


R0004 A document must contain only a single level of span element within a p element

 >> In EBU-TT following could be quite common <span style="style1">This is one <span style="style2">caption</span>.</span> This could be translated into <span style="style1">This is one </span><span style="style2">caption</span><span style="style1">.</span>


5.2 Use of Constrained TTML Feature content
R0007 A document must contain only the following elements:

     tt
     head
     body
     div
     p
     span
     br
     set
     layout
     styling
     region
     style

 >> The tt:metadata is not supported. All information that users put in EBU-TT documents by extension of tt:metadata will be lost. It could be translated into XML Comments.


6.6 Use of Constrained TTML Feature fontSize

R0025 document must only specify tts:fontSize(s) using a value of 50, 75%, 100%, 150% or 200%.

 >> This needs more investigation.


The default font height of the player must be 5% of the root container height
Note: This is equivalent to one cell using the default grid. A player is however not required to support the grid metric.

 >> In TTML the default is 32 columns and 15 rows. In EBU-TT a different grid will be used.


6.10 Use of Constrained TTML Feature styling-*

R0030 Style elements must not reference other styles.
	
7.1 Use of Constrained TTML Feature layout

R0038 Active region(s) must not overlap.

R0039 No more than 4 region(s) must be active concurrently.

R0040 No more than four lines of text must be selected into a region at any given time.

 >> For the most common "two row subtitle" this seems not a problem. But for some cases some transformations would be needed to match these requirements.
	

R0041  At most 32 characters must be present in any displayed text line.

 >> "New EBU-TT" and EBU-TT coming from STL do not have this restriction. The application of this constraint could result in unintended presentation.


R0042  At most one p element must be selected into a region at a time.

 >> If in an EBU-TT document more than one p is present in a region in a given time than some extra regions have to be defined.


C. Code points

 >> The required code point support would not be enough to map the current character sets supported by STL.
	

Best regards,


FRANS DE JONG 
Senior Engineer Technology & Innovation

 
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Received on Friday, 21 September 2012 13:32:15 UTC