Re: [TTML2] Current draft - ttm:item concept

> On Thu, Dec 11, 2014 at 4:01 AM, Andreas Tai <tai@irt.de 
> <mailto:tai@irt.de>> wrote:
>
>     Hi Glenn,
>
>     >From my view the introduction of the “ttm:item element concept”
>     needs more investigation. Currently I see problems a) with current
>     validation mechanisms and b) a good change to encounter the same
>     content in different parts of a TTML 2 document.
>
>     a) The semantic identity of a ttm:item element is defined by the
>     value of the name attribute instead of the element name (e.g.
>     there is a ttm:item element defined with the attribute name set to
>     "creationDate" instead of a new element creationDate). In this
>     scenario it is not possible to validate with XML Schema 1.0 that
>     the content of <ttm:item name"creationDate">[Content]</ttm:item>
>     conforms to the XML Schema 1 DataType "date". Although this is
>     possible with XML Schema 1.1 I do not see this as a validation
>     option. The use of XML Schema 1.1 is not very wide spread yet and
>     I do not know of any free parser that fully (!) supports XML
>     Schema 1.1.
>
>
> AFAIC, this is a non-issue: firstly, we don't specify a standard 
> schema system, but support two RNC and XSD (early on, we had actually 
> been explicit in favoring RNC as the primary, and XSD as secondary, 
> but this distinction has fallen away); secondly, as you well know, 
> there are *many* constraints in TTML itself and in its various 
> profiles that are not schema validatable, e.g., see the enumeration 
> "Additional Semantic Tests" in the TTV documentation [1]; thirdly, 
> there are tools, like TTV [1], that has no trouble verifying the 
> adherence of additional constraints;
>
> [1] https://github.com/skynav/ttv
>
> To take a case in point, the syntax of the <length> [2] and 
> <timeExpression> [3] value expressions are not validatable by any 
> schema system I have used, partly because additional constraints apply 
> depending on context of use.
>
> [2] http://www.w3.org/TR/2013/REC-ttml1-20130924/#style-value-length
> [3] 
> http://www.w3.org/TR/2013/REC-ttml1-20130924/#timing-value-timeExpression

Hi Glenn,

Thanks a lot for the detailed explanation of the design goals of the 
tt:item issue. For the validation part: it may be helpful to first agree 
of the importance of XML Schema 1 validation mechanisms for TTML 
vocabulary. In practice this is the most used one although other methods 
exists. Secondly it may be worth to discuss the importance of validation 
for metadata elements.

Best regards,

Andreas