Processor Profile, Content Profile and codecs

Hi all,

Consider the following scenario:
- someone has authored a TTML document
- someone else needs to generate the "codecs" attribute for this 
document (either because the document is served by an HTTP server which 
wants to compare the associated processor profile with what the HTTP 
client accepts, or because it needs to add a codecs attribute to a DASH 
MPD, or because the document is packaged in an MP4 and the MP4 is to be 
fed to a MediaSource Buffer or described in a DASH MPD).

The question is simple: how to find out which processor profile a given 
document conforms to?

For TTML2 [2], there is a definition of 'processor profile' and a 
process to find it out:
"Processor profiles are declared by using (1) the|ttp:processorProfiles| 
on the root|tt|element, (2) one or more|ttp:profile| 
of type|processor|, or (3) a combination of these two mechanisms. If not 
declared, a processor profile is inferred from a declared content 
profile or from adefault profile 
If the attribute is used, this seems clear to me.
If elements are used, the designator may not be present: "if not 
specified, the defined profile is considered to be an undesignated 
profile". I guess this means that the processor profile in that case is 
considered "not declared" and then inferred or defaulted.
If the profile is inferred or defaulted, there seems to be a way to find 
the associated designator, but I'm not sure, as it seems to be a complex 
task to do [3]. Are there restrictions in derived specs to always either 
use the attributes?

For TTML1 [1], the notion of processor profile does not exist. The spec 
however says:
"The profile of TTML that must be supported by a TTML/Content 
Processor/in order to process a/Document Instance/is determined either 
(1) by specifying a|ttp:profile|attribute on the root|tt|element, as 
defined by*6.2.8 ttp:profile* 
<>, or (2) by 
including one or more|ttp:profile|elements in the|head|element, in 
accordance with*6.1.1 ttp:profile* 
I understand that the notion of profile in TTML1 is equivalent to the 
notion of 'processor profile' in TTML1;.

The differences with TTML2 seems to be that:
- there is no default or inferred profile.
- the ttp:profile element does not have a designator attribute.
So for cases where the ttp:profile attribute is not used, how can the 
profile identifier be determined? Are there recommendations in derived 
TTML specifications (EBU, SMPTE, ...) to always set the ttp:profile 

Additionally, the "profile" parameter defined in TTML1's MIME type 
registration says:
"The document profile of a TTMLDocument Instance may be specified using 
an optional|profile|parameter, which, if specified, the value of which 
must adhere to the syntax and semantics of|ttp:profile|parameter defined 
by Section*6.2.8 ttp:profile* 
<>of the 
published specification."

IIUC, this means that for TTML1 the 'profile' MIME parameter is 
equivalent (but with long URI values) to the 'codecs' we are discussing.



Cyril Concolato
Multimedia Group / Telecom ParisTech

Received on Tuesday, 6 October 2015 11:40:02 UTC