W3C home > Mailing lists > Public > public-tt@w3.org > May 2014

Re: Draft TTML Codecs Registry - Issue-305

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Thu, 15 May 2014 19:28:48 +0000
To: Michael Dolan <mdolan@newtbt.com>, "'TTWG'" <public-tt@w3.org>
Message-ID: <CF9AC7B1.1DB90%nigel.megitt@bbc.co.uk>
Since namespaces and schemas define and constrain document contents without defining processing behaviour the quoted text defines a content profile declaration. It isn't asking for anything concerning specific processor capabilities but is merely describing  the contents of the document. The information may be used for downstream processing by context aware processors. The reference to namespace-aware compression makes clear that the mapping from whatever label scheme we choose to namespaces and schemas is important.

However it's clear that we expect the receiving system to use the information to direct its processing, as described previously.

Consider that the specification of a TTML variant x consists of the union of a content profile Cx and a description of processing behaviour Bx, which I'll express as S = C + B. The content profile shall itself reference one or more namespaces and schema locations. The processing behaviour may or may not be expressed in terms of TTML1-style profile features. There's no other language other than prose available for this purpose (that I know of).

It is possible to define two specifications S1 and S2 where S1 = Cx + Bx and S2 = Cx + By, i.e. the same contents are processed with different behaviour. By the quoted text there is no need to differentiate between them from an ISO 14496 perspective. However we understand from our knowledge of the problem space that it may be useful to signal to a receiving system which behaviour set is desirable. And it may be helpful in a receiving system to differentiate between the available behaviours in order to provide the optimal experience.

Would it be contrary to the spirit of the ISO wording to assign short labels each corresponding to some Specification, and for receiving systems to be expected to dereference (using a cached lookup table!) from those labels to the namespaces and schema locations contained within that specification's content profile? This would satisfy the ISO requirements and permit us to signal additionally the processor features and behaviours. At this stage the expression of those is not our concern  just that there is a document somewhere that describes how the implementation should work.

Going back to the previous example, if a document conforms to Cx then it could be signalled either as S1 or S2 or both, and if the content provider has verified that presentation will be acceptable either way then both S1 and S2 would be declared, otherwise just one of them (or neither if there's some other Sn that also uses Cx).

With this scheme combinatorial logic wouldn't really make sense  you could infer something about unions and intersections of content profiles but since the language used to describe processor behaviours can't be mandated (okay it could in theory, but it wouldn't be accepted in practice) it wouldn't be a well defined operation. Incidentally this is in no way a critique of the effort put in by Glenn, and its outcomes, in terms of defining content and processor profiles  though it might be nice to verify that this simple expression can be expanded into that scheme should a specification writer choose to do so.

This implies that every combination of content profiles and behaviours must be considered carefully and registered as a new specification with a new label. It also implies that if a document declares conformance with a set of specifications then it must conform to every member of the set of content profiles and it may be processed according to any one of the set of processing behaviours.

The expression of that set is as described previously, where we pick our favourite delimiter out of a hat made out of ampersands.

Also: this topic was discussed in summary briefly on the call today and a new suggestion arose, that some guidance for 'reasons why the TTWG would reject an application for registration' would be helpful. When requiring combinations to be registered separately there's a greater need to ensure that the registration process is quick and painless, and this guidance would help us and those who may follow to expedite it.


On 15/05/2014 18:00, "Michael Dolan" <mdolan@newtbt.com<mailto:mdolan@newtbt.com>> wrote:

I believe the problem statement is to replace the potentially unwieldy long strings in the namespace & schema_location fields defined in 14496-12 and 14496-30 with a more compact string suitable for the DASH manifest codecs field.

>From 14496-12, AMD2:

namespace is a null-terminated field consisting of a space-separated list, in UTF-8 characters, of
one or more XML namespaces to which the sample documents conform. When used for metadata,
this is needed for identifying its type, e.g. gBSD or AQoS [MPEG-21-7] and for decoding using XML
aware encoding mechanisms such as BiM.

schema_location is an optional null-terminated field consisting of a space-separated list, in UTF-
8 characters, of zero or more URLs for XML schema(s) to which the sample document conforms. If
there is one namespace and one schema, then this field shall be the URL of the one schema. If there
is more than one namespace, then the syntax of this field shall adhere to that for xsi:schemaLocation
attribute as defined by [XML]. When used for metadata, this is needed for decoding of the timed
metadata by XML aware encoding mechanisms such as BiM.

Im warming up to the idea of requiring TTML content profiles be created for the combinations.


From: Glenn Adams [mailto:glenn@skynav.com]
Sent: Thursday, May 15, 2014 9:15 AM
To: Nigel Megitt
Cc: Michael Dolan; TTWG
Subject: Re: Draft TTML Codecs Registry

My understanding from Dave was that the problem is how to answer the following method:

boolean canPlay(String contentTypeWithParameters)

I have not seen any statement of a problem that relates to signaling content conformance.

As for requirements driving the ability to express a combination of profiles, we already have (in TTML1) and will have more (in TTML2) that permits a user to characterize processing requirements by means of a combination of existing profiles. Consequently, any shorthand signaling of first-order processor support needs to be able to repeat the expression of such combinations.

I don't buy any "its too complex" argument thus far, primarily because nobody has stated what is (overly) complex in sufficient detail to understand if there is a problem or not.

My perception of the TTML profile mechanism is that it is easy to understand and implement, and, further, that it is a heck of lot easier to understand and implement than XML Schemas.

On Thu, May 15, 2014 at 9:58 AM, Nigel Megitt <nigel.megitt@bbc.co.uk<mailto:nigel.megitt@bbc.co.uk>> wrote:
Agreed there's a gulf of understanding/expectation that we need to bridge.

Can anyone volunteer to draft a set of requirements for this functionality, in the first instance being the smallest set needed to meet the ISO specs? (Mike, I guess I'm thinking of you, following our discussion at the weekly meeting earlier)

On 15/05/2014 16:48, "Glenn Adams" <glenn@skynav.com<mailto:glenn@skynav.com>> wrote:

i can see this subject is not going to be resolved easily as we clearly have a large gap about requirements; e.g., i think there are no requirements to signal content conformance, but only client processor requirements, i think we must use the TTML profile mechanism, etc

On Thursday, May 15, 2014, Michael Dolan <mdolan@newtbt.com<mailto:mdolan@newtbt.com>> wrote:
Maybe "highly undesirable", but if we don't address the A + B signaling
explicitly, then profiles need to be created for all the combinitorics of
namespaces in practice. Not the end of the world, but virtually prevents the
simple signaling of 3rd party namespaces already provided by the
namespace/schemaLocation mechanism today. No I am not proposing we use that
- I am pointing out a deficiency in this proposal that we already address
today in 14496.

Anyway, we need to go through the points in my email a week ago - if not
today, then on the 29th.


-----Original Message-----
From: David Singer [mailto:singer@mac.com]
Sent: Thursday, May 15, 2014 5:20 AM
To: Glenn Adams
Subject: Re: Draft TTML Codecs Registry


Though it will be a sub-parameter of the codecs parameter for the MP4 file
type, from the point of view of TTML it's actually a profile short name
registry rather than codecs registry, so I think we should rename it.

the values here should be usable in both
a) the profiles parameter for the TTML mime type
b) the codecs parameter for the MP4 mime type

so, also "named codecs" -> "named profiles"

I agree with Cyril that we only need a single operator here (implement one
of these profiles and you're good to go), both because we don't need the
complexity, and because a "implement both/all of these" is effectively
inviting file authors to make up new profiles ("to process this document you
have to implement both A and B"), which is (IMHO) highly undesirable.

On May 15, 2014, at 9:55 , Glenn Adams <glenn@skynav.com<mailto:glenn@skynav.com>> wrote:

> See [1].
> [1] https://www.w3.org/wiki/TTML/CodecsRegistry

Dave Singer



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Received on Thursday, 15 May 2014 19:29:21 UTC

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