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Re: Draft TTML Codecs Registry - Issue-305

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Mon, 19 May 2014 11:15:43 +0000
To: Glenn Adams <glenn@skynav.com>
CC: Michael Dolan <mdolan@newtbt.com>, TTWG <public-tt@w3.org>
Message-ID: <CF9FA4B4.1DD39%nigel.megitt@bbc.co.uk>
On 19/05/2014 12:05, "Glenn Adams" <glenn@skynav.com<mailto:glenn@skynav.com>> wrote:
On Mon, May 19, 2014 at 6:27 PM, Nigel Megitt <nigel.megitt@bbc.co.uk<mailto:nigel.megitt@bbc.co.uk>> wrote:
On 18/05/2014 02:39, "Glenn Adams" <glenn@skynav.com<mailto:glenn@skynav.com>> wrote:


On Fri, May 16, 2014 at 2:51 AM, Nigel Megitt <nigel.megitt@bbc.co.uk<mailto:nigel.megitt@bbc.co.uk>> wrote:
On 15/05/2014 23:45, "Glenn Adams" <glenn@skynav.com<mailto:glenn@skynav.com>> wrote:

Could you cite the exact documents/sections that you are referring to by "quoted text"?

I was referring to the text from ISO/IEC 14496-12, AMD2 that Mike included in his email.

I assume you refer to:

Correct.


>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.

This tells me nothing of why one would want to signal content profile or why one would want to communicate namespace usage separately (from XMLNS declarations found in the document).

The common text "When used for metadata, this is needed for  decoding using XML aware encoding mechanisms such as BiM." in both paragraphs appears to indicate the intended purpose of providing the information. Arguably in our case we don't need a generic XML processing feature in which case perhaps no value for this attribute is needed at all. That doesn't seem to be the case though given the level of interest in this topic.


Regarding

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).

If a specification defines processing semantics that must be supported in order for a processor to conform to the specification, and if that specification does not define any feature/extension, then I would firstly view that as a broken specification; however, another potential interpretation is that the specification implies an otherwise unnamed feature/extension whose feature/extension designation corresponds to the profile designation. That is, the profile designation serves as a high-level, un-subdivided designation of the set of semantics mandated by compliance with the defined profile.

Concerning 'broken' I note also TTML1SE 3.3 [1] does require an implementation compliance statement (ICS) to support claims of compliance  it would seem reasonable to require this as an input to the registration process. Or in TTML2 weaken this requirement.

[1] http://www.w3.org/TR/ttml1/#claims


This might be a way out of this without having to have such specifications define individual, fine-grained feature/extension designations.

Yes, that would be helpful to lower the barrier to entry.


Anyway, I'm still waiting for a someone to articulate a use case for signaling a content profile, or any aspect of a content profile (e.g., namespaces, schemas).

Did Mike's email including the relevant sections from 14496-12 not do this?

No, it does not. I repeat, signaling content profile can only have two purposes in the context of decoding/processing as far as I can tell:

(1) to validate incoming document, which is not yet done by any TTML processor, though we are looking at adding a @validation attribute in TTML2 that could be used to require this;

(2) to imply a processor (decoder) profile in lieu of explicitly signaling of a processor profile;

(3) to support generic XML processing functionality not specific to TTML.

Then it would seem to be outside the scope of TTML.

If we define an alternative to the generic set of {namespace, schemaLocation} then we need to explain how this requirement can be met otherwise we'll potentially break that generic requirement. It may be outside the scope of TTML but not outside the scope of what we've been asked to solve.



In the context of the current thread, it seems only the second of these is potentially relevant. However, I have to ask why one wouldn't simply signal a processor profile instead of using a more complex process of signaling a content profile and then having the decoder/processor infer a processor profile from that content profile.

My proposal is to signal a specification not a content profile.

I don't know what that means.

I defined it in my previous email on this thread at [1], also copied in to this email below, at "Thu, May 15, 2014 at 1:28 PM".

http://lists.w3.org/Archives/Public/public-tt/2014May/0075.html


This meets both the stated requirement in 14496-12 of identifying set of {namespace, schema} and the requirement to direct the choice of processor profile.

I don't think so.

So you don't think that a proposal that you don't understand does something. I don't know what to do with that information!


The content provider may not want the receiving system to infer 'any possible' processor profile from the content profile but instead describe 'must be one of these' processor profiles.

Sure, in which case the content author should reference or specify a processor profile in the document, in which case signaling a content profile has no purpose.

That's fine but out of scope of the discussion, which is about what is specified externally to the document.





If there are other reasons for signaling content profile (in the context of the current thread) then I haven't seen them articulated.







On Thu, May 15, 2014 at 1:28 PM, Nigel Megitt <nigel.megitt@bbc.co.uk<mailto:nigel.megitt@bbc.co.uk>> wrote:
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.

Nigel


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.

                Mike

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.

        Mike

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

OK

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

singer@mac.com<mailto:singer@mac.com>





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Received on Monday, 19 May 2014 11:17:08 UTC

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