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Re: Draft TTML Codecs Registry - Issue-305

From: Glenn Adams <glenn@skynav.com>
Date: Tue, 20 May 2014 08:25:49 +0900
Message-ID: <CACQ=j+e=UbFpJgC-mkY0J+p8H5y5Ghhg5+3BQO4JOi8ePDZ9kw@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: Michael Dolan <mdolan@newtbt.com>, TTWG <public-tt@w3.org>
On Mon, May 19, 2014 at 10:54 PM, Nigel Megitt <nigel.megitt@bbc.co.uk>wrote:

>  On 19/05/2014 12:35, "Glenn Adams" <glenn@skynav.com> wrote:
>
>
> On Mon, May 19, 2014 at 8:15 PM, Nigel Megitt <nigel.megitt@bbc.co.uk>wrote:
>
>>   On 19/05/2014 12:05, "Glenn Adams" <glenn@skynav.com> wrote:
>>
>>   On Mon, May 19, 2014 at 6:27 PM, Nigel Megitt <nigel.megitt@bbc.co.uk>wrote:
>>
>>>  On 18/05/2014 02:39, "Glenn Adams" <glenn@skynav.com> wrote:
>>>
>>>
>>> On Fri, May 16, 2014 at 2:51 AM, Nigel Megitt <nigel.megitt@bbc.co.uk>wrote:
>>>
>>>>  On 15/05/2014 23:45, "Glenn Adams" <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 URL’s 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.
>>
>
>  I don't think so. First, I don't agree there is a generic requirement
> that we need to solve. Or I haven't seen a clear explanation of such
> requirement or why we need to address it. [Assuming for a moment you are
> talking about namespaces, and not talking about the other discussion about
> MIME types.]
>
>
>  It seems clear to me from reading the ISO 14496 excerpt that this
> generic requirement exists for those who specify values to go in the
> @codecs parameter, which is what we're being asked to do to improve TTML
> interoperability in an ISO Base Media File Format context. Perhaps someone
> closer to that spec can explain if this is in fact the case?
>
>
>
>
>>  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
>>
>
>  I read that email, but it did nothing to specify the problem that folks
> are attempting to solve. For example, it does not indicate how namespaces
> are related to content profiles.
>
>
>  "The content profile shall itself reference one or more namespaces and
> schema locations. " to quote myself.
>
>     My position is that namespaces are not related to content profiles,
> and the former say nothing about the latter (in general). I'm not even sure
> there is a minimal mapping to a content profile that can be inferred from a
> set of namespace definitions.
>
>
>  Agreed.
>
>
>  So, if someone is suggesting that a list of namespaces serve as a
> shorthand for a content profile, I'm just not seeing it.
>
>
>  Agreed.
>
>
>>
>>>  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!
>>
>
>  If we are going to stop moving around in circles, I need to see a
> simple, complete problem statement. I have yet to see one.
>
>
>  Agreed.
>
>  The closest I have at the moment (synthesised from what others have
> written) is:
>
>    - Define a set of short labels and combinatorial operators to be used
>    in the first instance as extensions to the stpp.ttml prefix as used in the
>    ISO/IEC 14496-12 @codecs parameter and as a suffix to the
>    application/ttml+xml MIME type, and therefore conforming to any
>    requirements defined by the specifications for those.
>
> I agree with the part up to, but not including "and therefore conforming
to...".

>
>    - ISO/IEC 14496-12 requires signalling of the set of {namespace,
>    schemaLocation} used by XML documents.
>
> I'm not familiar with this spec or why it wants to signal namespaces. I
don't know why it wants such a list. In any case, this seems something
outside the scope of the TTML spec itself, in the realm of "how does
14496-12 deal with a particular XML format", which seems a general
question, and completely unrelated to the discussion of profiles.

>
>    - A receiving system must be able to to make processing choices based
>    on the values prior to parsing a TTML document.
>
> Must is a bit too strong. Should is better. There is nothing that prevents
a receiver from fully decoding TTML before doing anything else, though it
may be inefficient.

>
>    - It must be possible to specify receiving systems in terms of the
>    parameter values that must minimally be supported.
>
> Again, should is better than must. By "parameter values" here are you
referring to MIME Content-Type parameters? If so, then I would characterize
this as a first-order approximation to answering the question - can I
process? This might return true, but during the actual parsing it may turn
into false.

>
>    - It should be possible for those who wish, to be able to validate
>    documents against the conformance claims made for them by the value of this
>    parameter. (I think this is a weaker requirement than the others because
>    this would necessarily require parsing the document so it is reasonable to
>    use features internal to the document for validation.)
>
> If you are referring to an external parameter on Content-Type, then I
would not agree. There is no need to base validation processing on an
external parameter that signals content profile. Only a document internal
parameter, e.g., ttp:contentProfile, should be used for this purpose for a
simple reason: external data is more likely to be wrong that internal data.

>
>    - Content providers should be able to derive acceptable values for
>    this parameter given a previously unknown document, but may not need to if
>    they know something already about how the document was created.
>
> Where is this requirement coming from? What is the use case?


>
>
>
>>
>>
>>
>>>  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>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> 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 URL’s 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.
>>>>>
>>>>>
>>>>>
>>>>> I’m warming up to the idea of requiring TTML content profiles be
>>>>> created for the combinations.
>>>>>
>>>>>
>>>>>
>>>>>                 Mike
>>>>>
>>>>>
>>>>>
>>>>> *From:* Glenn Adams [mailto:glenn@skynav.com <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>
>>>>> 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> 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> 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 <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> wrote:
>>>>>
>>>>> > See [1].
>>>>> >
>>>>> > [1] https://www.w3.org/wiki/TTML/CodecsRegistry
>>>>>
>>>>> Dave Singer
>>>>>
>>>>> singer@mac.com
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> ----------------------------
>>>>>
>>>>>
>>>>>
>>>>> http://www.bbc.co.uk
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>>>>> ---------------------
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>
>
Received on Monday, 19 May 2014 23:26:40 UTC

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