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Re: Draft TTML Codecs Registry - Issue-305

From: Glenn Adams <glenn@skynav.com>
Date: Tue, 20 May 2014 08:33:00 +0900
Message-ID: <CACQ=j+czS+3NpywxAKsRf56fc_rLgFy0UE9JHzNGOhitgx-sPQ@mail.gmail.com>
To: Nigel Megitt <nigel.megitt@bbc.co.uk>
Cc: Michael Dolan <mdolan@newtbt.com>, TTWG <public-tt@w3.org>
I should add that this list seems to be a collection of somewhat unrelated
requirements, which makes having a coherent discussion painful.


On Tue, May 20, 2014 at 8:25 AM, Glenn Adams <glenn@skynav.com> wrote:

>
>
>
> 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
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> ----------------------------
>>>>>>
>>>>>>
>>>>>>
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>>>>>> ---------------------
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
Received on Monday, 19 May 2014 23:33:50 UTC

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