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Re: Draft TTML Codecs Registry - Issue-305

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Mon, 19 May 2014 16:11:36 +0000
To: David Singer <singer@mac.com>, Glenn Adams <glenn@skynav.com>
CC: Michael Dolan <mdolan@newtbt.com>, TTWG <public-tt@w3.org>
Message-ID: <CF9FEB16.1DD91%nigel.megitt@bbc.co.uk>
If we can definitively ditch the generic XML requirement that certainly
gives us more freedom. What do we need in order to confirm that?

It looks like the change from 'description of document' to 'set of
acceptable processors' is a bigger one though - are we okay to make that
leap too?



On 19/05/2014 17:02, "David Singer" <singer@mac.com> wrote:

>In Part 12 we were hamstrung by XML.  We *wanted* something that would
>tell the user of the file what they needed to know to determine whether
>they could process, but all XML offers is the schema (great, now I know I
>donıt have the exact schema ‹ but do I have a satisgactory one?) and
>namespaces (great, so there is stuff from other namespaces in here ‹ but
>is it mandatory to recognize or discardable?)
>
>We can do better with TTML, I hope.
>
>On May 18, 2014, at 3:39 , 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:
>>
>> 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 URLıs 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).
>>
>>
>>
>>
>> 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;
>>
>> 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.
>>
>> 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 URLıs 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.
>>
>>
>>
>> Iım 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>
>>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]
>> 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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>>
>
>Dave Singer
>
>singer@mac.com
>



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Received on Monday, 19 May 2014 16:12:13 UTC

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