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Re: Liaison response - template on MIME type parameter for TimedText - ISSUE 305

From: Nigel Megitt <nigel.megitt@bbc.co.uk>
Date: Tue, 13 May 2014 10:06:01 +0000
To: Glenn Adams <glenn@skynav.com>, David Singer <singer@apple.com>
CC: Cyril Concolato <cyril.concolato@telecom-paristech.fr>, TTWG <public-tt@w3.org>
Message-ID: <CF97A1F6.1D91A%nigel.megitt@bbc.co.uk>
Hi Glenn,

Can we make this simpler? Comments and suggestions inline below…


On 12/05/2014 20:15, "Glenn Adams" <glenn@skynav.com<mailto:glenn@skynav.com>> wrote:

On Mon, May 12, 2014 at 10:37 AM, David Singer <singer@apple.com<mailto:singer@apple.com>> wrote:
Hi Glenn, comments and questions inline…

On May 12, 2014, at 18:21 , Glenn Adams <glenn@skynav.com<mailto:glenn@skynav.com>> wrote:

> You say singular “the”, but a document can be conformant with more than one profile, can’t it?  How do I indicate that?
> In TTML1, it is not possible. However, we do have an open issue to add support to TTML2 to allow defining a profile by referencing multiple referenced profiles [1]. This mechanism may be used to refer to such a combined profile, where the profile designator makes reference to the definition of the combined profile, e.g., [using the mechanisms for defining processor profile]
> #1 referencing a combination processor profile

Got it, but that doesn’t enable a content author, but a profile definer…

> #2 referencing multiple processor profiles from ttp:profile attribute
> <tt ttp:profile="http://example.com/ttml/profile/Ahttp://example.com/ttml/profile/Bhttp://example.com/ttml/profile/C" ttp:profileCombination="leastRestrictive”>

yes, that works

> #3 embedding multiple processor profiles with ttp:profile element
> <tt ttp:profileCombination="leastRestrictive">
> <head>
> <ttp:profile use="http://example.com/ttml/profile/A"/>
> <ttp:profile use="http://example.com/ttml/profile/B"/>
> <ttp:profile use="http://example.com/ttml/profile/C"/>
> </head>
> ...
> </tt>

that works too

> note that TTML1 already allows this type of combination profile definition but defines a hard-wired (rather than author specified) combination method

sorry, you lost me

TTML1 already allows including multiple ttp:profile elements [1] and defines a hardwired combination method:

If more than one ttp:profile element appears in a Document Instance, then all specified profiles apply simultaneously. In such a case, if some feature or some extension is specified by one profile to be used (mandatory and enabled) and by another profile to be required (mandatory) or optional (voluntary), then that feature or extension must be considered to be used (mandatory and enabled); if some feature or some extension is specified by one profile to be merely required (mandatory) and by another profile to be optional (voluntary), then that feature or extension must be considered to be required (mandatory).

This is equivalent to specifying ttp:profileCombination="replace" as currently defined in TTML2 [2].

[1] http://www.w3.org/TR/2013/REC-ttml1-20130924/#vocabulary-profiles
[2] https://dvcs.w3.org/hg/ttml/raw-file/tip/ttml2/spec/ttml2.html#parameter-attribute-profileCombination

> However, we need to be clear about the purpose of using a profile here. It is *not* to specify conformance, at least from the way I see people discussing this matter. Rather, it is to specify what processor profile is required to process the document. In other words, what features must be supported by processor according to author's requirements. This is distinct from what profile(s) the document conforms to. For example, a document may conform to a profile in which a feature is optionally used, but then require that feature be supported in order for it to be processed.

I am not sure I get the distinction.

* This processor profile is required to process the document.

If this can be rephrased as follows, then yes:

"A processor must abort processing (unless overridden) when the effective processor profile specifies a feature/extension is required and the processor does not support that feature/extension."

where "effective processor profile" is the result of combining all processor profiles referenced/defined by a document, and the method of combination is specified by ttp:profileCombination.

* This document conforms to this profile.

If this can be rephrased as follows, then yes:

"A document declares it satisfies (or otherwise conforms with) the effective content profile. In addition, in the absence of declared processor profile, a processor may infer a processor profile from this effective content profile."

where "effective content profile" is the result of combining all content profiles referenced/defined by a document, and the method of combination is specified by ttp:contentProfileCombination.

What is the practical difference here, for a client trying to decide “I support profiles X, Y, Z; can/should I process this document?”.

To answer this question in general, the client must determine the effective processor profile by combining all referenced/included/inferred processor profiles. A content profile declaration would only apply in the absence of an explicitly referenced or included processor profile, i.e., only when it is necessary to infer a processor profile from the effective content profile.

Isn’t it just a question of how conformance is defined (as a format question or as a processing question)?

The definitions of content conformance and processor conformance are distinct.

A document may (or not) conform to a content profile, a determination that can be made by a content validator/verifier using some set of specifications, including, e.g., schemas, custom verification tools, etc.

A processor on the other hand doesn't, strictly speaking, conform to a processor profile. It conforms to general semantic requirements of TTML, e.g., the ability to compute a document's effective processor profile and test whether it (the processor) supports that profile's required features/extensions. Thus, it is better to ask whether a processor "supports" or "satisfies" a given processor profile, and not whether a processor "conforms" with a processor profile.

Also, with respect to content profiles, it is better to ask whether a processor "supports a processor profile implied by (inferred from) a content profile".

Why do these concepts of processor profile and content profile need to be distinct? It is best to give an example:

Let's say that content profile C defines a feature F to be optional, meaning it may but need not be present. By itself this doesn't say anything about whether a processor must support F. Now, an author may decide to use F in some documents but not others, all of which conform to C. Now, let's say an author wants all processors that may process these documents to be able to correctly support F if it is present. So the author defines a processor profile P that specifies that support for F is required. Now this P is not the same as C, since the former says (support for) F is required, and the latter says (use of) F is optional.

If the author were to only make reference to content profile C, and not reference a processor profile, then a processor profile would need to be inferred from C, in which case a determination must be made as to whether support for F is required or optional. Depending on how we define this default inference process, the inferred processor profile may or may not meet the original requirements of the author (that F must be supported by processor whether or not F is used in a document), in which case the author might need to specify the suggested ttp:inferProcessorProfile.

So to summarize, the author has the following options (without considering an external codecs/profile hint):

#1 declare only a processor profile;
#2 declare only a content profile,  in which case a processor profile is inferred at processing time;
#3 declare both processor and content profiles;
#4 declare neither processor nor content profile, in which case a default processor profile is determined by the document interchange context, or, if no context or the context doesn't specify a default, then choose a default based on the type of processing (transform vs presentation) and the version of TTML that applies;

We're defining information here that can be used consistently in the document interchange context for #4 only. I have no expectation that any real world presentation processors will inspect profile features declared in the TTML documents and make processing choices based on them. Rather, it is more likely that a processor will need to be selected and initialised to expect to process a set of documents that meet a known specification, including any behaviours not expressed as features, and then be given that set to display. (I use the term 'document set' to cover the streaming scenario in which a time ordered set of documents is made available rather than a single document)

There are 2 decision points in the workflow:

#1 How should the content provider's packager label the documents? This will require some configuration or inspection of the document sets, perhaps using the profile information in the documents.
#2 What should the receiving system do with the available content?

For #2 here I would expect the decision to be made as follows:

* Given the set of specifications for which document sets are available, do I have a processor that can process any of them?
* If none, do not fetch the resource
* Else if exactly one and user wishes to display the content, initialise the processor for that specification, fetch the document set and provide it to the processor.
* Else if more than one use some implementation-defined logic to work out which specification is preferable, initialise the processor for it, fetch the document set and provide it to the processor.

It's even possible that a document will make use of optional features that aren't supported by the selected processor – in which case the processor behaviour may be defined by the relevant specification or if not, by the implementor.

I've not made any distinction between content profiles and processor profiles here because they're equivalent for this purpose: we're looking to establish a 'contract' between a document's claim to content profile conformance and a processor's claim to processor profile conformance, using identical labels each of which represents some kind of specification.

When processing on client, the processor looks only at the processor profile, unless it is performing validation processing, in which case it would look at the content profile. If there is no declared content profile, then no validation is possible, i.e., content profile is never inferred from processor profile, etc.

* Any one of these processor profiles are required to process the document.
* This document conforms to these profiles.

and these are the same with higher cardinality.

> The only utility of a statement of content profile conformance is to (1) perform validation processing, and/or (2) to imply a processor profile in the absence of an explicit declaration of processor profile. From what I can tell in this discussion, folks are primarily thinking about the second of these uses of a content profile conformance declaration. Furthermore, it appears that, in regard to discussing references to multiple content profiles, folks are assuming that a disjunction combinator applies; namely, that the least restrictive expression of any given feature usage requirement would apply to creating a corresponding processor support requirement.
> For example, say we have three content profiles P, Q, and R that define one feature F, where P makes F prohibited, Q makes F optional, and R makes F required.

Then it’s only possible to make a document that conforms to 2 of them (F is absent: PQ;  F is present; QR).

"conforms simultaneously to 2 of them"

> If we then had an expression of conformance (where "leastRestrictive" profile is similar to an an "or" or "union" operation), e.g.,
> <tt ttp:contentProfile="P Q R" ttp:contentProfileCombination="leastRestrictive”/>

No, stop, we’re not asking that.

<tt ttp:contentProfile="P Q R" ttp:contentProfileCombination="leastRestrictive”/>

would mean
a) everything in the document is either permitted by P Q and R (or is ignorable — permitted ignorable stuff under P Q and R)

actually, leastRestrictive is defined to mean that when merging values component-wise for a given feature F (or extension E), then choose the least restrictive value (optional > required > prohibited);

so for the example, the combination of P, Q, R would be F: optional; if said document does use F, then it conforms to the combination, but not to P; if said document does not use F, then it conforms to the combination, but not to R

in contrast ttp:contentProfileCombination="mostRestrictive” means choose most restrictive (prohibited > required > optional);

so for the example, the combination of P, Q, R would be F: prohibited; if said document does use F, then it does not conform to the combination, but does conform individually to Q and R; if said document does not use F, then it conforms to the combination, but not to R individually;

b) there is nothing in the document contrary to any of P Q or R

it is up to the author to determine how they want to declare conformance; in this (admittedly) complex example, the author has declared conformance with two mutually conflicting content profiles, P and R; the combination methods are defined to produce reproducible results, which they do accomplish;

c) if you implement at least one of P Q or R, then you can process the document, ignoring stuff that is not in the profile(s) you support, and the result is OK by the content author.

in the above example, the author has only declared a (set of) content profile(s), so it will be necessary to infer a processor profile from the effective content profile, i.e., the content profile produced by performing the content profile combination method on the declared content profiles;

if we alter our example and say that P, Q, and R are not pair-wise mutually conflicting, and ttp:inferProcessProfile is not specified, in which case it would default to 'loose', then we would end up with an effective processor profile that requires support for a feature F only if all of P, Q, and R require support for F, otherwise support for F is optional;

if ttp:contentProfileCombination were specified as 'mostRestrictive', then the same configuration would require support for a feature F if any of P, Q, or R require support for F, otherwise support for F is optional;

> I understand this desire. However, it is also clear that we are not going to define a calculus for use in a codecs parameter that is equivalent (in terms of expressibility) with the formal definition mechanism for processor and content profiles in TTML2.

I don’t see any calculus required.

If we define the syntax P+Q+R as a suffix of codecs (following stpp.ttml, e.g., "stpp.ttml.P+Q+R") to mean:

  1.  P, Q, and R are identifiers that map to pair-wise non-conflicting content profiles;
  2.  determine an effective processor profile as follows:
     *   compute effective content profile ECP by combining P, Q, and R using content profile combination method "leastRestrictive"
     *   infer an effective processor profile EPP from ECP using the "loose" inference method
  3.  if EPP contains a required feature/extension that is not supported by the processor, then do not fetch resource
  4.  otherwise, fetch resource and commence processing of profile declarations contained in resource (in tt:tt and tt:head elements);
  5.  if processing of profile declarations contained in resource result in processing being aborted, then do not fetch remainder of resource and continue to next candidate resource

This appears to be too complex. I don't think we can require receiving systems to perform combinatorial logic on profiles prior to fetching documents, at run-time. Nor can we define implementation behaviour if a required feature is not supported – a specification may require that a feature be supported in the knowledge that non-support results in inferior presentation albeit better than nothing. There's no benefit to a middle 'preferably support' tag on a feature though!

Using this method, the codecs parameter could serve as a pre-filter on fetching the resource and the the full TTML profile processing semantics (based on declarations found in the resource) could occur next. So there are two opportunities to reject and pass over a candidate resource in a list of resources: once using the codecs parameter, and once using the TTML resource's profile declarations.

It's reasonable to provide these two possibilities however I suspect that real world processors, especially embedded into e.g. set top boxes,  won't bother with the latter, and if they did reject a document part way through processing there wouldn't be a feedback mechanism for a higher level system to try an alternative document+processor combination.

A conservative client can ignore the codecs parameter and fetch each resource to perform the TTML specified processor profile semantics; a liberal client can perform the codecs base pre-filtering step and reject fetches without performing the TTML specified profile processing.

The above procedure is a "limited calculus" that would appear to expose the desirable filtering while not exposing the need to fetch or perform the full TTML profile computations (in the pre-fetch state).

Look, we have a number of TTML profiles already with significant overlap.  If I write a document that stays in that intersection, someone implementing EBU TT, SMPTE TT, W3C DFXP, and so on, should all be fine.  I suspect many simple cases will fall into this intersection.  Documents ought to be able to say so.

I agree they should be able to do this internally, and they certainly will be able to do so in a fuller manner in the TTML2 context. Not so much in TTML1.

Defining the codecs parameter and a higher level (fetch filtering) proposal such as described above should be able to handle this both for existing TTML1 and new TTML2 resources.

David Singer
Manager, Software Standards, Apple Inc.


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Received on Tuesday, 13 May 2014 10:07:03 UTC

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