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Re: Timed Text Authoring Format - Distribution Format Exchange Pr ofile (DFXP)

From: John Birch <johnb@screen.subtitling.com>
Date: Thu, 17 Mar 2005 23:35:24 -0000
Message-ID: <01f401c52b49$ff9aa8e0$30216551@win2000itx>
To: <public-tt@w3.org>
RE: Timed Text Authoring Format - Distribution Format Exchange Profile (DFXP)Glenn,

I guess I was hoping that DFXP would be a purely semantic document format, rather than a more presentational one.

DFXP will be an adequate format for interchange of existing file formats, [they also have the same limitation (they are presentational not semantic)], but existing file formats can already be interchanged.

But this really isn't the issue... existing file formats are recognised as being limited. There is a real desire to move to a more sophisticated abstraction for the storage of this type of content, to simplify and promote the reuse of media assets across a wide range of distribution formats. Using DFXP will not necessarily resolve issues that arise from the reuse of stored content in a different distribution domain.

This desire to re-use content (assets) quickly and cost effectively fuels a requirement to **appropriately** algorithmically modify exchanged documents to suit the new distribution mechanism. Performing this correctly requires an understanding (or a record) of why a specific presentational style was originally chosen. 

This might be achieved by over-using the style id to retain the information required - but this is not an elegant solution... nor is using extensions to DFXP or conventions in document layout or element usage.

It is evident that a fully applicative style model in DFXP would require more sophisticated implementation, however the adoption of a fully applicative style model was not what I proposed... I had something far simpler in mind.

best regards

John Birch
  ----- Original Message ----- 
  From: Glenn A. Adams 
  To: Johnb@screen.subtitling.com ; public-tt@w3.org 
  Sent: Thursday, March 17, 2005 5:55 PM
  Subject: RE: Timed Text Authoring Format - Distribution Format Exchange Pr ofile (DFXP)




  From: Johnb@screen.subtitling.com [mailto:Johnb@screen.subtitling.com] 
  Sent: Thursday, March 17, 2005 12:33 PM
  To: public-tt@w3.org
  Subject: RE: Timed Text Authoring Format - Distribution Format Exchange Pr ofile (DFXP)




  Comments inline

    -----Original Message-----
    From: Glenn A. Adams [mailto:gadams@xfsi.com]
    Sent: 17 March 2005 16:11
    To: Johnb@screen.subtitling.com; public-tt@w3.org
    Subject: RE: Timed Text Authoring Format - Distribution Format Exchange Pr ofile (DFXP)




    From: Johnb@screen.subtitling.com [mailto:Johnb@screen.subtitling.com] 
    Sent: Thursday, March 17, 2005 9:53 AM
    To: public-tt@w3.org
    Subject: RE: Timed Text Authoring Format - Distribution Format Exchange Pr ofile (DFXP)




    As defined, the use of referential styles already requires head fragments to be repeated throughout a stream transmission to permit mid-stream acquistition. A stream unit is not directly parsable if it uses referential styling, because it will require lookup in this 'head' fragment.

    So it would seem that the sole reason for not including class based (or rule based) styling is the need for "re-evaluating all rules for each content unit that arrives".


    [GA] Repeating a fragment that contains <head/> or <styling/> is expected in a streaming delivery scenario. This would be required in general in order to interpret any fragment that has a semantic dependency on <head/> or <tt/>. 


    Exactly, and that is true for referential styling too!


    [GA] Yes. This is understood, and is acceptable (and different from the general model).


    Another, and more primary reason for not including rule based styling in DFXP is that the WG made a conscious choice to simplify DFXP, particularly since the expected mechanism to be used for applicative styling will be the use of XPath expressions to select the content to which styles will apply. The use of XPath necessitates, in the general case, that the entire document is memory resident in order to construct complex predicates. 


    Obviously a decision was taken by the WG, my point is whether it was the correct one ;-)

    I understand the restriction created by the use of XPath, and also see the greatly increased complexity its use will allow in document instances. It is unlikely that practical inserters will be developed IMHO to process AFXP to true on-the-wire distribution format - this is what DFXP was intended for. For my marketplace AFXP is of little relevance, the workstation product will always be custom to the role of subtitling - I see little to be gained by adopting the extreme sophistication allowed by AFXP in a preparation workstation, only to throw most of it away in the transition to DFXP. My interest is in a distribution format that solves some of the interchange problems that are faced now by the marketplace. If DFXP does not contain features that provide improvement over existing formats, what will prompt it's adoption over those formats? If you are suggesting that distribution be performed using AFXP (or a sub-profile of it), for what is currently the largest single target for DFXP (subtitling), then what future is there for DFXP? 


    [GA] Clearly, the WG members believe that DFXP is more than adequate to serve as an interchange format among existing distribution formats. If you can present a concrete case why this is not true, then I'm certain the WG will carefully consider. Also, keep in mind that you can use arbitrary extensions in DFXP provided they are in a different namespace. This will allow you and others to customize their uses. If it appears that there is a common extension desired by many parties, then we can consider standardization.


    The WG rejected the use of a non-general, special case mode of application such as you suggest, preferring instead to support a general approach in AFXP. 


    I don't see rule based styling as non-general or special case - it's a powerful feature of CSS.


    [GA] And it will be similarly powerful in AFXP, but not DFXP.


    I am not personally convinced that this is more onerous than supporting a referential style... YMMV ! 


    Speaking as an implementor, I can assure you that it is more simple to implement referential styling. 

    Hmmm! I was also speaking as a potential implementor. Why do you think searching for an applicable rule is more difficult than searching for an applicable style reference?


    [GA] Because looking up a referential style does not require traversing the document instance. It merely requires a hashtable lookup on the set of styles already received in the fragment that contains the <styling/> element. In contrast, applicative styling potentially requires evaluating every node of the DOM in order to match a single rule.


    Not including this feature in DFXP does make restyling of DFXP content somewhat more onerous.... since any relationship between a role and a style will be lost by transition into DFXP. Consequently, this mandates the use of AFXP for exchange and pre-distribution storage if the intention is to support these relatively minor 'presentation' changes at output time.


    If you examine the TTAF System Model in Figure 1, you will see there is a compilation step when going from general AFXP to DFXP. Compilation usually involves a loss of abstraction, in order to construct a simpler equivalent expression. This is the model followed with DFXP. 


    Yet this is more than a loss of abstraction, it is a real loss of data. The relationship between the style and the metadata is lost.


    [GA] What you call "data" I call "abstraction". It does not lose "content". Furthermore, if a compiler wishes to do so, it can add non-standardized decorations that allows it to recover the abstraction. However, standardization of such reversible transform is not a requirement for DFXP, and, indeed, was an explicit non-requirement.


    I may seem to be 'pedantic' on this point, but one of the major limitations of existing formats is that they do not support easy transitions between real on the wire distribution formats - where the distribution formats do not provide equivalent support for presentation options - simply because they also do not convey this connection between style and role. If there is no connection between the role / agent metadata and the style in DFXP - then there is little point in including the role and agent metadata IMHO.


    There is no normative use of role/agent in DFXP; it was included to permit passing through this metadata from AFXP for use by non-standardized processing, or potentially future standardize processing. An AFXP to DFXP compiler is free to not include this metadata in DFXP. However, it is there in order to permit an author to interchange it on an end-to-end basis. 


    Yes, I understand. But exchange between **authors** should be at an AFXP level surely?


    [GA] It is not the intent of the WG or its specs to dictate to authors how they should use the different profiles. It is their choice. The two profiles have different design centers. DFXP is explicitly intended to be cooked/flattened/compiled.


    DFXP is targeted to support conversion into multiple true distribution output formats. This one to many relationship requires that the one format (the source) contains a sufficient richness, or a sufficiently high level of abstraction to support the variations in output formats, but still retain the original intention of the author.


    [GA] Then you will want to use AFXP for such abstraction level, or add proprietary extensions to DFXP.


    The intention of the author (in subtitling at least) is NOT that a particular word be red, or italicised, but that it be different from the surrounding context. Or put another way, what is important is not **THAT** the style exists, but **WHY** the style change exists. Further, there are very fixed conventions as to the styling used to represent different contexts (dialogue, shouting, sound effects, music), and those conventions differ from true on-the-wire distribution format to format - and from user to user! But these conventions exist for the same purpose, regardless of distribution format, and it is that **purpose** that needs to be preserved (and IMHO enforced) in DFXP.


    [GA] I'm afraid you have a different idea of the intention of DFXP than the TT WG. The intention you ascribe to DFXP is what the TT WG ascribes to AFXP.


    My concern Glenn is this.


    Once you make the context optional, you effectively have thrown it away. Without a strong emphasis on the relationship between style and 'role', DFXP seems to be heading in a direction that (almost) encourages the development of 'cooked' documents. IMHO this is the antithesis of what is required in a true multi-target distribution format. I would personally dare to suggest that DFXP should drop inline style and style references **totally**, in favour of ONLY a class based style mechanism - simply to enforce the relationship between style and context/role.


    This is because in order to support the conversions that would be anticipated, the style mechanism would have to also carry the role aspect as part of the style ID.... thus creating an explosion in style definitions. Further, each fragment of content that required identification would need to carry a style reference.




    IMHO In this aspect, DFXP is too cooked. I prefer mine raw!


    Then please contribute and support the development of AFXP. 

    I intend to, but I'm not so interested as to fund joining the W3C out of my own pocket ;-)


    [GA] Then you will have to be satisfied with what the TT WG produces, while, of course, taking due consideration of yours and other comments from the public. I would note that some very small companies (mine for instance) is willing to make this investment out of pocket.


    BTW - Where does this streaming issue come from, a DFXP file is likely to be trivial in size compared to ANY companion stream. (e.g. video or audio). I would suggest that any composite stream that included a TT content stream would simply do so by reference and require the target to pull down the entire file.


    [GA] There are many real-world use cases where it would be useful or important to integrate DFXP content into a streaming data context, particularly in unidirectional delivery contexts.


    regards John Birch.


    -----Original Message-----
    From: Glenn A. Adams [mailto:gadams@xfsi.com]
    Sent: 17 March 2005 14:02
    To: Johnb@screen.subtitling.com; public-tt@w3.org
    Subject: RE: Timed Text Authoring Format - Distribution Format Exchange Pr ofile (DFXP)

      Actually, DFXP does not support out-of-line styling in the traditional sense (e.g., CSS sense). The fact that one can place style specifications in <head/> and share their use among multiple content elements is merely an optimization of expressing inline styles (by reference). We call this referential styling.


      What you are requesting is a form of rule based applicative styling that applies independent style rules to content based on matching criteria. This mechanism will be defined in AFXP, but was explicitly ruled out for DFXP since it requires either (1) having all content available to apply rules to, or (2) repeatedly re-evaluating all rules for each content unit that arrives (e.g., in a streaming scenario).


      The basic model for DFXP is completely inlined styles, but the referential styles were defined as an optimization to allow:


      (1)     aggregation and sharing of common inline styles

      (2)     pre-delivery or separate packaging of a fragment containing referential styles from fragments containing content


      The decision to simplify DFXP was based on the desire that DFXP content be more concrete and directly parsable/renderable in a potential streaming context. The general use of out-of-line applicative style rules is antithetical to this approach.





      From: Johnb@screen.subtitling.com [mailto:Johnb@screen.subtitling.com] 
      Sent: Thursday, March 17, 2005 7:56 AM
      To: public-tt@w3.org
      Subject: RE: Timed Text Authoring Format - Distribution Format Exchange Pr ofile (DFXP)


      Glenn, et al, 

      The DXFP specification includes support for styling, both in-line and out-of-line styling. 
      However it does not support a class based styling model. 

      In subtitling, styles are most often associated with changes in the text 'role' (e.g. dialogue differs in presentation from music) or 'speaker' (Joe - red, Frank - blue).

      Could a mechanism be added to support this? 

      E.g. This might be represented in DXFP by utilising a class based style mechanism that was sensitive to ttm:role and ttm:agent. Thus:

      <style id="s1" style tts:color="white" tts:fontFamily="monospace-serif"/> 
      <style id="intro" style="s1" tts:fontSize="4%"/> 
      <style id="documentary" style="s1" tts:fontSize="10%" tts:fontFamily="sans-serif"/> 
      <style id="music" ttm:role="music" tts:fontStyle="oblique"/> 
      <style id="joe" ttm:agent="joe" tts:color="red"/> 

      <div style="intro"> 
      <!-- all text 4% high --> 
      <!-- all text monospace-serif --> 
      <p ttm:role="music">Quiet Violin music</p> 
      <div style="documentary"> 
      <!-- all text 5% high --> 
      <!-- all text sans-serif --> 
      <p>White Large sans-serif</p> 
      <p ttm:role="music">White Oblique Large sans-serif</p> 
      <p ttm:agent="joe">Red Large sans-serif</p> 

      the ttm:role and ttm:agent attributes could be considered as implicitly adding inline style attribute(s) to their container....


      John Birch 
      Senior Software Engineer, 
      Screen Subtitling Systems Limited, 
      The Old Rectory, Claydon Church Lane, 
      Claydon, Ipswich, Suffolk. 
      IP6 OEQ 
      Tel: +44 1473 831700 
      Fax:+44 1473 830078 

      See us at NAB Las Vegas April 18-21st Stand No. SU8956 

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      -----Original Message----- 
      From: Glenn A. Adams [mailto:gadams@xfsi.com] 
      Sent: 14 March 2005 16:51 
      To: public-tt@w3.org 
      Subject: Timed Text Authoring Format - Distribution Format Exchange 
      Profile (DFXP) 


      A new update of the Timed Text Authoring Format 1.0 - Distribution 
      Format Exchange Profile (DFXP), is now available at [1]: 


      The TT WG solicits your comments on this new draft as soon as possible, 
      as a very rapid turn-around is expected in order to publish a first Last 
      Call (LC) draft. 

      Please sent comments either to this list or, if you prefer privacy, to 
      me directly. 

      Glenn Adams 

Received on Thursday, 17 March 2005 23:54:07 UTC

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