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RE: Question on the boundaries of content negotiation in the context of the Web of Data

From: Williams, Stuart (HP Labs, Bristol) <skw@hp.com>
Date: Thu, 12 Feb 2009 17:02:54 +0000
To: "wangxiao@musc.edu" <wangxiao@musc.edu>
CC: Michael Hausenblas <michael.hausenblas@deri.org>, "www-tag@w3.org" <www-tag@w3.org>
Message-ID: <233101CD2D78D64E8C6691E90030E5C8293C589043@GVW1120EXC.americas.hpqcorp.net>

Hello Xiashou,

I'll venture just one response and note that we repeatedly go round this loop, making little if any progress other than to increase the size of the permthead...

> -----Original Message-----
> From: Xiaoshu Wang [mailto:wangxiao@musc.edu] 
> Sent: 12 February 2009 15:54
> To: Williams, Stuart (HP Labs, Bristol)
> Cc: Michael Hausenblas; www-tag@w3.org
> Subject: Re: Question on the boundaries of content 
> negotiation in the context of the Web of Data
> Williams, Stuart (HP Labs, Bristol) wrote:
> > Hello Michael,
> >
> > <snip/>
> >
> >   
> >> Please note that I don't ask if this works. It does. Obviously. The
> >> question, to put it in other words, is: is the PNG *representation* derived
> >> via conneg from the generic resource <http://sw-app.org/sandbox/house>
> >> equivalent to the RDF in Turtle?
> >>     
> >
> > Does't look like it to me...
> >
> >   
> >> If not, why not?
> >>     
> >
> > The 'house' depicted in the PNG appears to have a red roof. 
> The redness of the roof (amongst may other properties of the 
> 'house') are not apparent in the turtle.
> >
> > The only possible coherent fragment of information that 
> could conceivably be the same between the two representations 
> is a subject URI. In the case of the PNG (not that I'm 
> familiar with the internal encoding of information - 
> particularly if its more structured than an array of coloured 
> dots or a list of lines, shapes and fills) whilst a human 
> interpreting the picture might intuit that the juxtaposition 
> of what appears to be a text string close to what appears to 
> be a 'house' may be intended as naming the house with the 
> string and that that named thing is the subject of, well 
> what, I suppose the art, if we can call it art :-). However 
> one is quite some distance from a machine readable assertion 
> that http://swa-app.org/house.tll#my names any particular 
> thing far less a house. There is also the question of what 
> the media-type registration for PNG has to say about what the 
> fragId should be interpreted as referring to; I don't know, I 
> haven't looked.
> >
> > The turtle representation, gives a machine readable 
> assertion that claims http://swa-app.org/house.tll#my as a 
> http://swa-app.org/house.tll#House (whatever one of those is).
> >
> >   
> >> If it is, can you please point me to a finding, note, a
> >> specification, etc. that 'normatively' defines what 
> >> 'equivalency' really is?
> >>     
> >
> > Do you think those representations are equivalent 
> representations of whatever it is that you intend 
> http://swa-app.org/house.tll#my to be an identifying URI of - 
> (a conceptual red roofed house? a depiction of that concept? 
> an actual particular house?... )
> >
> > Equivalent for ALL purposes? or just for some narrowly 
> defined purpose?
> >
> > I guess that it could be argued that the subject of both 
> representations is the same 'house' - but they are 
> representations of two different descriptions/depictions of the same thing. 
> I think if the equivalence is defined with regard to "representation", 
> then they are different.  But I don't think that is the problem. The 
> real question (and problem) is if they are equivalent with regard to the 
> thing that the URI denotes. 
> > IMO you shouldn't be conneg-ing between these at all. One 
> might wriggle and provide a ttl representation that used some 
> drawing vocabulary to describe the lines and shapes in the 
> picture, adn I think given such a vocab - it would be 
> reasonable to regard a PNG and such a detailed ttl account in 
> a suitably grounded vocab - as equivalent representations of 
> a depiction of... well, a house (if that's what it is).
> >   
> Michael's turtle file is simplified a bit, but I don't think there is 
> any reason to say it is wrong.

I did not say that the turtle file was 'wrong'. I know of now decision procedure to determine the truth or otherwise of the statements therein (it doesn't say very much).

> If his turtle representation also 
> describe the house's details, e.g., the red roof, shape etc., then would 
> it be allowed to conneg?

Well... I don't know what Michael intends that his URI designate. If it were and image depicting a house then ttl about an actual house or the class of houses with red roofs would be about something different. 

Granting that Michael probably intended the URI be about either some conceptual house or some particular conceptual house I'd contend that the ttl and png awww:representations were/are awww:representations of *distinct* descriptive/depictive (respectively) resources that have a common subject (Michaels actual or conceptual house).

> If the answer is not, then where is the line 
> between a coarse description and a detailed one.  (This goes to the 
> definition of information resource).  In addition, what about for those 
> visually disabled persons? If you give it (it doesn't matter if the "it" 
> here is the house or the image) a different URI, then the implication is 
> that they must be two different things.

There are (at least) three things here: 1) whatever resource http://swa-app.org/house.ttl#my is intended to refer; 2) and http://swa-app.org/house.ttl an RDF graph (I presume), a resource, that contains at least one RDF statement about whatever resource is referred to by http://swa-app.org/house.ttl#my 3) http://swa-app.org/house.png an image (resource) that appears depict an intended association between http://swa-app.org/house.ttl#my a graphic that depicts the idea of a house. The resources in 2 and 3 are not houses; the resource in 2) is not a picture/image.

[I think that's consistent with my take in all our previous discussions, doesn't contribute any new information - and likewise your position has not moved either...]

> The answer, IMHO, can only be "yes". The Web is simply a medium for 
> communication.   If you are the owner of the resource, and if you think 
> it helps you to communicate with your users by grouping different 
> representations under the same URI, just do it as long as it works.
> On the other hand, if you don't think such kind of technique don't help 
> your cause, just don't do it. I don't think it is about *right* or 
> *wrong* here. It is about making the Web pragmatic. 

Personnally, I think that its is quite a stretch to cast an image/picture and a graph as being the same thing. I'm sure that you could carefully construct a corner-case scenario where it was harder to argue the distinction - eg. I offer one where the ttl in some suitably grounded vocabulary describe the pixel colours of each dot in the image or provided a vector representation with sufficient information to allow reasonable facsimile to be drawn - then you'd have representations that were near equivalent.

> > Xiaoshu 
> > BTW: what is it about the depiction that makes it a 
> depiction of a house - quite apart from web architecture, 
> what are the rules for examining two depictions/pictures and 
> determining them to be equivalent? An if you can resolve that 
> is the ttl representation an admissable equivalent under those rules?
> >
> >   
> >> Cheers,
> >>       Michael
> >>
> >> -- 
> >> Dr. Michael Hausenblas
> >> DERI - Digital Enterprise Research Institute
> >> National University of Ireland, Lower Dangan,
> >> Galway, Ireland, Europe
> >> Tel. +353 91 495730
> >> http://sw-app.org/about.html
> >>     
> >
> > Regards
> >
> > Stuart
> > --

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Received on Thursday, 12 February 2009 17:07:49 UTC

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