RE: New Editors Draft of the httpRange-14 Finding

Hello Alan, 
 
My impression is that the language in AWWW is deliberately that way
because there is no hard and fast rule about what defines 'equivalent'
representations. I suppose that we might say that authors make assertions
about equivalence when they create multiple representations and offer them
via content negotiation.
 
By the way, I'm not convinced that lossless transformation is the right
model for this. For example, I believe that representations that differ
only in language are appropriate to serve via content negotiation. HTTP is
explicitly set up to support language as one of the criteria for such
negotiation [1]. The HTTP specification notes that the reason it's called
content negotiation and not format negotiation is precisely because
representations are not necessarily different formats of the same content.
 
Actually, there are some intersting use cases in accessibility and in
support for small mobile devices that are related. Suppose I mint a URI
and assert that it identifies a resource that let's you find a barbeque
I'm holding this weekend (chance would be a fine thing with the summer
we've had!)
 
Anyhow, for users with traditional Web access, I provide a representation
that includes a map, delivered as a large, colour image, with a big arrow
showing the location. If I want to support users with visual disabilites,
or people accessing my site with a small, text only mobile phone, this
won't be much use. I might want them to receive directions to the event
instead. I might deliver them as text or even as an audio clip.
 
Now, the question is, can I legitimately serve the map, the textual
directions and the audio as representations of the same URI? If I claim
that the URI identifies a map of the location of the barbeque, then I
think the answer is no. However, if I claim it identifies a resource that
let's users find the barbeque, I suspect the answer might well be yes,
though I would not be surprised to find that opinions differ.
 
In short, I think equivalence is in the eye of the author.
 
On the specific question of whether a jpeg is a representation of a
person, I think the consensus is no. There has been a lot of discussion
about related topics on this list over the last couple of months. If you
mean person the way I think you mean it, then that would be a
non-information resource, as described in AWWW. These have no
representations and the current feeling is that it would be misleading to
serve a representation if the associated URI is accessed. The range-14
finding is about what should be returned if such an access occurs.
 
Of course, you could content negotiate between a jpeg, gif, png...(pick
your favourite image format) for a URI that you claim is a picture of a
person.  
 
Best wishes
Rhys
 
[1] http://www.ietf.org/rfc/rfc2616.txt (Section 12 Content Negotiation)

  _____  

From: Alan Ruttenberg [mailto:alanruttenberg@gmail.com] 
Sent: 24 August 2007 05:48
To: Rhys Lewis
Cc: www-tag; Jonathan Rees
Subject: Re: New Editors Draft of the httpRange-14 Finding


Hello Rhys, 

The problem I have always had is knowing what the conditions are for two
representations to be of the same resource (and are the resources
information entities, or only the representations? - can I content
negotiate for the jpeg "representation" of a person?)

Using translations as an example is particularly problematic, as most
language translations are not exact because there are inevitably cultural
attachments to the words that can not be easily understood by non-native
speakers.

I can understand some much more constrained than what I perceive to be
extremely loose language the AWWW, and this document as well, use. For
instance we could say that two representations are of the same resource if
there is a documented algorithm implementable by a computer that
losslessly transforms one into the other, as with a byte sequence and its
gzip compressed version.

My worry has been that unless there is some way for someone to say: "No,
you are wrong, these two things are *not* representations of the the same
thing" then the term "representation" is meaningless.

Regards,
Alan

On Aug 23, 2007, at 3:00 AM, Rhys Lewis wrote:


Hello everyone, 

Could I just take a moment to thank Roy and David for their extensive
comments on the latest draft. They provide excellent input for the
forthcoming TAG face to face meeting next month.

I hope to make progress on specific points before that meeting. If so,
I'll respond here on particular topics.

Thanks again

Best wishes

Rhys