Re: Display requirements

From: Stefano Mazzocchi <>
Date: Mon, 13 Sep 2004 21:52:56 -0400


thanks for following up on this.

David R. Karger wrote:

> hold it! This is where I have a problem: I think you want a template
> language for RDF in RDF.
> What I would like to have is a declarative visual ontology but
> definately *NOT* a procedural template language since there are already
> so many out there.
> The fact is that foaf:person cannot be visualized because there is no
> visual information (therefore, you cannot decide how to present this).
> You said "cascading RDF stylesheets" and that hits the nail on the head,
> but when you say "you must also have the company logo", that freaks me out.
> one thing is to have "content" in the CSS level (very useful in :before
> and :after CSS selectors), because, for example, "Warning: " before a
> <warning>...</warning> element is style, not content.
> But a company logo, a header, a footer, they are al content, not style
> and content should be dealt with a template language, not a stylesheet
> language and I really think that writing a template language in RDF that
> would be abstracted from the actual presentation markup would be
> *waaaaay* abusive (and would reinvent half a dozen wheels)
> We did it this way in haystack (using rdf to specify layouts in
> rows/columns, sized of regions, etc). It may have been a mistake.
> It's very powerful (let's user customize the views by using
> view-views) but as you say, it reinvents many wheels.

I wouldn't call it a "mistake" per-se, but I am concerned (being the
goal of this project to come up with something that average programmers,
not hard-core semweb geeks, should be able to use, adopt and customize)
about the "abuse" of RDF for things where hierarchical markup is
normally used (and with great success and lots of followers) for that

In terms of "optimal and immediate adoption", I feel that using an RDF
template language would be a serious drawback, if not a mistake.

In terms of "optimal solution and technology", I honestly have no idea.

> We also consider it important for a view to be able to invoke
> arbitrary code---ie, we don't assume that all presentation of data can
> be done using the standard widgets, and we want a view to be able to
> include its own special-purpose widgets, such as a graph-layout
> widget, and invoke them at the right time.

Hmmm, interesting point, but I wonder if it really applies at this
level. I mean, in the "what" description, obviously this doesn't
apply... but in the "how" description, well, the ability to invoque
arbitrary code is strongly influenced by the media against which we want
to render.

This could be interpreted as a serious issue with a cross-media RDF
publishing system, or simply as a known deficiency of some rendering
media (for example, PDF which cannot invoke applets, for example).

At the same time, I think that this is a serious concern and it is
should be indeed possible for the presentation layer to have pluggable
widgets, but I'm scared about the complexity of the conceptualization of
such visualizing layer.

> > 11. Handling of Missing Values
> >
> > It should be possible to specify alternatives for missing values e.g.
> > display foaf:firstname and foaf:last name if foaf:name is missing. Or
> > display dc:title if rdfs:label is missing.
> Again, same thing: conditionals are not part of style because
> conditionals are procedural not declarative.
> right. if, after all the equivalences have been worked out, there is
> no appropriate value, then hatstack shows "null"

why "null" instead of nothing at all? [just curious, "null" seems to be
pretty problematic in usability terms, as many people would find it

> It is not up to the 'display ontology' to indicate "what" information
> should be there, but "how" the information present should be displayed.
> this is a little blurry (see my last post). eg, a lot of haystack
> lenses are "property set lenses" which just give a list of the
> properties of the object to show; they are then shown using a
> relatively generic presentation. Is this lens deciding what or how?

I would say "what" if they contain just a list of properties and a mix
of "what and how" if they contain both the list of properties and, for
example, the font size to show them or their box-nesting hierarchy.

In that case, I would be pleased if we could come up with a way to
cleanly separate the two.

Stefano Mazzocchi
Research Scientist                 Digital Libraries Research Group
Massachusetts Institute of Technology            location: E25-131C
77 Massachusetts Ave                   telephone: +1 (617) 253-1096
Cambridge, MA  02139-4307              email: stefanom at mit . edu
Received on Tue Sep 14 2004 - 01:52:53 EDT

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