SGML: SGML and HyTime

SGML: SGML and HyTime


Subject: Re: SGML contra HyTime

Date: 9 Jan 1997 16:52:05 GMT

From: "W. Eliot Kimber" <drmacro@drmacro.com>

Newsgroup: comp.text.sgml



andreas knapp <knapp@iassnb.irp.uni-stuttgart.de> wrote in article
<5b29et$3cam@info4.rus.uni-stuttgart.de>...
> Hi all, 
> =======
> 
> is there anyone out there who could tell me the real difference between
> SGML and HyTime, yes, I know somehow HyTime is an SGML application, but
> what exactly does it do for me if I want to create Hypermedia documents
> automatically, would it be better for me to use SGML of HyTime?

[Eliot]

HyTime is an application of SGML: to oversimplify, HyTime defines a set of
element types that you can use with your existing SGML document types to
provide hyperlinking and other facilities in a standardized way.

In other words, HyTime effectively extends SGML by defining a standard set
of facilities for doing hypertext and multimedia presentations.  

HyTime does not compete with or change SGML in any way--it simply builds on
the basic facilities SGML provides.  In particular, HyTime provides the
following hypertext-related facilities:

1. A general model of hyperlinks and syntax for hyperlink
   representation using SGML elements.  This is much more than
   simple ID/IDREF.
2. Facilities for addressing things other than elements with
   IDs in the local document: cross-document ID refs, references
   to elements without IDs, references to data characters,
   references to multiple objects at once.

These are facilities that all hypertext applications need.  HyTime provides
a general and robust mechanism for adding them to SGML documents and SGML
processing applications.  HyTime can also be integrated with other
mechanisms, such as TEI extended pointers or HTTP URLs.  

There is much more to HyTime than this, but the above are probably of
immediate interest and utility to most SGML document type designers.

Technicaly, HyTime is an "enabling architecture".  In particular, rather
than defining element types that you must use directly in your DTD (as you
would if you used part of a standard DTD, say the 12083 Maths elements), it
provides a set of 'meta-element types' that serve as templates or
supertypes from which you derive your own element types with their own
element type names and specialized attributes.  As long as your element
types conform to the minimum requirements of the HyTime-defined
meta-element types ("architectural forms"), a HyTime-aware processor will
be able to provide HyTime-related services, such as as finding the anchors
of hyperlinks.

Say you want to provide a general hyperlinking element in your document
type.  To make it possible for a HyTime processor to process this hyperlink
it has to be told that your linking element is "derived from" a
HyTime-defined hyperlink element form.  You make this association by using
an attribute to name the HyTime element form your hyperlink element is
derived from.  For the HyTime standard, this attribue is normally called
"HyTime".  In your declarations you do something like this:

<!-- My personal hyperlinking element, derived from the HyTime
     "contextual link" (clink) form: -->
<!ELEMENT CrossRef   - - (#PCDATA) -- Cross reference -->
<!ATTLIST CrossRef
          linkend    IDREF #REQUIRED
          -- Names ID of element cross reference is to --
          HyTime     NAME  #FIXED "clink"
          -- Tell HyTime processor this is a HyTime clink --
>

The HyTime declaration for the contextual link element form is (more or
less):

<!element clink   -- Contextual link -- 
                  - O (%ArcCFC;)* 
                  -- Can contain any elements or data -->
<!attlist clink
          linkend   -- Address of other end of link --
                    IDREFS #REQUIRED
>

The rules for deriving element types from element forms are fairly simple: 
For each required attribute of the element form (clink,
in this case), the dervied element (CrossRef) must have the same attribute
with the same name and its value must be consistent with the architectural
attributes value prescription.  The content of the client must be
consistent with the content model of the meta element type (in this case,
clink can contain anything or nothing, so it's impossible to be
inconsistent with it).

Thus, we can see that the CrossRef element meets these requirements because
the clink form requires an attribute called "linkend" with a value
prescription fo IDREFS and the CrossRef element has an attribute called
"linkend" with a value prescription of IDREF.  The attribute name is the
same and the value prescription "IDREF" is consistent with IDREFS (every
IDREF is a valid value for an IDREFS attribute).

The HyTime attribute of the CrossRef element indicates that this element
type is derived from the HyTime clink element form--this is enough
information for a HyTime processor, such as SoftQuad's Panorama or
TechnoTeacher's HyBrowse, to know how to process the CrossRef element as a
HyTime hyperlink.

If you want to use different attribute names in your element types, you can
map the HyTime-defined names to your own names using the HyNames attribute.
 For example, your authors might find an attribute like "section" more
intuitive than "linkend" for the CrossRef element.  You can do this:

<!-- My personal hyperlinking element, derived from the HyTime
     "contextual link" (clink) form: -->
<!ELEMENT CrossRef   - - (#PCDATA) -- Cross reference -->
<!ATTLIST CrossRef
          section    IDREF #REQUIRED
          -- Names element cross reference is to, e.g., "Section" --
          HyTime     NAME  #FIXED "clink"
          -- Tell HyTime processor this is a HyTime clink --
          HyNames    CDATA #FIXED "linkend section"
          -- Map HyTime attribute name "linkend" to our preferred
             name "section" --
>

A HyTime processor will now know that the attribute "section" is really the
HyTime attribute "linkend" and will know what to do (otherwise the HyTime
processor would have no idea what the section attribute was for and would
ignore it).

You can learn much more about HyTime from the TechnoTeacher Web site,
"http://www.techno.com", and from my personal Web site,
"http://www.drmacro.com/hyperlink".  You can get a free version of Panorama
from SoftQuad at "http://www.sq.com".  You can also get my Perl HyTime
library Hy-Lib from the ISOGEN Web site at
"http://www.isogen.com/demos/hy-lib.html".

-- 
<Address HyTime=bibloc homepage="http://www.drmacro.com">
W. Eliot Kimber, eliot@isogen.com
Senior SGML Consulting Engineer, Highland Consulting
2200 North Lamar Street, Suite 230, Dallas, Texas 75202
+1-214-953-0004 +1-214-953-3152 (fax) 
http://www.isogen.com (work)</Address>
"Rats in the morning, rats in the afternoon...if they don't go away, 
I'll be reducated soon..."   --Austin Lounge Lizards, "1984 Blues" 
(http://www.webcom.com/~yeolde/all/lllhome.html)