Characterstic Specification

Subject: Characterstic Specification
From: Vivek Agrawala <vivek@xxxxxxxxxxxxxxx>
Date: Mon, 19 May 1997 14:30:13 -0400
Section 12.4.6 of the DSSSL standard has the following paragraph:

>  "An expression specifying the value of a characterstic shall be
>   evaluated with respect to two flow objects, which are referred 
>   to as the _value flow object_ and the _specification flow object_.
>   The value of a characterstic for a flow object is determined by
>   evaluating the expression specifying that characterstic with
>   both the value flow object and the specification flow object
>   equal to that flow object."

What does it mean to "evaluate an expression with respect to two
flow objects" ?  Is the evaluation of expressions in the DSSSL
style language defined somewhere ?

Also, are the terms "value flow object" and "specfication flow object"
defined somewhere ?

Further down in the same section, the standard says:

>  "[182] application-characterstic-declaration = (declare-characterstic
>   _identifier_  _string_  _expression_)
>   ....  The _string_ is a public identifier specifying the semantics
>   of the characterstic. If an implementation does not recognize the
>   specified public identifier, it shall ignore uses of the
>   characteristic.  .... "

The mechanism for specifying semantics of user-defined characterstics
seems to be implementation defined.
When the standard refers to an 'implementation' does it include
only the style engine, or also the backends ?

Jade's RTF backend currently supports some additional characteristics.
I looked at the implementation, and it seems that the semantics are
defined by the backend when it defines the FOTBuilder::Extensions. It
is not clear to me how one defines that a certain charateristic is
only available on a certain flow-object class.
Currently, the public ids for these characteristics are of the form:
  "UNREGISTERED::James Clark//Characteristic::heading-level"
Would it be possible to have more general public ids like,
  "UNREGISTERED::James Clark//Characteristic::integer" ?
The name of the characteristic is already available as the first
argument to declare-characteristic.

-- Vivek Agrawala, Ph.D.
Siemens Corporate Research, Inc.	email: vivek@xxxxxxxxxxxxxxx

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