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Percentages + absolute lengths

From: Peter B. West <pbwest@powerup.com.au>
Date: Fri, 07 Feb 2003 02:21:32 +1000
Message-ID: <3E428B8C.3060709@powerup.com.au>
To: xsl-editors <xsl-editors@w3.org>

Please clarify the editors' expectation for the resolution of
an expression like "25% + 3pt" an FO where the relative value
is resolved in terms of an enclosing reference area.

The difficulty with respect to such expressions that I have
experienced in implementing FO tree building is that they force
property resolution into dependency on Area tree construction.
Even in the simplest cases, the evaluation of such expressions
assumes the availability of page-based information, as opposed to
the FO tree's flow and static-content view.  I those cases where
the dimensions of areas may be dependent on look-ahead layout
and retries, e.g. during attempts to resolve column widths for
"auto" column layout, the dependency becomes even more tortuous.

The end result is that such expressions must be carried around in
the constructed FO tree, and only resolved as the applicable FO's
areas are generated and returned, or re-generated and returned.
This greatly complicates the expression parsing which must be
done early for the FO tree building to be able to proceed.

While a simple percentage, e.g., "25%", suffers from the same
uncertainties, it can at least be reduced to a single datum,
whose resolution makes no further demands of the expression
parser, which in turn simplifies the parser.

My current implementation of FO tree building rejects
expressions such as "25% + 3pt" on the basis that (part of)
"...the expression value cannot be converted to the necessary
type for the property value," (5.9.12) within the context of
the building of the FO tree.  This is in spite of the fact that
the spec states, fatuously in my view, that, "Properties are
evaluated against a property-specific context. This context
provides: ... Conversions from relative numerics by type to
absolute numerics within additive expressions."

The context of this conversion is not property-specific as much
as area-tree specific.

Peter B. West  pbwest@powerup.com.au  http://www.powerup.com.au/~pbwest/
"Lord, to whom shall we go?"
Received on Thursday, 6 February 2003 11:22:36 UTC

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