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From Apache Wiki <wikidi...@apache.org>
Subject [Xmlgraphics-fop Wiki] Update of "TableLayout/TableMarkers" by JeremiasMaerki
Date Thu, 27 Jan 2011 08:14:45 GMT
Dear Wiki user,

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The "TableLayout/TableMarkers" page has been changed by JeremiasMaerki.
The comment on this change is: Some notes on table markers and friends.


New page:
= Notes on Table Markers and Friends =

Apache FOP 0.20.5 had a proprietary extension {{{fox:continued-label}}} to add some inline
content on the non-first parts of a table. Table markers from XSL 1.1 could take over that
task but they haven't been implemented, yet. RenderX uses another proprietary extension attribute
({{{rx:table-omit-initial-header}}}) on {{{fo:table}}} which, as the name says, omit the first
table header if activated. That way the user can add a different table header for the first

{{{fox:continued-label}}} is slightly more powerful and probably easier to comprehend (for
the user) than {{{rx:table-omit-initial-header}}}. Probably even simpler
than table-markers. Still, the goal is obviously to support table markers at some point.

At first, it would appear that a {{{fox:table-omit-initial-header}}} should be much easier
to implement than a fox:continued-label. The problem with the latter: we have to implement
a complete layout manager for the continued-label element which is an inline-level object.
Furthermore, the table layouter has to put some kind of signal in the layout context on which
the !ContinuedLabelLayoutManager has to react and not return any Knuth elements. The table
layout manager has to run the layout for the header
at least twice, once with the signal and once without.

But then one would think that for fox:table-omit-initial-header it should be easy to suppress
the Knuth Elements for the header at the first break. But at the time of element generation,
we don't know whether a break will be the first break of the table or not. And that seems
to make both approaches pretty much impossible with total-fit.

Here's how we do headers and footers:

 * P=penalty
 * H=header
 * Hc=header continued
 * F=footer
 * S=step (i.e. some table content)

Simplified element List for table with no break:

{{{S S S S H F}}}

Real element list for a table:

{{{S P(H+F) S P(H+F) S P(H+F) S P(H+F) S H F}}}

== Investigating the implementation of table-omit-initial-header ==

For table-omit-initial-header, we would have to modify the H for certain penalties but we
don't know which will cause the break for the first page.

The last "H" serves two cases:

 1. no break: H is the single table header.
 2. break: H is the last table header.

For case 1 we'd actually have to suppress the final H. And that we can't do at the time of
element list generation. Only in ''first-fit'' could we react on such a case. And that will
also make table markers really hard if not impossible for ''total-fit''.

For case 2, the final "H" is actually rather "Hc". So, in one case it should represent "H"
and in another "Hc". As soon as the heights of the two variants are not the same, the problem

== Additional thoughts ==

For very special cases, it would be possible to take a similar approach as taken for page-number-citation.
But where page numbers have a relatively small width change, "continued" labels are usually
much wider which can produce problems. And it would be a big hack, of course.

Maybe a new Knuth element would have to be introduced that would allow adding stuff after
a break. We may have talked about something like that a few years back.

In the end, it looks like many page breaking problems could be solved by refactoring the layout
engine's page breaking from ''total-fit'' to ''first-fit'' or ''best-fit''. That would also
offer to optimize memory usage of FOP, but obviously, that would require a major work investment.

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