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<?xml version="1.0" encoding="ISO-8859-1"?>
<!-- $Id$ -->
<!--
<!DOCTYPE document SYSTEM "../../xml-docs/dtd/document-v10.dtd">
-->
<document>
<header>
<title>Keeps and breaks</title>
<authors>
<person name="Peter B. West" email="pbwest@powerup.com.au"/>
</authors>
</header>
<body>
<!-- one of (anchor s1) -->
<s1 title="Keeps and breaks in layout galleys">
<p>
The <link href= "galleys.html" >layout galleys</link> and the
<link href= "galleys.html#layout-tree" >layout tree</link>
which is their context have been discussed elsewhere. Here we
discuss a possible method of implementing keeps and breaks
within the context of layout galleys and the layout tree.
</p>
<s2 title="Breaks">
<p>
Breaks may be handled by inserting a column- or page-break
pseudo-object into the galley stream. For break-before, the
object would be inserted before the area in which the flow
object, to which the property is attached, is leading. If
the flow object is leading in no ancestor context, the
pseudo-object is inserted before the object itself.
Corresponding considerations apply for break-after.
Selection of the position for these objects will be further
examined in the discussion on keeps.
</p>
</s2>
<s2 title="Keeps">
<p>
Conceptually, all keeps can be represented by a
keep-together pseudo-area. The keep-together property
itself is expressed during layout by wrapping all of the
generated areas in a keep-together area. Keep-with-previous
on formatting object A becomes a keep-together area spanning
the first non-blank normal area leaf node, L, generated by A
or its offspring, and the last non-blank normal area leaf
node preceding L in the area tree. Likewise, keep-with-next
on formatting object A becomes a keep-together area spanning
the last non-blank normal area leaf node, L, generated by A
or its offspring, and the first non-blank normal area leaf
node following L in the area tree.
<br/>TODO REWORK THIS for block vs inline
</p>
<p>
The obvious problem with this arrangement is that the
keep-together area violate the hierarachical arrangement of
the layout tree. They form a concurrent structure focussed
on the leaf nodes. This seems to be the essential problem
of handling keep-with-(previous/next); that it cuts across
the otherwise tree-structured flow of processing. Such
problems are endemic in page layout.
</p>
<p>
In any case, it seems that the relationships between areas
that are of interest in keep processing need some form of
direct expression, parallel to the layout tree itself.
Restricting ourselves too block-level elements, and looking
only at the simple block stacking cases, we get a diagram
like the attached PNG. In order to track the relationships
through the tree, we need four sets of links.
</p>
<p>
<strong>Figure 1</strong>
</p>
<anchor id="Figure1"/>
<figure src="block-stacking.png" alt="Simple block-stacking
diagram"/>
<p>
The three basic links are:
</p>
<ul>
<!-- one of (dl sl ul ol li) -->
<li>Leading edge to leading edge of first normal child.</li>
<li>Trailing edge to leading edge of next normal
sibling.</li>
<li>Trailing edge to trailing edge of parent.</li>
</ul>
<p>
Superimposed on the basic links are bridging links which
span adjacent sets of links. These spanning links are the
tree violators, and give direct access to the areas which
are of interest in keep processing. They could be
implemented as double-linked lists, either within the layout
tree nodes or as separate structures. Gaps in the spanning
links are joined by simply reproducing the single links, as
in the diagram. The whole layout tree for a page is
effectively threaded in order of interest, as far as keeps
are concerned.
</p>
<p>
The bonus of this structure is that it looks like a superset
of the stacking constraints. It gives direct access to all
sets of adjacent edges and sets of edges whose space
specifiers need to be resolved. Fences can be easily enough
detected during the process of space resolution.
</p>
</s2>
</s1>
</body>
</document>
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