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<?xml version="1.0" standalone="no"?>
<!-- Overview -->
<document>
<header>
<title>Area Tree</title>
<subtitle>All you wanted to know about the Area Tree !</subtitle>
<authors> <person name="Keiron Liddle" email="keiron@aftexsw.com"/>
</authors>
</header>
<body><s1 title="Area Tree">
<s2 title="Area Tree">
<p>The Area Tree is an internal representation of the result document. This
is a set of java classes that can put together a set of objects that
represent the pages and their contents.</p>
<p>This information is created by the layout managers and is rendered to the
output using a renderer.</p>
<p>The Area Tree follows the description of the area tree in the XSL:FO
specification.</p>
<p>The Area Tree consists of a set of pages, the actual implemenation places
these in
a set of page sequences.</p></s2>
<s2 title="Pages">
<p>A page consists of a page+viewport pair.</p>
<p>The PageViewPort and Page with the regions is created by the
LayoutMasterSet. The contents are then placed by the layout managers. Once
the layout of a page is complete then it is added to the Area Tree.</p>
<p>Inside the page is a set of RegionViewport+Region pairs for each region on
the page.</p></s2>
<s2 title="Blocks">
<p>Block level areas contain either other blocks or line areas (which is a
special block area).</p><p>
A block is either positoned or stacked with other block areas.</p>
</s2>
<s2 title="Inline Areas">
<p>Inline areas are stacked in a line area. Inline areas are objects such as
character, viewport, inline-container, leader and space. A special inline
area Word is also used for a group of consecutive characters.</p>
<p>The image and instream foreign object areas are placed inside a viewport.
The leader (with use content) and unresolved page number areas are
resolved to other inline areas.</p>
<p>Once a LineArea is filled with inline areas then the inline areas need to
be aligned and adjusted to fill the line properly.</p></s2>
<s2 title="Traits">
<p>A trait is information associated with an area. This could be information
such as text colour or is-first.</p>
<p>Traits provide information about an area. The traits are derived from
properties on the formatting object or are generated during the layout
process. Many of the layout traits do not have actual values but can be
derived from the Area Tree. Other traits that apply when rendering the
areas are set on the area. Since setting the same value on every area
would use a lot of memory then the traits are derived from default or
parent values.</p>
<p>A dominant trait on a block area is set, for example font colour, so that
every line area with the same dominant value can derive it. The text
inline areas then get the font colour set on the inline area or from the
line area or from the block area.</p></s2>
<s2 title="Forward References">
<p>The Area Tree maintains a set of mappings from the reference to pages.</p>
<p>The PageViewPort holds the list of forward references that need resolving
so that if a references is resolved during layout the page can be easily
found and then fixed. Once all the forward references are resolved then
the page is ready to be rendered.</p>
<p>To layout a page any areas that cannot be resolved need to reserve space.
Once the inline area is resolved then the complete line should be adjusted
to accomodate any change in space used by the area.</p></s2>
<s2 title="Caching">
<p>We may need to cache pages due to forward references or when keeping all
pages.</p>
<p>This is done by serializing the Page. The PageViewport is retained to be
used as a key for page references and backward references.
The Page is serialized to an object stream and then all of the page
contents are released. The Page is then recoved by reading from the object
stream.</p>
<p>The PageViewport retains information about id areas for easy access.</p>
</s2>
<s2 title="Extensions">
<p>The Area Tree holds the Output Document extensions. This is information
such as pdf bookmarks or other output document specific information that
is not handled by XSL:FO.</p>
<p>It is also possible to create custom areas that extend a normal area. The
actual data that is rendered could be set in a different way or depend on
resolving a forward reference.</p>
</s2>
<s2 title="Area Tree Handlers">
<p>To handle different situations the handler for the Area Tree handles each
page as it is added.</p><p>
The RenderPagesModel sends the page directly to the renderer if the page
is ready to be rendered. Once a page is rendered it is discarded.
The StorePagesModel stores all the pages so that any page can be later
accessed.</p>
<p>The Area Tree retains the concept of page sequences (this is not in the
area tree in the spec) so that this information can be passed to the
renderer. This is useful for setting the title and organising the groups
of page sequences.</p></s2>
<s2 title="Associated Tasks">
<ul><li>Sort out extensions concept, access to AreaTree.</li>
<li>Sort out trait handling.</li>
<li>Caching implementation.</li></ul></s2>
</s1>
</body></document>
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