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<?xml version="1.0" standalone="no"?>
<!DOCTYPE document PUBLIC "-//APACHE//DTD Documentation V1.1//EN"
"http://cvs.apache.org/viewcvs.cgi/*checkout*/xml-forrest/src/core/context/resources/schema/dtd/document-v12.dtd">
<document>
<header>
<title>Galleys</title>
<authors>
<person name="Peter B. West" email="pbwest@powerup.com.au"/>
</authors>
</header>
<body>
<section>
<title>Layout galleys in FOP</title>
<section>
<title>Galleys in Lout</title>
<p>
Jeffrey H. Kingston, in <link href =
"http://snark.niif.spb.su/~uwe/lout/design.pdf" ><em>The
Design and Implementation of the Lout Document Formatting
Language</em> Section 5</link>, describes the
<strong>galley</strong> abstraction which he implemented in
<em>Lout</em>. A document to be formatted is a stream of
text and symbols, some of which are <strong>receptive
symbols</strong>. The output file is the first receptive
symbol; the formatting document is the first galley. The
archetypical example of a receptive symbol is
<strong>@FootPlace</strong> and its corresponding galley
definition, <strong>@FootNote</strong>.
</p>
<p>
Each galley should be thought of as a concurrent process, and
each is associated with a semaphore (or synchronisation
object.) Galleys are free to "promote" components into
receptive targets as long as</p>
<ul>
<li>
an appropriate target has been encountered in the file,
</li>
<li>
the component being promoted contains no unresolved galley
targets itself, and
</li>
<li>
there is sufficient room for the galley component at the
target.
</li>
</ul>
<p>
If these conditions are not met, the galley blocks on its
semaphore. When conditions change so that further progress
may be possible, the semaphore is signalled. Note that the
galleys are a hierarchy, and that the processing and
promotion of galley contents happens <em>bottom-up</em>.
</p>
</section>
<section>
<title>Some features of galleys</title>
<p>
It is essential to note that galleys are self-managing; they
are effectively layout <em>bots</em> which require only a
receptive area. If a galley fills a receptive area (say, at
the completion of a page), the galley will wait on its
semaphore, and will remain stalled until a new receptive
area is uncovered in the continued processing (say, as the
filled page is flushed to output and a new empty page is
generated.)
</p>
<p>
Difficulties with this approach become evident when there
are mutual dependencies between receptive areas which
require negotiation between the respective galleys, and, in
some cases, arbitrary deadlock breaking when there is no
clear-cut resolution to conflicting demands. Footnote
processing and side floats are examples. A thornier example
is table column layout in <em>auto</em> mode, where the
column widths are determined by the contents. In
implementing galleys in FOP, these difficulties must be
taken into account, and some solutions proposed.
</p>
<p>
Galleys model the whole of the process of creating the final
formatted output; the document as a whole is regarded as a
galley which flushes in to the output file.
</p>
</section>
<section>
<title>The layout tree</title>
<anchor id="layout-tree"/>
<p>
This proposal for implementing galleys in FOP makes use of a
<strong>layout tree</strong>. As with the <link href=
"../layout.html" >layout managers</link><em></em> already
proposed, the layout tree acts as a bridge between the <link
href= "../fotree.html" >FO Tree</link> and the <link href=
"../areas.html" >Area Tree</link>. If the elements of the
FO Tree are FO nodes, and the elements of the Area Tree are
Area nodes, representing areas to be drawn on the output
medium, the elements of the layout tree are <strong>galley
nodes</strong> and <strong>area tree fragments</strong>.
The area tree fragments are the final stages of the
resolution of the galleys; the output of the galleys will be
inserted directly into the Area Tree. The tree structure
makes it clear that the whole of the formatting process in
FOP, under this model, is a hierarchical series of galleys.
The dynamic data comes from fo:flow and fo:static-content,
and the higher-level receptive areas are derived from the
<em>layout-master-set</em>.
</p>
</section>
<section>
<title>Processing galleys</title>
<p>
Galleys are processed in two basic processing environments:
</p>
<section>
<title>Inline- and block-progression dimensions known</title>
<p>
The galley at set-up is provided with both an
<em>inline-progression-dimension</em> (<em>i-p-d</em>) and
a <em>block-progression-dimension</em> (<em>b-p-d</em>).
In this case, no further intervention is necessary to lay
out the galley. The galley has the possibility of laying
itself out, creating all necessary area nodes. This does
not preclude the possibility that some children of this
galley will not be able to be so directly laid out, and
will fall into the second category.
</p>
<p>
While the option of "automatic" layout exists, to use
such a method would relinquish the possibility of
monitoring the results of such layout and performing
fine-tuning.
</p>
</section>
<section>
<title>Inline- ior block-progression-dimensions unknown</title>
<p>
The galley cannot immediately be provided with an i-p-d
ior a b-p-d. This will occur in some of the difficult
cases mentioned earlier. In these cases, the parent
galley acts as a layout manager, similar to the sense used
in <link href= "../layout.html" >another
discussion</link>. The children, lacking full receptive
area dimensions, will proceed with galley pre-processing,
a procedure which will, of necessity, be followed
recursively by all of its children down to the atomic
elements of the galley. These atomic elements are the
individual <em>fo:character</em> nodes and images of fixed
dimensions.
</p>
</section>
</section>
<section>
<title>Galley pre-processing</title>
<anchor id="pre-processing"/>
<p>
Galley pre-processing involves the spatial resolution of
objects from the flows to the greatest extent possible
without information on the dimensions of the target area.
Line-areas have a block progression dimension which is
determined by their contents. To achieve full generality in
layouts of indeterminate dimensions, the contents of
line-areas should be laid out as though their inline
progression dimension were limited only by their content.
In terms of inline-areas, galleys would process text and
resolve the dimensions of included images. Text would be
collected into runs with the same alignment
characteristics. In the process, all possible "natural" and
hyphenation break-points can be determined. Where a
line-area contains mixed fonts or embedded images, the b-p-d
of the individual line-areas which are eventually stacked
will, in general, depend on the line break points, but the
advantage of this approach is that such actual selections
can be backed out and new break points selected with a
minimum of re-calculation. This can potentially occur
whenever a first attempt at page layout is backed out.
<br/><br/>
<strong>Figure 1</strong>
</p>
<figure
src="images/design/alt.design/galley-preprocessing.png"
alt="Galley pre-processing diagram"/>
<p>
Once this pre-processing has been achieved, it is
envisaged that a layout manager might make requests to the
galley of its ability to fill an area of a given
inline-progression-dimension. A positive response would
be accompanied by the block-progression-dimension. The
other possibilities are a partial fill, which would also
require b-p-d data, and a failure due to insufficient
i-p-d, in which case the minimum i-p-d requirement would
be returned. Note that decisions about the
actual dimensions of line-areas to be filled can be
deferred until all options have been tested.
</p>
<p>
The other primary form of information provided by a
pre-processed galley is its minimum and maximum i-p-d, so
that decisions can be made by the parent on the spacing of
table columns. Apart from information requests,
higher-level processes can either make requests of the
galleys for chunks of nominated sizes, or simply provide the
galley with an i-p-d and b-p-d, which will trigger the
flushing of the galley components into Area nodes. Until
they have flushed, the galleys must be able to respond to a
sequence of information requests, more or less in the manner
of a request iterator, and separately manage the flushing of
objects into the area tree. The purpose of the "request
iterator" would be to support "incremental" information
requests like <em>getNextBreakPosition</em>.
</p>
</section>
</section>
</body>
</document>
|
