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<section id="backends">
<sectioninfo>
<title>FoDoc and xmlroff Backends</title>
<subtitle>Backends</subtitle>
<abstract><para>Backends for writing xmlroff output</para></abstract>
</sectioninfo>
<section id="backends-text">
<title>Backends</title>
<para>xmlroff is built with one or more backends that do the work of
writing the output file, e.g., PostScript or PDF output.</para>
<para>The backends are based on other open source graphics libraries
that handle the details of the file formats. xmlroff could have been
written to natively write PostScript, PDF, etc., but there's little
point doing that when libraries for doing that already exist (or now
exist, since, for example, the Cairo library on which the Cairo
backend is based is much newer than xmlroff).</para>
<para>The key requirement for a backend is that it is possible to
render Pango layouts using the backend.</para>
</section>
<section id="yapc">
<title>FoDoc is YAPC (Yet Another PostScript Clone)</title>
<para>Graphics libraries seemingly all implement some variation on the
PostScript imaging model:</para>
<variablelist>
<varlistentry>
<term>Drawing is done at an active "point"</term>
<listitem><para>The "point" has a position, and you can move the
"point" to an absolute or relative position on the drawing surface.</para></listitem>
</varlistentry>
<varlistentry>
<term>There are a number of primitive graphic operations</term>
<listitem><para>Primitive operations exist for moving the point,
drawing lines, arcs, and rectangles, and translating the origin of
the coordinate system.</para></listitem>
</varlistentry>
<varlistentry>
<term>Drawing is stateful</term>
<listitem><para>You can set, for example, the stroke colour, and
that colour will be used by other drawing operations until you
change it again.</para></listitem>
</varlistentry>
<varlistentry>
<term>Graphic states can be saved on a stack</term>
<listitem><para>Graphic states can be saved and restored so you
can, for example, save the state, change the stroke colour, draw
something, then restore the state and, by doing so, restore the previous stroke
colour, etc.</para></listitem>
</varlistentry>
</variablelist>
<para>Along with their similarities, graphic libraries have their
differences. xmlroff hides the differences between backends by
implementing an abstract FoDoc object type that has functions for the
graphic operations that xmlroff uses. Each backend is
implemented as a subtype of FoDoc.</para>
<para>So, as you would expect, FoDoc is yet another PostScript
clone.</para>
</section>
</section>
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