Extension subsystem

General

The Extension system is a way to add functionality to Inkscape.  The
design of the extension system follows the bridge design pattern,
breaking apart the functionality that is being provided, and the
implementation of that functionality.  The term extension is used to
describe all of this, the functionality and the implementation.

Functionality Provided

There are several types of functionality provided by the extensions
system today, and more are slated for the future.
-- Input.  Input extensions take data from a file and bring it into
Inkscape.  Even reading SVG files (Inkscape's native format) is
implemented as an internal extension.  Input extensions can be chained
so that extensions can be created that don't make it all the way to SVG
directly.
-- Output.  Output extensions take the data from Inkscape and turn it
into a file.  SVG Output is an internal extension.  Output extensions
can also be chained.
-- Effects.  Effects are extension that take a document, and possibly a
selection, and change it in some way.  This could be anything from
changing colors to generating fractals.  These are similar to Filters in
The GIMP.
-- Path Effects.  Path effects provide a way to change a path.  They
allow a user to edit a path using the standard Inkscape path editing
tools, but then have it look differently than just a line.  This could
include everything from adding hash marks to making the line look like
it's on fire.
-- Tools.  Tools are the ways that you draw in Inkscape.  Rectangle
tool, circle tool, star tool...  the goal is to eventually have the
ability to add tools as extensions.

Implementation Types

While the functionality that the extension provides is described above,
how that gets done is a different story.  The Inkscape extensions system
allows for programmers to implement their extension in  a variety of
ways, trying to match their particular skillset.
-- Internal.  Any extension type can be implement in C or C++ and
directly linked into the Inkscape codebase.  While in general the goal
is to move as much out of the main Inkscape binary, this makes sense for
many extensions including things like SVG input/output.
-- Scripts.  Scripts are where Inkscape uses standard in and standard
out on an external executable to implement the functionality.  This
allow Inkscape to reuse a variety of utilities that already do format
conversion or implement cool things in SVG.  Typically the scripts that
are effects use a SVG DOM library in their programming language of
choice.
-- XSLT.  This implementation is done in the XSLT language, and uses
libxml's XSLT parser that is already linked with Inkscape.
-- DOM Scripts.  As Inkscape finishes its DOM implementation the goal
will be to provide this directly to scripts.  This will allow for faster
execution, but also other access to Inkscape's internals and algorithms.

See Also

http://sourceforge.net/mailarchive/message.php?msg_id=15742559