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<h2><span class="refentrytitle">Plug-in Proxy Overview</span></h2>
<p>Plug-in Proxy Overview —
Overview of <span class="application">Gwyddion</span> plug-ins and the plugin-proxy module
</p>
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<div class="refsect1">
<a name="id-1.4.2.3"></a><h2>Plug-ins Overview</h2>
<p>
Plug-ins are external programs that can be executed by <span class="application">Gwyddion</span>
to either perform some operation on the data or to read or write
data in a third-party file format.
</p>
<p>
Generally it is preferable to extend <span class="application">Gwyddion</span> functionality by
modules, because modules are dynamic libraries linked directly
to <span class="application">Gwyddion</span> at run-time allowing much more versatile interaction
with the application, and they are also faster (for the same reason).
For example, plug-ins generally cannot make use of existing <span class="application">Gwyddion</span>
data processing functions and cannot modify data in-place, a new
window is always created for the result.
Programming of modules is also no harder than programming of plug-ins,
maybe it is even easier (assuming you know C).
</p>
<p>
They are however two reasons to use plug-ins:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
Instability. A crash in a module is a crash in <span class="application">Gwyddion</span>
because modules, once loaded, become part of the application.
When a plug-in fails, one gets garbage or nothing instead of
data, but it does not affect the application.
</li>
<li class="listitem">
Licensing. Since modules link with <span class="application">Gwyddion</span> they are derived
works of it and thus they have to be licensed under GNU General
Public License, if they are to be distributed. The plug-in
interface was on the other hand designed specifically to allow
plug-ins to be independent works and thus not bound by GNU GPL
(you are of course still encouraged to use GNU GPL for your
plug-ins).
The interface consists only of a small set of command line
arguments and a very simple data file format used for data
exchange.
</li>
<li class="listitem">
Testing. While module unloading and reloading is theoretically
possible, it is not implemented. Thus one has to quit and restart
<span class="application">Gwyddion</span> for changes in a module to take effect. Plug-ins are
separate programs, so they are run in whatever state they currently
are each time they are used.
</li>
<li class="listitem">
Language issues. <span class="application">Gwyddion</span> currently has only C API.
If you already have your data processing routines written in
FORTRAN, Java, Ruby, Delphi, or whatever, you may find easy to make
a plug-in from them, while conversion to a module can require
substantial changes and/or rewrite to C.
</li>
</ul></div>
<p>
</p>
</div>
<div class="refsect1">
<a name="id-1.4.2.4"></a><h2>Plug-in Proxy</h2>
<p>
The capability to use plug-ins is not a built-in <span class="application">Gwyddion</span> feature,
instead it is provided a by a smart module called plug-in proxy.
With the small exception there is a dedicated directory for plug-ins
defined directly in <span class="application">Gwyddion</span>, the plug-in proxy module is in no way
privileged or otherwise special.
It can be removed and/or other plug-in proxies can be added and
happily coexist with the current one.
</p>
<p>
When the plug-in proxy module registeres self, it scans the plug-in
directories, requests information from the plug-ins it finds and passes
the information as its own features.
Likewise when it is run to process some data it executes
the appropriate plug-in to process it and passes the result back to
<span class="application">Gwyddion</span>. All plug-in features thus look as plug-in proxy features
to <span class="application">Gwyddion</span>, as you can see yourself in the Module Browser.
</p>
<p>
FIXME: More to be said.
</p>
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