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<td valign="top">
<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>
</td>
<td valign="top" align="right"></td>
</tr></table></div>
<div class="refsect1">
<a name="id629899"></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" 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="id598066"></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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