The latest version of 'default.preloop' can be downloaded from

Pre-loop code is the code that gets executed once when an RFX filter is applied to a range of frames. In the case of RFX-GIMP filters, the pre-loop code typically performs the following steps:

<ul>
<li>Check if GIMP's Script-fu server is running and connect to it. If this connection fails then a new instance of GIMP (running Script-fu server) is started in the background.</li>
<li>Define a Perl subroutine, 'rfx_sendmsg', that sends messages to the Script-fu server and waits for the response.</li>
<li>Sends a message to the server that defines a Script-fu procedure. This procedure is to be used by [loop-code|loop code] when saving an individual

This basic functionality is provided by the file, and including this default pre-loop code should be sufficient for most all RFX-GIMP filters. However, there is no reason a RFX-GIMP filter can't provide its own custom version of the pre-loop code if the filter needs to do so.