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

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:

This basic functionality is provided by the default.preloop 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.