Watch Part 1 first: http://www.youtube.com/watch?v=dAuJJ6P8Jxs
Part 3: http://www.youtube.com/watch?v=l2yhdaJhje8

Sorry, the audio seems to get slightly out-of-sync towards the end of the video.

The code for this Smooth Threshold script is included in the completed Splatter Brush script, so download it here if you want a closer look at it:
http://monsoonami.deviantart.com/art/Splatter-Brush-Generator-GIMP-Script-308796378

video made with:
CamStudio http://camstudio.org/
VirtualDub http://www.virtualdub.org/
Audacity http://audacity.sourceforge.net/