Hello.
I always wondered why the filter Lights and Shadows > Xach-effect not allow negative values ​​in the shadow offsets. To place the shadow in some positions I had to rotate or mirror the image before applying the filter.
Well, I searched and I could find the solution here (dated 2004 ) :
http://www.graphicsgroups.com/16-gimp/a ... 01fc1a.htm

You should find where you installed the script "xach-effect.scm". In GNU/Linux is usually in "/usr/share/gimp/2.0/scripts/" and you will need permissions to modify it. In Windows you look under "C:\Program Files" the folder named "gimp", "gimp-2" or "gimp-2.8" or something. Inside that folder in "\share\gimp\2.0\scripts".
Open the file with a text editor and locate the following lines:

And replace with:


Save the file.

That is all.