Script-Fu -> High Pass
This is an adjustable script,
high pass filters have a long and varied history, this one is a re-work of script by Rob Antonishen. His blog post describing the original filter is here:
High Pass is often used as a sharpening tool, but it is a bit more at home in the quot;contrast" department. To ease its use as a sharpening tool, however, I include the "overlay" option in the settings dialog.
Here's a list of the different modes:
Colour - I think this is the same as photoshop's high pass filter effect. Preserve DC - like colour, but adds the average image colour back in. Greyscale - this desaturates the layer before high pass filtering. Greyscale, Apply Chroma - as above, but blends it in with the source layer colours Redrobes - (named after the technique described by Redrobes)
After - default settings
After - overlay (sharpen)