I’m fairly new to GIMP, but I’ve been using Photoshop for a while. I’ve run across some useful tips and tricks over time or Photoshop and one I’ve come to rely on pretty heavily is the high pass filter (via InformIT). This filter comes in handy when you’re trying to sharpen an image without affecting the original. There are plenty of sites that cover the details on this:

• The High Pass Way to Sharpen in Photoshop
• Improving Details in Photos

GIMP doesn’t have a high pass filter built in so I attempted to find a way to perform a similar operation. here’s what I came up with.

1. Copy image layer twice
2. Run Edge-Detect->Difference of Gaussians
3. On one layer set Radius 1 to 6, Radius 2 to 7, and check normalize and invert
4. One the second layer switch the radius values and keep the remainder of settings
5. Set the top layer’s mode to “grain extract”
6. Merge the two new layers
7. Set the resulting layer to overlay and modify its opacity to suite the level of sharpening needed

Of course, once I finished adjusting a number of images in this manner I found some information relating to how to perform a high pass filter in GIMP. (D’OH!)

• Vidar’s GIMPy high pass filter sketch affect
• Gimp High Pass Filter Plugin

And some general high pass goodness:

• The Power of the High Pass Filter