High Pass Filter Plugin

GIMP doesn't have a built in high pass filter. While it is trivial enough to implement manually, I figured I could implement a plugin^[?] and throw in a few bonus features. Much of the process comes this article on making a "Sketch Effect" using a high pass filter. More recently, I came across a few external posts on sharpening images using high pass filter overlays, rather than un-sharp masking (for Photoshop). There is even a GIMP plugin to high-pass sharpen.

Then Redrobes at the Cartographers' Guild posted a great thread on creating seamless textures that relied heavily on high pass filters. Because of all that I decided to write a general purpose high-pass filter plugin. It shows up as Filters->Generic->High Pass Filter. You can select a blur radius that sets the size of detail to be passed by the filter, a contrast adjustment, as well as an option to keep the source layer or replace it.

It actually implements 5 different high pass modes:

1. Colour - I think this is the same as photoshop's high pass filter effect.
2. Preserve DC - like colour, but adds the average image colour back in.
3. Greyscale - this desaturates the layer before high pass filtering.
4. Greyscale, Apply Chroma - as above, but blends it in with the source layer colours
5. Redrobes - (named after the technique described by Redrobes)

They are all straightforward, using the basic "duplicate, blur, invert, 50% opacity, merge down" process, with a few small details added for each, except the Redrobes mode. The steps he described were:

Get an image 1.
Blur it quite a lot 2.
Blur it until its a constant colour 3.
Use Image arithmetic to generate 2 - 3 => 4
Use Image arithmetic to generate 3 - 2 => 5
Take 1 and subtract 4 and add 5 => 6

Here are samples that display the 5 filter modes, various blur amounts, and the default blur (10) with different contrast adjustments:

Modes

Amount

Contrast

The plugin can be downloaded here, or at the GIMP plugin repository.