High Pass Filter

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, as well as an option to keep the source layer or replace it.

EDIT: Updated to use grain extract rather than the previous method, This looses less histogram data and is closer to the Photoshop version...

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 a basic "duplicate, blur, grain extract, 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

While this is supposed to be the same as a basic filter with preserve DC offset, it seems to give slightly different results, so I left it in :)

For more details and sample images, see my blog post:
http://www.silent9.com/blog/archives/152-High-Pass-Filter-Plugin.html