High Pass FIlter

RobA — Fri, 25 Jul 2008 16:59:51 +0200

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:

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)

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

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://ffaat.pointclark.net/blog/archives/152-High-Pass-Filter-Plugin.ht...

Highpass Sharpen

eg — Mon, 07 Jan 2008 13:48:19 +0100

Implement a highpass filter sharpening

Infos about highpass sharpening:
http://www.gimp.org/tutorials/Sketch_Effect/
http://www.retouchpro.com/forums/showthread.php?s=&threadid=3844&highlig...