High Pass Filter in GIMP (new plugin)

[RobA]

I am not sure if this should be in the resources section or in the tutorial section, but here it is, anyway.

GIMP doesn't have a high pass filter. I had previously posted about "faking" a high pass filter with GIMP to get rid of pattern noise in this post.

Then Redrobes posted a great thread on creating seamless textures that relied heavily on high pass filters.

More recently, I cam across a few external posts on sharpenning images using high pass filter overlays, rather than unsharp masking (for photoshop).

Because of all that I decided to write a high pass filter plugin. Just unzip the attached file into your scripts directory. It shows up asFilters->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.

I actually implemented 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 greyscales the layer before high pass filtering.
4. Greyscale, Apply Chroma - as above, but blends it in with the source layer colours
5. Redrobes - (for lack of a better name ) follows the procedure in post #17 of this thread

Here are samples of the 5 filter modes:
hpfsamples1.png

And various blur amounts:
hpfsamples2.png

The default blur (10) with different contrast adjustments:
hpfsamples3.png

And a sample how a high pass filter can be used to make a nifty overlay for a map:
hpfsamples4.png

-Rob A>

[Redrobes]

I have a mode named after me... sniff... its all too much, im gonna cry.

This is great Rob. The only bit I cant quite figure is what is the basic difference between the Preserve DC and 'my' mode since I was trying to make a filter to high pass with preserve DC. It does look similar in the results in any case.

Interesting bit about the stairs. Although I have never procedurally created the staircase pattern I do use one as a transparency to pop over any other type of terrain - like dungeon stone for example.

Have you tried to use the FFT expansion with a DC preserve - ill attach the kind of filter you need. You will have to adjust the levels to suit Gimp tho. Note the center pixel is 'no change' color - DC preserve. Oh yes should also note - if you get that pixel out by one in any direction it all goes to pot. So try moving it about a little if it doesn't work. The way you can get a definitive answer of course is to give the FFT something all white and get its spectrum and that should have just the one single pixel lit - well thats the one that must be no change.

[RobA]

I thought you'd appreciate the honour, especially when I post this up at the Gimp plugin repository 10 years from now someone will say "why is that mode called redrobes?"

I think the difference comes from adding in the extra blur-average, to the average-blur.

The basic algorithm I used for all of them (except the redrobes mode) was:

-duplicate layer
-blur the duplicate
-invert the duplicate
-set the duplicate to 50% opacity, keeping the mode "normal"
-merge it down.

This gives the hpf with a base 10 50% grey.

EDIT: Thanks for the filter. I'm also working to add all of your FFT maps as pick lists in a front-end to the gimp FFT plugin.

-Rob A>