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  1. #1
    Community Leader RobA's Avatar
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    Apr 2007
    Toronto, Canada

    Info High Pass Filter in GIMP (new plugin)

    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:
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    And various blur amounts:
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    The default blur (10) with different contrast adjustments:
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    And a sample how a high pass filter can be used to make a nifty overlay for a map:
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    -Rob A>
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  2. #2
    Administrator Redrobes's Avatar
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    Dec 2007
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    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.
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    Last edited by Redrobes; 07-24-2008 at 06:30 PM.

  3. #3
    Community Leader RobA's Avatar
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    Apr 2007
    Toronto, Canada


    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>

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