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    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:
    Click image for larger version. 

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    And various blur amounts:
    Click image for larger version. 

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    The default blur (10) with different contrast adjustments:
    Click image for larger version. 

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    And a sample how a high pass filter can be used to make a nifty overlay for a map:
    Click image for larger version. 

Name:	hpfsamples4.png 
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    -Rob A>
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