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I'm primarily a gimp user, but I want something to blur an image similarly to the way a lens would blur an out of focus background. AFAIK this is essentially a circular blur/convolution (assuming circular aperture and ignoring diffraction).

The operation is very straight forward, but I've been searching for a while and the only thing I can come up with is ImageMagick:

convert image.png -define convolve:scale=! -morphology Convolve Disk:20 blurred-image.png

This is great for automating tasks, but is rather cumbersome with trial and error edits. I'd also like an alternative just to double check results. Does anyone know of a tool that does circular blur?

If someone with rep could add more tags, please do: convolution, depth-of-field

[EDIT] An example...

original gamma

circular blur undo gamma, final

standard gaussian blur

Custom convolutions give quite a small matrix (for example nothing close to 30x30) and I can't be bothered typing in weights. I'm also very much a fan of live previews with easy adjustments.

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  • any tool capable of making a custom convolution, so Photoshop, Gimp, After effects, fusion, nuke, shake, matlab, mathematica etc.
    – joojaa
    Commented May 19, 2014 at 16:37
  • FYI blurred backgrounds in photos are related to depth of field: en.wikipedia.org/wiki/Depth_of_field They are not literally blurring in a circular manner (though that's a common way to try and fake it, which can work to varying degrees depending on the photo being used)
    – DA01
    Commented May 19, 2014 at 20:04
  • @DA01 how do we lnow he does not have a depth channel? Or indeed that he isnt painting one. Biggest problem with good bokkeh is lack of hdr data for the blur to blow intense parts right.
    – joojaa
    Commented May 19, 2014 at 20:58
  • @joojaa I'm not sure I follow. I don't now that the OP is asking about bokkeh per se--but regardless, the effect is still done the same way via the camera--narrowing the depth of field.
    – DA01
    Commented May 19, 2014 at 21:00
  • @DA01 imagine that the entire picture is distant.
    – joojaa
    Commented May 20, 2014 at 3:28

3 Answers 3

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Quite a versatile tool to generate a circular blur comes from the GREY's Magic for Image Computing (G'MIC) plugin for Gimp.

After installation we have access to this tool through G'MIC > Degradations > Blur [depth-of-field] with many convenient settings to control the blur effect:

enter image description here

enter image description here

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  • Great plugin! Thanks for pointing it out, though I haven't found a circular blur yet.
    – jozxyqk
    Commented May 20, 2014 at 14:09
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The Focus Blur GIMP plug-in does exactly this:

Screenshot of GIMP Focus Blur plug-in

If you're using Debian / Ubuntu Linux, you can get this and a whole bunch of other useful GIMP plugins (including the totally awesome Resynthesizer) by installing the gimp-plugin-registry package.

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  • Exactly what I'm after, thanks! If only gimp-plugin-registry was available for fedora so I wouldn't have to compile/install everything individually.
    – jozxyqk
    Commented May 21, 2014 at 7:30
  • @jozxyqk: You could always try installing it with alien. Mind you, I've never tried that, so use it at your own risk. It might work, or it might not.
    – Ilmari Karonen
    Commented May 21, 2014 at 7:55
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In addition to Takkat's answer, there is an even more robust and versatile tool from GREY's Magic for Image Computing (G'MIC): it is called Blur by Color, and not only includes the capability to do a standard gradiented depth of field blur (see the first example on the Blur by Color page), but also will let you do circular blurs, motion blurs and any shaped combination of the above, using a depth/color-direction map.

Screenshot from the Blur by Color page: example 1.

screenshot from the Blur by Color page

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  • 1
    Please do not only link to a web page, explain in your own words how one can solve the given issue with this software ... As it stands now this is a link-only-answer ...
    – Mensch
    Commented May 29, 2019 at 15:04
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    @Kurt: This is far from a link-only answer. It is still useful with all the links removed. Please see this FAQ.
    – Wrzlprmft ♦
    Commented May 30, 2019 at 8:03

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