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I have a screenshot of a web UI and I wish to blur the parts that are not relevant to the point being demonstrated, is there a way to blur / pixelate part of an image using Mac Preview?

Note: the top ranking google result offers a highly substandard solution resulting in an obtrusive rectangle in place of the parts you want blurred - this is really not the same as subtle blurring that obscures information with minimal aesthetic interference with the look of the image.

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    Do you want to make the blurred information unrecoverable, or do you just want to de-emphasize the irrelevant parts? – CodesInChaos Jun 14 at 10:46
  • @CodesInChaos what's being obscured isn't necessarily sensitive information, but there's simply no need for the audience to know what's behind the blur, so I'd say unrecoverable is best. – stevec Jun 14 at 10:49
  • If something is not to be shown, remove it. Do not try to apply any creative filter. – Trang Oul Jun 14 at 13:05
  • @TrangOul I wanted to keep it as close to what uses will expect to see as possible. Check out the method linked to in the question, how obtrusive the big grey box is. It completely draws attention away from the main image. The subtlety of the blurring effect avoids that distraction. – stevec Jun 14 at 13:12
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    @stevec I see; I just wanted to point out that blurring is not secure, yet often used with confidential data. – Trang Oul Jun 14 at 13:14
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Thanks @benwiggy for correctly pointing out that Preview is insufficiently complex to perform a blur.

The application I used was free, and available on App Store - it's called 'Skitch': https://apps.apple.com/au/app/skitch-snap-mark-up-share/id425955336

The bar on the left hand side as a 'blur' icon, simply press it and blur out whatever you need to:

enter image description here

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Preview is not a fully-fledged image editor, and has very limited capabilities.

Conceivably, you could export duplicate copies of the image with low-quality JPEG-ing until the image is unrecognisable, and then paste the relevant area onto the original. It might also be possible to lower the resolution of the duplicate image sufficiently with built-in OS tools.

But ideally, you will need a third-party image editing app with a Blur Tool.

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    Gimp would be the obvious, free solution. It has a specific pixellizer - docs.gimp.org/2.6/en/plug-in-pixelize.html – Tetsujin Jun 13 at 10:50
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    I don't use Gimp, I just suspected it might & so googled that. I use Photoshop, which actually can't do it natively, but you can script it to do so. – Tetsujin Jun 13 at 10:52
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    Just realised, by looking at my script, that it actually does use an internal function, Mosaic. – Tetsujin Jun 13 at 10:59
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    Skitch is a great free app and although it's designed to work with Evernote you do not have to use it with an Evernote account. I especially like the markup tooling. Snagit is great if you need to create a lot of screenshots and if that's the case, it's worth the money. – James Brickley Jun 13 at 17:14
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    @Tetsujin Of course Photoshop natively supports blurring. There is even a whole category of filters, one called "Blur" and one called "Pixelize". – MechMK1 Jun 14 at 11:57
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Gimp has an option to do this - https://docs.gimp.org/2.6/en/plug-in-pixelize.html - though I've not tested it myself.
There's a more Mac-like port of Gimp, fully maintained, if you don't like the usual Windowsy look - https://www.partha.com/

Personally, I use Photoshop - it has a Mosaic function which can be applied to any selection.

enter image description here

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    It works fine in OP's case, but note that it shouldn't be used for sensitive information (e.g. passwords). It's possible to brute-force every character, blur/mosaic the image and compare with your result. For passwords, a completely black rectangle should be used instead. – Eric Duminil Jun 14 at 20:39
  • @EricDuminil that's pretty incredible, I had no idea, but it makes sense. How accurate is the brute forcing? Any recommended links for some beginner reading on the topic? – stevec Jun 15 at 11:06
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    @stevec: I'll try to find the article I read many years ago. I remember that depending on the blur, it worked fine even for long passwords : since one letter on the left doesn't influence the blur on the right part, it's possible to detect letters alone or by pairs. It makes it much easier to find the password, without having to bruteforce every combination. Edit : Found something similar: github.com/beurtschipper/Depix linkedin.com/pulse/… – Eric Duminil Jun 15 at 11:09
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    @stevec Also : lifehacker.com/… – Eric Duminil Jun 15 at 11:12
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    This is more a topic for Information Security, where it's already been covered- security.stackexchange.com/questions/129683/… – Tetsujin Jun 15 at 11:19
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Open your image, shrink the window until the part of the image is unrecognizable, and take a screenshot of the part of the image. Then paste this screenshot back into preview and resize it over the section you wish to be pixelated and save this as a new image.

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