GIMP Version: 2.8.14
Operating System: Windows
GIMP Experience: New User

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Something that has always bothered me with gimp is how when i select something , then use the grow or shrink selection, what is resulting is an horribly aliased selection, by example :

I have an antialiased colored thing, i use the "Select by color" tool, i make sure "Antialiased" is enabled in the tool.

Then i Select -> Grow for some pixels.

While in other image editor i have used , the result would still be antialiased, in Gimp the resulting selection has completely lost the antialiasing of the original selection, in case i explain badly i fill the result with a blue color so you can see how very bad it is :


Gimp has a supposed Filter-> Enhance -> Antialias but it simply does nothing in this case.

That's just a pain when you have more difficult cases to deal with than just the example i posted, so there must be a way i am unaware about. I know photoshop does not have that problem as if your original selection is antialiased, your expansion of such selection keep the antialiasing, but in Gimp not at all from what i see.

The only workaround i found was to fill the resulting selection with a color, then reselect that color with the color tool with feather edge selected while being set to a very low value, then refill that color with the same color a dozen of time, and the final result looks antialiased, but very very unprecise.

Basically it's just a pain.
So how do you avoid this very ugly problem when you just want/need to grow/shrink a selection ?

Was looking on google for a while, and typical as soon as i post because i can't find it myself ... i just find it on google :
http://gimpforums.com/thread-a-method-f ... on-borders

Works greatly, i wish there was an antialias option for the grow/shrink to cut on the workaround

Another way is to anti-alias the original selection. Select>To Path followed by Select>From path seems to do the job.

Do you still have the xcf? I would have tried to gaussian blur the different colours

the gaussian blur trick is a part of the method i linked to, but gaussian blur alone will not fix the whole problem as it will still display a noticable blur in many case instead of a seemingly nicely sharp transition, you'll need to "sharpen" the blured color after that, as mentionned in the tutorial linked by using a brightness/contrast trick.

I didn't tried the path method yet, as the linked tutorial has worked perfectly for my needs , but i will have to test if the path method can cut on the time spent in this workaround.

Thanks for the replies.

You could probably also feather the selection by a few pixels

I had tried the feathering, it unfortunately isn't doing a satisfying result, very far from what you would want to have with antialiasing, so far the technique from the linked tutorial is the closest to having a real antialiased grow/shrink, it's just more steps.

I just had to try this for myself.
Yet another reason to love paths.

Got some time to test the path method, and it works splendidly, and it's less step than the motion blur+contrastbrigthness+colortoalpha trick.

Only oddity is that before doing a Select -> Selection to Path after enabling the path tool, i need to click once on the canvas to make the 1st path control point to appear, if i do not do that Select -> Selection to Path does nothing.

But anyways, it's faster that the other method and works as well.
Thanks

You don't need (or want) to "enable the path tool" before doing Select > To path

I just select the right click the created path and select Path To Selection (I think it does the same).