GIMP Version: 2.10.12
Operating System: Windows
OS Version: win 10
GIMP Experience: New User

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Hi

I’m kind of very new to gimp and photo editing, but I’ll try to be clear.

I use:
Gimp 2.10.12
Win 10

I have an image, which I want crop and feather on a white background. I get unwanted white lines in the picture.

Steps I do:
1. Open image in gimp
2. Make a white layer as background
3. Add alpha to picture layer
4. Corp the picture (invert selection) and add feather by hitting delete x 5.
Original picture is 1024x1024
Feather is set to 100px

I then get this strange white line effect. (See picture)

I don't quite understand all your steps - especially step 4.

Could you attach the original 1024x1024 image - before any editing.

The banding of colors you get, happens probably because your image is not smooth enough.
Wasn't hard to find the image you tried with, so I used the same.

What I did first was G'mic anisotropic smooth. Some other smooth filter would also work, but don't use blur, because that would blur the clouds on the bottom.
Then circle selection, feather 100px, invert selection, delete. Add white background.



Original attached if you want to take look, circle path was used for the round selection.

Wow, that was fast, and some very clear answers. Thanks very much for you guys time.

The picture you made is what i am trying to do. I opened your file nidhogg, so i could see and do it for my self, but i can not find G'mic anisotropic smooth in gimp.
I also reproduced the problem in my original post in a very confussing way, so that might be it. Sry, i'll try again.
The picture clear sky is just a random picture i grabbed to practise things.

What I am trying to do is to take a photo, put a layer of white over it and poke holes in it with feathered edges. (basicly) And the "white lines" apear in my white layer.
I have made an .xcf file and attached it.

This is known as "banding". When you have almost uniform areas, round-off errors during transforms translate into sharp transitions that are rather visible (in 8-bit/channel, you have only 256 values to work with). You can try to use 32-bit precision to mitigate this.

Okay, so i tried to make a "Test to show v2". xcf, but choseing 32-bit integer instead of the 8-bit integer, and then following the same steps as before.
Then opening them side by side to compare on my monitors. It seems like it has fixed / migated the issue alot, and given a larger file size, wich is logical. Thx
"Banding", now i can sleep tonight (joke). It feels nice to find out why you had problems, and how to fix them.

I would still like to be able to reproduce what you did nidhogg. You worked in a 8-bit channel, and used G'mic anisotropic smooth filter.

**Tried to attach the .xcf file, but it was over 6 mb.

Edit KL. 21:30 2019.09.11:
Made a "Test to show" 8, 16 and 32 bit. Put them up side by side on my monitors, and the 8 bit very bad banding, the 16 and 32 bit looked ok.
So i think i am gonna make working in 16 bit my standard. Maybe even 32 bit, but the file size do go up quite a bit.

Internally Gimp does all its processing in 32-bit floating point now, so if you use that as your format it could be significantly faster since it would avoid conversions.

Hi

I'll give 32-bit floating a go, nice size on those files.

As there is no upvote system. I would like to say thx to ofnuts and Nidhogg for they're time in giveing med advice and help.

Nidhogg i was able to do your thing.
Finally got around to checking out G'mic.
Now I just need to be better at it all.

Glad to hear lt03.
Yes, G'mic is must-have. You will find dozens of great effects of the 500+ it currently includes, and you can save custom presets.
Here is how I normally would make feathered selection without needing to add alpha.
1. Make the selection.
2. Select: Feather 100px.
3. Edit: Copy
4. Edit: Paste as...new layer in place.
5. Add the white layer.