Get rid of thin pic border after Crop

[GMP]

I cropped a pic and it looks fine except I didn't quite get the very outside border of the pic. So there's a tiny line at the top and the right. I did it right on the left. I tried to fix it with the Crop tool, but it didn't work. I tried the Eraser tool, but I couldn't get it to work, since I've never used it before. What the best way to fix this?

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I tried the Eraser tool again, and this time it worked. Not sure what I was doing wrong, except this time I chose the tool from the icons on the left

[rich2005]

I would not use the eraser tool, it leaves a line of transparent pixels which in certain circumstances shows up.

Since you have already done that, follow up with Image -> Crop to Content or Image -> Zealous Crop

Since I already did this, and FWIW Go back to using the Crop Tool.

Make sure View -> Snap to Guides is on (It should be default unless you turned it off)
Zoom in to top right corner and set a horizontal and a vertical guide.
Use the crop tool with Deleted cropped pixels on, and drag to set the boundary. Zoom it to check that top right corner.
Click on canvas to apply. 40 second example animation https://i.imgur.com/n2tTgQv.mp4

[GMP]

Actually I used the free select tool to crop the pic not the crop tool - sorry my mistake. I wanted to keep the inside of the pic, but when I hit delete, it deleted the inside and not the outside. I think there's a way to reverse it where it deletes the outside of the pic. Can that be done? I ended up selecting the part outside the pic so the selection had what I wanted to remove and that worked.

[rich2005]

Edit: Just realised the issue: Free select around around a foreground shape, the background is white (or some other colour) and cutting the selection leaves a thin perimeter line from the background. This is a very common problem

The usual way is:
Make your selection as close as possible to the foreground shape.
Shrink the selection by 2 or 3 pixels (depends on the size of the image) Select -> Shrink
Invert the selection
Clear (or Cut) the selection. If necessary follow up with the eraser if you were not so good with the initial selection

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If you make a selection free / rectangular / ellipical to swop from inside selection to outside selection use Select -> Invert Then use clear or delete.

A safer way is make your selection then Edit -> Copy followed by Edit -> Paste as -> New Image and that saves the original, just in case.

[zeuspaul]

For removing a tiny border I resize the canvas by a few pixels.  Either by a best guess or by first using the measure tool to determine the number of pixels. If the border isn't on the right side or the bottom I move the image left or right to capture the border. Then using rectangular select I capture the entire image.  I make a broad selection well outside the extent of the image.  Then in the image menu I select crop to selection.