How to crop a rectangle with rounded corners.

[Nick]

Hi folks, I need to add an image to my current project, but I need the image to have rounded corners.
What's the best way to achieve this?

[Ofnuts]

There are only rectangles... but some rectangles can have transparent pixels in the corners. If you open the image in Gimp, you can use the rectangle selection (with its Rounder corners and companion Radius option) in two ways for this:

1. Make a selection, copy, switch to the target image, paste
   
2. Make a selection, invert the selection  (Select ➤ Invert) and [delete] then drag to your target image.
   

[PixLab]

One of the way I do it is with the Rectangle Select tool   , then in the option tool I tick the "Rounded corners" a slider appears below, play with it
then menu > Select / Invert > hit that delete key or add a mask (I prefer the mask)
   

Also you can do Ctrl+A, then Ctrl+C then go in the menu Select / Rounded rectangle... a window opens input your percentage, then like above > Select/Invert > Mask or delete

[rich2005]

There are a few ways: Relatively simple is (more-or-less the same as previous but no need to invert selection)

1) Select -> Select All
2) Use the Rectangular Select tool and click in the canvas to activate.
3) Enable Rounded corners in the tool options and adjust the slider
4) Edit -> Copy
5) Into your project file and Edit -> Paste.
6) Move into position and then Layer -> To New Layer

30 second example. https://i.imgur.com/XYqSArb.mp4

If you want to keep a copy of the image with rounded corners to use later then:
Edit -> Paste as -> New Imge and Export as a .png to keep the transparency.
Add to your project with File -> Open as Layers.