06-01-2022, 08:34 PM
If it is a smaller layer over a larger layer I would introduce a new layer in between and size it to make the border.
Gimp is a raster editor and works in pixels so 3 mm can be exact or not depending on the pixels-per.xxx resolution. However the units can be changed during use if that is easier for you ....
1. Duplicate the small layer
2. Fill it with the border colour
3. Scale the layer. Layer -> Scale Layer set the units to millimetres, increase the width and height + 6 mm (3 mm all round)
example: https://i.imgur.com/xhcSN4q.mp4
Gimp is a raster editor and works in pixels so 3 mm can be exact or not depending on the pixels-per.xxx resolution. However the units can be changed during use if that is easier for you ....
1. Duplicate the small layer
2. Fill it with the border colour
3. Scale the layer. Layer -> Scale Layer set the units to millimetres, increase the width and height + 6 mm (3 mm all round)
example: https://i.imgur.com/xhcSN4q.mp4