I'm trying to create a bordered, transparent circle.
I created a circle with the ellipsis tool, then Select->Border, then Edit->Fill with FG Color. The result is a terribly jagged border.
I tried adding a gaussian blur, but it looks terrible, looks like the image is popping up. It just seems like very poor quality, how can I increase the number of pixels being used for the color around the edes?
I mean look at the "so jagged" text, the lines are so smooth for the letters.
Colour depth is the number of bits/bytes used to display a colour. Presently you're using an 8-bit depth, giving you 256 colours (0-255). This is an indexed colour approach, the different values (from 0-255) are paletted, meaning that the each of the 256 values points to a colour representation stored in the header of the graphics document you're working on.
There are a number of other formats which will allow you to produce the required smooth edges. Try changing your image format to 24 or 32-bit depth.
In GIMP you can do this like so...
You'll then need to tick the Feather option in the Select Border dialog box.
Once you have an RGB format document you can turn anti-aliasing on or off for the selected area:
It's also possible, from here, to create your oval using guides and remove the centre of your selection.
I've played a bit more with this, and it is possible to create a nicely antialiased circle or oval by following these steps...
Different to selecting the Gimp Select > Border tool does not allow for anti-aliasing. This leads to hard to overcome aliasing artifacts on working with this selection.
Feathering the selection border does not lead to the desired result.
The only way I know of on how to overcome this is to make the selection on an up-scaled version of our image.
Note that in the example above I did not fill the border prior to down-scaling. I only scaled the selection which then will have appropriate anti-aliasing. The image was in RGB, not indexed mode.