Gimp Jagged Borders


Question

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.

enter image description here

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.

1
5
4/5/2016 7:21:00 AM

Accepted Answer

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...

Change to RGB

You'll then need to tick the Feather option in the Select Border dialog box.

Update...

Once you have an RGB format document you can turn anti-aliasing on or off for the selected area:

Toolbox showing anti-aliasing setting for selection tool

It's also possible, from here, to create your oval using guides and remove the centre of your selection.

Update 2...

I've played a bit more with this, and it is possible to create a nicely antialiased circle or oval by following these steps...

  1. Create a new layer.
  2. Add guides to the shape to reflect the width and height of the outside and inside of the shape...

Move guides into position

  1. Select the oval selection tool and select the outside of the shape...

Select the outside

  1. Deselect the active selection by clicking outside of the selected area somewhere.
  2. Change the selection mode of the oval selection tool to *Subtract from current selection...

Subtract mode

  1. Select the inside of your oval...

Select inside

  1. Pick the brush or fill tool and paint the inside of the selection...

Fill in the selection

  1. Take a look at your newly anti-aliased shape...

Anti-aliased

1
4/7/2016 8:15:00 AM

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.

  1. Upscale your source to e.g. 400%
  2. Make the selection, then create the selection border
  3. Downscale the image to original size (e.g. 25%)
  4. Then fill the selection.

enter image description here
Left: border selection at 100% size Right: selection at 400% size

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.


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