I often want to blur a circle either internally (and not externally) or externally (and not internally). In the example below, I have applied Gaussian blur to an image, selecting only the circle so as to preserve the sharpness of its outer edge—that is, so as not to allow the effect to ‘bleed’ outside the circle.
However, whilst the effect is subtle, the Gaussian blur algorithm leaves the image with higher light levels in the axial directions—top and bottom, left and right—as shown. These artefacts become even more obvious when the image is resized (reduced), and to the keener eye are annoying.
How can these artefacts be avoided?