I tried it, and it looks very wrong indeed. Try creating the circle,
then blurring at 48x48, then using treshold. The circle has changed into
a weird shape. I took a look at the gaussian blur plugin but was unable
to positively identify a bug. I do however have an idea as to what might
be happening: gimp only takes pixels into account within a rectangle
enclosing the selection. Pixels inside the rectangle but outside the
selection are taken into account as if they have a colour of 128, 128,
128, pixels outside the rectangle are not counted at all. Therefor, if
you consider a pixel at the top of the circle, the gaussian blur is
calculated by taking into account the white pixels inside the circle,
and a few grey ones that are outside the circle but inside the bounding
rectangle. Compare this to a pixel on the edge at 45 degrees along the
circle, now we take into account the same amount of white pixels from
the inside of the circle (the gaussian filter is circular so it's the
overlapping area is equally large), but many more grey pixels inside the
bounding box but outside the circle, resulting in a darker colour.

A solution if this turns out to be the case would be to enlarge the
bounding rectangle on all sides by the blur radius, or to simply count
pixels outside the bounding box as 128, 128, 128 as well.

I've CC'd this mail to the gimp-developer mailinglist, hopefully the
maintainer of the plugin or one of the Gimp developers can shed a light
on this. I'll send a summary of the replies on gimp-developer to GimpI
to keep everybody informed.


> Andrew J Fortune wrote:
> Hi all,
> Try the following :
> (1) Create a new image with a black background;
> (2) Draw a circle from the center and fill with white;
> (3) Leave the circular selection on;
> (4) Gaussian Blur IIR, using 48x48;
> (5) Repeat blur with 24x24;
> (6) Repeat blur with 12x12.
> The result is that you get a nasty side-effect. Instead of blurring
> evenly (radially), it leaves a bump on the top, bottom, left and right
> extremities of the white circle (see attached).
> What is causing this, and how can it be rectified ?
> regards,
> Andrew
Gimp-developer mailing list

Reply via email to