Gaussian Blur using only the selected area
I've used the GIMP for several years and I'm trying to repeat a
technique I'm sure I used to be able to do. Often in a photo, I want to
select an area and gaussian blur it to reduce noise (for example select
the sky and clean up the CCD noise).
Current versions of the GIMP (I'm using 2.2.11 from the Ubuntu 6.06
AMD64 repository) perform the gaussian blur on only the selected pixels
but they pick up colour values from outside the selected area, causing
bleed into the selection. I would like the gaussian blur to only use
colours from inside the selection.
I do not want to use a more "automatic" technique like Selective
Gaussian Blur - that is not the solution I need.
Just so there is no confusion, here's a quick way to see exactly what I
mean.
Create a new image - 128x128 will be fine.
Fill the background with Black (if it isn't already).
Switch to Pencil (press N) and put a white blob on top of the black
background.
Use the "Select Contiguous Pixels" (press Z) to select the white blob.
Choose the Filters->Blur->Gaussian Blur plugin.
Using a 15 pixel horizontal and vertical blur, blur the image.
You will see that the black background bleeds into the white blob inside
the selection. What I would like to happen is that no visible change
should occur in this example - the white selected blob is already a
constant shade and should not show any change under a gaussian blur.
I've tried tricks like cutting out the selected area into a new layer
and performing the blur on the new layer. That is no better. My
searches seem to suggest that Photoshop can limit the blurring to the
selected area's colours if the layer is locked (which is also pretty
obscure).
The worst part is I'm sure that the gimp blur routines used to only take
colour values from the selected area. Maybe I'm just going nuts...
Thanks,
Toby Haynes
