Calculating a new selection from the current selection would be faster 
(I think):

def create_border(image, selection_type='rectangle', size=1):
     is_selection, x1, y1, x2, y2 = pdb.gimp_selection_bounds(image)

     if is_selection:
         if size < 1:
             size = 1
         if selection_type =='rectangle':
             pdb.gimp_rect_select(image, x1+size, y1+size, 
(x2-x1)-(size*2), (y2-y1)-(size*2), CHANNEL_OP_SUBTRACT, False, 0)
         elif selection_type == 'ellipse':
             pdb.gimp_ellipse_select(image, x1+size, y1+size, 
(x2-x1)-(size*2), (y2-y1)-(size*2), CHANNEL_OP_SUBTRACT, True, False, 0)
         gimp.message("Please make a selection...")

def test_plugin(image, drawable):

     create_border(image)    # A rectangular selecion with default 
     #create_border(image, selection_type='ellipse', size=3)     # Or 
Ellipse with custom settings...

     # Fill
     pdb.gimp_edit_fill(drawable, 0)


On 09/24/2010 10:19 AM, Ofnuts wrote:
>    Hi,
> My code needs to do a one-pixel-wide selection, at distance "x" from the
> current selection.  This looks a lot like a border selection except that
> the border selection creates at best a two-pixel wide ribbon and I only
> want one (but if I'm wrong, please tell me how to :-)
> So far my code goes like this:
> # Selects pixels that are between x and x+1 pixels from
> # the original selection. Bumping the selection by one
> # each time doesn't work, a small circle degenerates into
> # a square with rounded corners instead of a big circle.
>       def select_ribbon(self,image,selection,dist):
>           pdb.gimp_selection_load(selection)
>           pdb.gimp_selection_grow(image,dist+1)
>           outer=pdb.gimp_selection_save(image)
>           pdb.gimp_selection_load(selection)
>           pdb.gimp_selection_grow(image,dist)
>           inner=pdb.gimp_selection_save(image)
>           pdb.gimp_channel_combine_masks(outer,inner,CHANNEL_OP_SUBTRACT,0,0)
>           pdb.gimp_selection_load(outer)
>           image.remove_channel(outer)
>           image.remove_channel(inner)
> That works, but can be slow (especially since it's at the core of a
> loop). Is there any better way? Or useless code to jettison?
> Next improvement is to create a 3-pixels selection and feather it one
> pixel. Anything to be wary of?
> --
> Ofnuts
> _______________________________________________
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> Gimp-developer@lists.XCF.Berkeley.EDU
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