Quoting Ofnuts <ofn...@laposte.net>:

> 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?

There is no need to create (and later delete) the 'outer' channel.  
Just use the selection itself. In Script-fu, this would be done as  
follows:

   (gimp-selection-load selection)
   (gimp-selection-grow image dist)
   (set! inner (car (gimp-selection-save image)))
   (gimp-selection-load selection)
   (gimp-selection-grow image (+ dist 1))
   (gimp-channel-combine-masks (car (gimp-image-get-selection image))
                               inner
                               CHANNEL-OP-SUBTRACT 0 0)
   (gimp-image-remove-channel image inner)

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