How to smoothen alpha borders?

[Simone Avogadro]

I've an image which has "hard" alpha-transparency (=pixels are either fully transparent or not transparent at all).

I'd like to smoothen the transition so that the "last" pixels on the frontiers become progressively more transparent, so that once overlapped with the background it will blend more softly.

 

let me represent this on one dimension:

 

Currently I've

Image: 000000DDDDDDD   (0=empty, D=data)
Alpha: 0000009999999   (0= fully transparent ...  9=not transparent at all)

 

Id' want to get to:

Image: 000000DDDDDDD   (0=empty, D=data)
Alpha: 0000001369999   (0= fully transparent ...  9=not transparent at all)

 

I guess there might be a plugin which does this but I couldn't find it.

I found plugins which allow feathering the selection but:

• this does effect the content of the image too, not just the alpha channel
• this does "expand" the selection, whereas here I need to create the softening in the inside (because there's no data pixel outside the image)

[frio]

AA's Assistant from dpy's plugin pack does preserve color data as far as I can tell.

Made an opaque red circle (255, 0, 0, 255) on transparent green background (0, 255, 0, 0) and after AA's assistant the opaque circle's edges have contracted and no green has bled into the red or vice versa.

 

There's also Alpha Blur from Tanel's pack that doesn't touch the other channels, only alpha, but it's a little more aggressive. Might contract the object too much in some cases without additional adjustment.