I have an interesting case where I have a circular logo which I overlay on a web-page. The logo needs to have a transparent background, however it is placed over many different elements (the background is not a solid color), so I can't use the smei-flatten code to antialias it.

Originally, we had just used flash for the logo, but not all browsers support transparency in flash, so I converted it to a gif (and lost the nice smooth edges)

My solution was to import an image of the webpage (without the logo) as a second layer, and do a 'smart' semi-flatten between the transparent logo layer and the background image). This results in antialiasing against any background (doesn't need to be a solid color) as long as it isn't dynamic.

Since I did this in Windows, and didn't want to compile everything, I wrote it in Script-Fu.

Below is the code if someone finds it useful (Since it is doing pixel-by-pixel checks, it is very slow, and should really be in C, but it works for my needs). Also, while I am familiar with lisp, this is my first attempt at Script-Fu, so the code is pretty cludgy)

.Geoff -------------------------------------------------------- ;; -*-scheme-*-
;; This script will apply semi-flatten against a background layer ;; instead of just a solid color

(define (script-fu-smart-semiflattern image drawable fg bkgnd) (gimp-image-undo-disable image)
(if (or (or (= fg -1) (= bkgnd -1)) (= fg bkgnd)) t (let ()
(set! drawable fg)
(set! width (car (gimp-drawable-width drawable))) (set! height (car (gimp-drawable-height drawable))) (set! y 0)
(while (< y height)
(gimp-progress-update (/ y height)) (set! x 0)
(while (< x width)
(set! pxll (gimp-drawable-get-pixel drawable x y)) (set! pxl (cadr pxll))
(set! alpha (aref pxl 3))
(if (or (= alpha 0) (= alpha -1)) t (let ()
;(print "alpha: ")
;(print x)
;(print y)
;(print alpha)
(set! bgpxl (cadr (gimp-drawable-get-pixel bkgnd x y))) (set! a (aref pxl 3)) (if (< a 0) (set! a (+ 256 a))) (set! r (aref pxl 0)) (if (< r 0) (set! r (+ 256 r))) (set! rb (aref bgpxl 0)) (if (< rb 0) (set! rb (+ 256 rb))) (set! g (aref pxl 1)) (if (< g 0) (set! g (+ 256 g))) (set! gb (aref bgpxl 1)) (if (< gb 0) (set! gb (+ 256 gb))) (set! b (aref pxl 2)) (if (< b 0) (set! b (+ 256 b))) (set! bb (aref bgpxl 2)) (if (< bb 0) (set! bb (+ 256 bb))) (aset pxl 0 (/ (+ (* a r) (* (- 255 a) rb)) 255)) (aset pxl 1 (/ (+ (* a g) (* (- 255 a) gb)) 255)) (aset pxl 2 (/ (+ (* a b) (* (- 255 a) bb)) 255)) (aset pxl 3 255)
(gimp-drawable-set-pixel drawable x y (car pxll) pxl) )
)
(set! x (+ x 1))
)
(set! y (+ y 1))
)
)
)
(gimp-image-undo-enable image)
(gimp-displays-flush)
)

(script-fu-register "script-fu-smart-semiflatten" _"_Smart Semi-Flatten" "Semi flatten against a background image." "Geoffrey Hausheer" "Geoffrey Hausheer, 2005. Public Domain." "January 2005"
""
SF-IMAGE "Image" 0 SF-DRAWABLE "Drawable" 0 SF-LAYER "Foreground Layer" -1 SF-LAYER "Background Layer" -1 )

(script-fu-menu-register "script-fu-smart-semiflatten" "/Script-Fu/Colors")