With Script-Fu, loading and saving a PNG loses alpha

Question

I'm trying to write a script in GIMP that will load a PNG file and save it again with maximum compression (I also plan on adding other processing steps). The following script, however, seems to destroy alpha information:

(define (process-png pattern)
  (let* (
      (filelist (cadr (file-glob pattern 1)))
    )
    (while (not (null? filelist))
      (begin
        (catch ()
          (let* (
              (filename (car filelist))
              (image (car (file-png-load RUN-NONINTERACTIVE filename filename)))
            )
            (begin
              (file-png-save2 RUN-NONINTERACTIVE
                  image (car (gimp-image-get-active-drawable image))
                  filename filename
                  0 9 0 0 0 0 0 0 0)
              (gimp-image-delete image)
            )
          )
        )
        (set! filelist (cdr filelist))
      )
    )
  )
)

For instance, the translucent pixels in JQuery icons all seem to become completely transparent, making everything aliased.

How can this be fixed?

Answer 1

The documentation of file-png-save2 says that the last parameter is for Preserve color of transparent pixels? which in your case is set to 0. Try to set this to 1 and it should just work fine.