[Scribus] transparency question

[jrm at kw.igs.net]

avox wrote:
> AFAIK PDF has supported hard masks for images from the beginning
> (that's also possible in PostScript using the imagemask 
> operator).
> What is only possible since 1.4 is softmasks (alpha channel),
>  since those require blending imagepixels with background pixels.
> 
> I don't know if the behaviour for images with an alpha 
> channel which contains only 0% and 100% values is defined, 
> better not rely on that. Using an indexed palette with a 
> special color for 'transparent' should work.

Thanks, Andreas, now I'm hoping for transparent light at the
end of the tunnel.  What I found with 1.3.3.9 is that my png
with a hard mask seemed to be respected even exporting to
PDF 1.3 ... which from my naive perspective looks like good
behaviour by Scribus.  For the time being, I'll export to 
PDF 1.4 to be on the safe side, but I'd like to avoid
painting myself into a corner by creating obstacles to moving 
to PDF/X-3.

When I open my png in GIMP, and open the palette editor, the 
way to create a transparent colour is not clear (if you'll
excuse a horrible pun) so I'm wondering whether it might
be more appropriate to use Inkscape, or something else, and
then avoid opening the image in GIMP?  Where might I (and
others) find an explanation of how to get transparent colours
into a palette?

Thanks!
John (MacPhail)