[Scribus] transparency question

[avox]

jrm wrote:
> 
> Hi,
> 
> This regards using png images with portions that are
> 100% transparent.  What seems to happen is that this
> kind of transparency is respected even when exporting
> to PDF 1.3 (as seen in acroread and kpdf), which is not
> what I was expecting.  So I'm wondering if this is
> too good to be true, that I may be able to use this
> kind of transparency without restriction on what PDF
> version to use, maybe even PDF/X-3...  The question is, 
> can I rely on this behaviour?
> 
> What I guess was expecting was that since "PDF 1.3 does
> not allow transparency" the transparent part of the png
> would be made white, or maybe black.
> 

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.

/Andreas

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