I have submitted a patch which (slightly) improves the quality of the print
plugin (see Bug #387604). The problem basically was that Postscript can't
cope with transparency. In contrast, PDF is able to do so.
The current implementation/patch now has a disadvantage: if you print to a
postscript target, the image has to be exported TWICE: once for the 'print
preview widget' (with alpha) and once for the postscript target (without
alpha). This is certainly not optimal regarding memory consumption.
So the question is, whether this is a real problem or not.
If yes, a solution could be to not distinguish between a Postscript and a
PDF target (i.e. to embed only opaque images into a PDF despite the fact
that PDF can handle images with alpha values). In this case, we only have to
export the image at most once.
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