Thomas Freitag authored Feb 14, 2012 and Albert Astals Cid committed Feb 14, 2012

It is an enhancement patch, a
merge fix and a bug fix in one: an enhancement, because it now completes
the implementation overprint mode and devicen in postscript, a merge
fix, because it fixes some bugs in the overprint implementation in
splash of xpdf 3.0.3 and has now the complete functionality (and more!)
of my implementation back again and a bug fix, because it fixes the use
of splash cmyk in postscript which never had worked.

1. Overprint implementation in postscript
To have a complete overprint implementation in the (pure) postscript
device there were just two things missing: overprint mode and the
implementation of the DeviceN colorspace in PostScript. I double checked
my implementation with the Ghent Test Suite with GhostScript (device
pdfwrite) and Acrobat X distiller, and all the tests now succeeds,
either in Acrobat X distiller or in GhostScript. As overprint is a
device dependent feature, it is up to the output device if it supports
overprint and what features of overprint are supported, and often You
have various configuration possibilities there. Nearly all PostScript
output of the Ghent tests show now the desired results if converting it
back to PDF with ghostscript pdfwrite, the implementation in ghostscript
is complete. On the other hand a few tests failed when using Acrobat X
distiller, all of them with the overprint mode switch. Funny, because
overprint mode 1 is often also called the illustrator overprint mode
because it was introduced by illustrator, but probably the destiller
only handles EPS correctly which comes from illustrator

2. Overprint implementation in postscript if using splash rasterization
Of course the postscript overprint implementation will only work if
pdftops doesn't decide to use splash to rasterize it because of the use
of transparencies in the PDF. But because overprint is device dependent
I decided to spend an additional parameter "overprint" to pdftops
(simular to pdftoppm). Switching it on (only available if compiled with
SPLASH_CMYK, because overprint is only in CMYK colorspace showable) will
use the overprint implementation in splash also if rasterizing the PDF.

3. Overprint implementation in splash
The overprint implementation in splash now uses the better designed
interface of xpdf and therefore now also works in transparency groups.
Thanks to the developper team of xpdf. I just fixed a small bug in it,
where it defies the technical specification. Of course it is still in
the nature of the splash implementation that it fails if CMYK values
should overprint spot colors, because spot colors are converted
immediately in their CMYK alternates. And in the implementation of
overprinting spot colors over CMYK colors I made a small assumption to
get it running which causes undesired results if this assumption fails,
but I didn't want to implement more and more configuration switches. I
still plan to implement a DeviceN output in splash and make a complete
implementation there, but this is of course a bigger task (but doable).
The overprint switch in pdftoppm is still only available if compiled
with the SPLASH_CMYK directive.