[Scribus] colours in pdf

[Craig Ringer]

Terence Chan wrote:
> Thanks to all those who replied to my previous post about colour
> management. I've just about got the hang of it now, apart from some
> fine tuning. My one remaining concern is that once I've exported
> to a pdf file, the colours look nothing like the scribus original in the
> Acrobat reader acoread.

How does it compare to the print preview in Scribus? How about the
on-page rendering in Scribus when the colour management preview is enabled?

> Indeed even B&W images for which colour
> management isn't an issue

Isn't it?

Pure B&W perhaps, but do you mean B&W or greyscale? AFAIK colour
management is an issue for greyscale images, as dot gain can change
their reproduction significantly. I don't think ink weight limits are a
factor in 1-channel images, so it'd pretty much be down to compensating
for dot gain. Come to think of it, it's probably an issue for pure B&W
images as well, but I'm not sure since I *never* deal with them.

So ... exactly what kind of image are you speaking of? What colour
format is it in? RGB? Greyscale? 1-bit B&W? This can matter quite a lot.

> appear quite differently in acroread. Is
> this just a problem with the way acroread displays images, or do I have
> to apply the CMYK profiles if I want to provide a pdf file to a
> professional printer?

Well, you haven't said (in this message at least) how you're exporting
currently. Please give details such as:

	- Scribus version
	- Platform, OS and version (eg "Fedora Core 6 on an Athlon 64")
	- PDF viewer and version (eg "Adobe Reader 7 on Linux")
	- Whether colour management is enabled in Scribus and if so
	  which profiles you are using.

and once you've provided that information we'll see about giving you
some useful answers.

If I *assume* that you are currently _not_ using colour management in
Scribus (given your question about whether you need to use a profile),
then I would expect you to see colour issues in your exported document.
Unprofiled RGB->CMYK conversions are iffy at best, and terrible if you
have images that are outside the CMYK gamut. A colour profile will not
magically make it all work (especially with out-of-gamut issues) but
will definitely help get a better conversion.

Remember that RGB colour != CMYK colour. Sometimes shifts are inevitable
simply because there are colours that can be represented in RGB but not
CMYK. Also, because of ink weight limits on the press, compensation for
dot gain, etc your images may be intentionally adjusted to get a better
print reproduction even though it won't look as similar on screen.

To say whether this is what is happening, or if something is going
wrong, we'll need details like I asked for above. Samples of your PDF
output and your original images would also be extremely helpful in
eliminating the guesswork from any answers someone on the list might
give you. We're not psychic you know ;-)

--
Craig Ringer

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