[scribus] Two color management questions

[Patrick Noffke]

On Thu, Jan 19, 2012 at 4:03 PM, Hadmut Danisch <hadmut at danisch.de> wrote:

> Am 19.01.2012 22:49, schrieb Patrick Noffke:
> > PDF/X-3 specifically allows the images to be saved in the file as RGB, as
> > long as they have a profile associated with them.  The profile(s) for RGB
> > images define what L*a*b* color you want for each RGB value for that
> image.
> >
> > Then the output intent tells how to convert from L*a*b* to CMYK.
>
> So the rendering intent is stored in the PDF/X-3 file?
>
> I believed the intent is used only locally to convert the given into the
> wanted colormodel (i.e. some RGB to some CMYK)
>
>
>
The output intent (different from the rendering intent) is stored in the
PDF/X-3.  Apparently, PDF/X-1a allows the file to contain an identifier for
a registered output intent  characterization data on color.org.  This can
only work if the file is 100% CMYK, as it must be for PDF/X-1a.  Otherwise,
to convert from RGB -> L*a*b* -> CMYK (as with PDF/X-3), you need the
output intent ICC profile stored in the PDF file.  I'm not sure if the spec
for PDF/X-3 allows you to store the identifier if the file is only CMYK.
 If so, there must be a profile somewhere to go along with that identifier.

The rendering intent for each image is also stored in the PDF/X-3.


>
>
> >
> > Your printer has said Fogra27L should be your output intent.  Did they
> give
> > you an ICC profile?
> No. They just said they use what all printing service providers use, "4C
> Euroskala". On their website they say they're using Fogra27L.
>
>
You can get the Fogra27L characterization data from here:
http://color.org/chardata/fogra27.xalter

But you will need an ICC profile to complete the process.  See below.


>
> > Any printer that accepts PDF/X-3 should convert the RGB images to CMYK
> > (possibly during their RIP) using first the image profiles to convert to
> > L*a*b*, and next the output intent profile to convert to CMYK.  This
> should
> > be supported by their RIP software or some step in their process.  They
> may
> > add a further step that does ink optimization (i.e. GCR), which is
> supposed
> > to leave the color (L*a*b* value) alone.
>
> That would make sense.
>
> But where in that process does the rendering intent (perceptual,...)
> play it's role? Do the image profiles depend on it?
>
>
>
The basic problem is the gamut for RGB (and L*a*b*) color space is much
wider than that for a printer.  The rendering intents allow you to "store"
different ways of doing the gamut compression, such as when going from
L*a*b* to CMYK.

The output intent characterization data (only a subset of what is stored in
an ICC profile) only defines what the printer needs to achieve (the L*a*b*
that they should achieve for any given CMYK value).  This is stored in the
"BToA" direction (CMYK to L*a*b*) in the ICC profile.  The absolute
colorimetric rendering intent tells you what L*a*b* value you should expect
to get if you measured your printed page with an instrument capable of
measuring L*a*b*.  The media-relative (or sometimes just "relative")
colorimetric intent is the same as the absolute intent, but divided by the
paper color in XYZ space.  This intent is how a lot of color tools render
images, and has the result of making paper appear the brightest white that
your monitor can display.  This rendering intent is quite useless in my
opinion, and for some reason is the default of many tools that try to "do"
L*a*b*.  If you want to soft-proof how an image will look when it is
printed, you must use the absolute colorimetric rendering intent (and of
course have a calibrated monitor).  But, I digress.

Because RGB images have such a wide gamut, the absolute/relative rendering
intents shouldn't be used to convert to CMYK.  Otherwise colors outside the
printable gamut will just get clipped to the closest printable color.  The
perceptual intent is typically used, and there is some secret sauce in the
tools that make these profiles as to how they do this gamut compression (so
it has a smooth mapping from RGB space to the printable gamut).

I've heard the saturation intent is mostly supposed to be used for things
like logos, not images from a camera.

To get to your issue, you will need to get a Fogra27L ICC profile.  I think
you can find one at the link below, but don't know how they setup the
profile for the "AToB" (L*a*b* to CMYK) direction.  Aside from the
perceptual and saturation rendering intents, it is also possible to do ink
optimization, as I mentioned earlier.  Perhaps you can find info on whether
that was done.
http://www.fogra.org/en/fogra-standardization/fogra-characterizationdata/a-icc-en.html

Otherwise, does anyone know if there are FLOSS tools for making profiles,
with options to customize the perceptual intent, GCR, and so on?

Pat
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