[scribus] Colour management (mis?)understood [was "No real CMYK black possible??", "mixing rgb and cmyk"]

[Hedley Finger]

All:

Clearly, I am not the only one who has trouble with colour management in 
Scribus.  In fact, I found it so baffling that I researched it on the 
Internet (thanks a million, Wikipedia) and in the on-line help for Adobe 
InDesign, Photoshop, Illustrator, and wrote myself an account to try to 
understand it.

Here is what I understand but it would be great if MrDocs or other 
people who work in graphic arts or printing could chime in to correct my 
understanding.

INPUT

Digital colour images come from digital cameras and 
reflection/transmission scanners.  A RAW image is a bunch of numbers 
representing colours absent a colour profile.  Otherwise, the image is a 
bunch of numbers plus an associated colour space profile that describes 
the extent of colours that the device can record and the meaning of the 
numbers within that colour space.

Adobe RGB (1998) is a popular colour space with a white point at D65 
(colour temperature of  6500K) used by digital cameras.  It is used as 
an absolute colour space although it is in theory device-dependent, 
except few real-world devices have its exact colour gamut.  Most 
scanners have their own colour space.

MANIPULATION, PROCESSING

When a continuous tone colour image is opened by a pixmap application 
(Gimp, Photoshop), if it has an associated colour profile, that profile 
is honoured and preserved.  For example, my scanner has  the Epson 
Stylus CX6900f profile (it's a multifunction printer that has a separate 
profile for CMYK inkjet printing).

Every colour managed graphics app has a default working space, often 
Adobe RGB (1998).  If a JPEG or other pixmap image does not have an 
associated profile, the working space colour profile is associated with 
it for purposes of manipulation.

A colour profile can be assigned to or associated with an image if it 
does not have one.  Thereafter, all apps will honour that colour profile.

All pixmaps with associated profiles retain their original numbers 
mediated through their profile.  Manipulating these graphics will change 
their numbers (because you are changing the colours), mediated through 
their profiles.

If a pixmap is converted from one profile to another, the numbers are 
actually changed and remapped to new numbers within the target profile.  
This is a one-way, irreversible operation that cannot be undone (note 
redundancy in previous statement for emphasis effects).

OUTPUT

This can be to the screen of your monitor, to your inkjet printer for 
draft proofing (i.e. just checking stuff is in the right place, layout, 
etc.) or to a press for printing.  If your inkjet printer is high-end 
(e.g. uses Pantone Hexachrome CMOYGK or six-colour CcMmYK) it can be 
soft proofing, i.e. you specify a press colour profile and the inkjet 
printer lays down a tint to represent the colour of the paper stock, and 
adjusts the black point and white points to be desaturated and generally 
prints the colours within the *press's* ink/paper colour profile -- that 
is, as a subset of the inkjet printer's own colour profile.

So when you are working on a colour image (possibly with an Ekta Space 
profile) in Gimp (with a working Adobe RGB (1998) profile, not applied 
to this graphic), you are viewing an image on your monitor (with an sRGB 
profile), and printing to your inkjet (with an  Epson IJ Printer 06 
profile), which is eventually printed on a web offset press (with  an  
US Web Coated (SWOP) v2 profile).

The point being that the numbers in the original image are unchanged and 
all the profiles (and associated rendering intents -- see below)  get 
the best possible result for a particular output.  The other point is 
that you do not have to prematurely convert your input graphics to the 
output press's profile.  This loses a lot of colour information and 
anyway, in a colour managed workflow, the colours will be converted 
during the platemaking stage.  Yet another point is that an RGB 
working/associated colour space usually has a larger gamut than a CMYK 
space (remember both are device dependent).

So moving everything to a CMYK space may leave you left footed when the 
Amazing 11 Colour Digital Ink is invented and your graphics cannot take 
advantage of its amazingly big colour space, but your RGB images will!

Yet another point is that if you have spot colour printed using CMYK 
rather than a specially mixed ink -- perhaps a corporate livery colour 
or a colour with an existing meaning such as red for /danger/ -- you 
need to take the press output profile into account.  In this case you 
need to set your target profile and use the colour picker to set an 
in-gamut colour that will not be distorted by a rendering intent (see 
following).

The Ekta Space profile has a gamut that is very much bigger than the far 
smaller US Web Coated (SWOP) v2 profile.  The output profile's 
*rendering intent* determines how a large gamut is crammed into a small 
one.

Perceptual intent desaturates all colours towards the output white point 
until the most saturated fits within the output gamut.  The downside is 
that some input colours which are already within the output gamut get 
desaturated anyway, so that the result looks duller than the original.  
The upside is that all colours have the same tonal relationships, so a 
colour that is brighter than another in the input will still be brighter 
in the output, although *both* will be desaturated from the original.

Relative intent is good for images where a large number of input colours 
already lie within the output gamut and only a few saturated input 
colours are outside the output gamut.  In this case, in-gamut colours 
are preserved and out-of-gamut colours are desaturated until they lie on 
the output gamut's boundary.  The downside is that if there are several 
out-of-gamut colours that are tones of the same hue, they will all be 
clipped to the same colour, giving rise to banding, e.g. very noticeable 
in intense blue skies.  The upside is that colours are more vibrant than 
perceptual intent if there are few or no saturated colours.  Think 
London on an overcast winter's day in a park with no flowers.

QUESTIONS

Here is where I hope the graphics arts heavy hitters will chime in!

@    What is Scribus's default working colour space?  Adobe RGB (1998)?  
Can you change this default working space a la Photoshop?

@    If an image file already has an attached/associated/assigned colour 
profile, say Adobe RGB (1998), is this honoured by Scribus or is the 
working space profile applied regardless?

@    What is the meaning of the settings in the Colour Management tab of 
the Preferences dialogue?

         System Profiles

         --  RGB  Images      Is this the default working space for RGB 
images with no profile?  Or are ALL RGB images assumed to be within this 
profile, whether assigned or unassigned?

         -- CMYK Images    Likewise is the default working space for 
CMYK images?  Is it applied regardless of any assigned profile?

         --  Solid Colours    This sounds like a default working space; 
so you probably should select a CMYK press profile to catch out-of-gamut 
colours.

         -- Monitor            Probably should be the manufacturer's 
profile, e.g. BenQ E2200W DVI.  Or sRGB for uncalibrated monitors?

         -- Printer        (Better labelled as "Press", otherwise 
confused with inkjet.)   Is this used for creating the PDF/X1a output?  
Is it used for softproofing on the inkjet?

         Rendering Intents

         (Straightforward.)

@    When the output PDF is created (PDF/X1a), are all profiles attached 
to graphics and solid colours honoured and the entire PDF is output to 
the chosen press profile with the appropriate rendering intent?

Regards,
Hedley (Confused in Camberwell)

--

Hedley Finger

28 Regent Street   Camberwell VIC 3124   Australia
Tel. +61 3 9809 1229   Fax. (call phone first)
Mob. (cell) +61 412 461 558
Email. "Hedley Finger" <hfinger at handholding.com.au>