[scribus] Colour management (mis?)understood [was "No real CMYK black possible??", "mixing rgb and cmyk"]
hfinger at handholding.com.au
Wed Jun 25 02:19:31 CEST 2008
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
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
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.
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
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).
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
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
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.
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?
-- 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
-- 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?
@ 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?
Hedley (Confused in Camberwell)
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>
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