LaCie 324 Installation guide

Color Management White Paper 4
The Basics on ICC Color Management Systems

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scanners & digital camera
• Display (mntr) profiles – for monitors
• Output (prtr) profiles – for printers & video
recorders
The second category gathers profiles designed for
special purposes:
• Device Link (link) profiles used for specific direct
connection of devices
• Color space conversion (spac) profiles used for
conversion between color spaces
• Named color (nmcl) profiles used for specific spot
colors such as Pantone
• Abstract (abst) profiles for abstract spaces
All device profiles (except scanner profiles, which
only support conversion from device to PCS) are
bidirectional: they gather information to convert from
device to PCS and vice versa.
Each profile contains multiple tables referred to as
“tags”. AtoB tags translate from the device space to
the PCS; BtoA tags translate from the PCS (L*a*b* or
XYZ) to the device space. There is a pair of AtoB and
BtoA tags for each rendering intent. These are used
by the CMM to set up correspondence between any
compatible ICC profiles. As a complete RGB table
has more than 16 million rows, a profile containing
them all would be too large (several megabytes).
Therefore, the CMM performs an interpolation of the
table.
Device profile information can be gathered in two ways:
• Using algorithms based on matrices and
linearization curves called Matrix Profiles
• Using lookup tables called LUT Profiles
(or Table Profiles)
ICC profiles, apart from abstract (abst) and device
link profiles (link), can be embedded into images
and saved in most image file formats (EPS, TIFF, GIF,
etc…) to be easily used in the color management
workflow.
4. Rendering Intent refers to the way the CMM
(Color Management Module) handles out-of-gamut
colors during a conversion from one color space to
another.
The ICC specification defines four different rendering
intents: Perceptual, Relative Colorimetric, Saturation
and Absolute Colorimetric.
Because each device can reproduce a certain range
of colors, a specific gamut is described in its ICC
profile. When printing a document displayed on a
monitor, for instance, the gamut of the original file
(source) has to fit to the color space of the output
device or printer (destination). The rendering intent
determines how the CMM will translate colors from
one device to another. It is based on the concept of
Gamut Mapping.
There are two main techniques for mapping out of
gamut colors:
• Gamut compression compresses the range of colors
that are out-of-gamut into the destination gamut
• Gamut clipping maps all out-of-gamut source
colors to the closest colors within the destination
COLOR MANAGEMENT SYSTEM (CMS)
The International Color Consortium (ICC) defines an
open standard for a Color Matching Module (CMM)
at the operating system level and color profiles (ICC
profiles) for devices and working spaces. Operating
systems provide a built-in framework—Apple
ColorSync on Mac or ICM (Image Color Management)
on Windows—for implementing and handling these
device profiles. Color measurement instruments are
used with CMS and CMS-supported software to
gather important data included in device profiles and
to periodically monitor and adjust device performance.
The Color Management System as defined by the
ICC is based on four main elements:
1. A Color Matching Module
2. A Profile Connection Space
3. Color Profi
les
4. A Rendering Intent
1. A Color Matching Module (CMM) is a software
engine embedded in a graphic application’s
software, operating system, and hardware driver. The
CMM addresses tables within profiles—describing
how conversion should occur.Depending on the OS
and software used, several CMMs are available.
Adobe, Apple, Kodak, Heidelberg and other
manufacturers provide their own CMMs. This is
due to the fact that ICC is an open standard, which
explains why the same transformation can produce
different results if carried out by two different CMMs.
2. A Profile Connection Space (PCS) is the device-
independent standard reference space into or out of
which color data is transformed. It is either L*a*b*
or CIE XYZ, and is the universal translator—allowing
color space conversion to be carried out from source
to destination color spaces. AtoB and BtoA tags
(tables) contained in the profiles are used in a PCS to
translate source into destination.
3. Color Profiles (ICC Profiles) are small digital files
containing a description of how a particular device
reproduces color. They also describe the device’s
color space to the color management system. Profiles
are obtained by performing calibration and profiling
with ICC-compatible tools (such as LaCie blue eye
pro). They also contain other information, such as
the preferred CMM, the preferred rendering intent,
the PCS used and the version. Multiple tables used
for color translation processes are also embedded in
the profiles.
The ICC recommends 7 different types of profiles,
which are classified into 2 categories.
The first category contains the device profiles:
• Input (scnr) profiles – specially designed for
As mentioned in preceding LaCie White Papers, the wide variety of imaging devices available today
makes it very difficult for a document created on one device to be rendered correctly on another.
The use of an efficient Color Management System (CMS) is the most accurate answer to this problem.
Gammut
Compression
Gammut
Clipping
Gamut Mappings methods compared
Two Color Management Policies
Above: Without CMS; Below: With CMS

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Both techniques are used for calculating rendering
intent. The rendering intents below all derive from
them. Each ICC profile is tagged with a default
(Profiles generated with LaCie blue eye pro use
“perceptual rendering intent” by default).
The following four rendering intents are used in
accordance with various imaging requirements:
– Perceptual rendering intent preserves color
relationships by scaling the entire source space
into the destination space, including those colors
that were in the source gamut. It produces the
most pleasing color results and is generally
recommended for continuous-tone images and
photographs—when reproducing exact hues is
not necessary.
– Saturation intent reproduces the relative saturation
of colors from gamut to gamut for the brightest,
most vivid results. It is recommended for businesses
and vector-based graphics.
– Relative Colorimetric changes only the colors
outside the gamut of the destination device. Colors
are scaled relative to the destination profile’s
white point; the whitest white of the source space
is mapped to the whitest white of the destination
space. It often produces a more appealing result
than the perceptual intent because it preserves
colors within gamut.
– Absolute Colorimetric rendering intent matches
in-gamut colors exactly, reproducing the white
point of the source profile on the destination
device, but clipping out-of-gamut colors to the
nearest hue. It’s best for signature colors, such as
LaCie Blue or Coca-Cola Red, and is most useful
for soft-proofing, especially when the proofing
device has a larger gamut than the final output.
APPLICATIONS: HOW DOES THIS PROCESS WORK?
To see how color translation works in practice, suppose
a document created in a computer and displayed on
a monitor in a particular color space (RGB) must be
converted to another (CMYK) in order to be printed.
• The first step is to obtain the two ICC profiles for
these devices (printer and monitor). Each RGB triplet
is first converted to the PCS using the RGB profile.
A color conversion always occurs between two
profiles; the first is the “source” profile and the
second is the “destination” profile. In the source
profile, the table is always read from RGB to L*a*b*;
in the destination profile, the table is read from
L*a*b* to CMYK.
• If necessary, the data is converted in the PCS
between L*a*b* and CIE XYZ.
• The data is then converted in the PCS to the four
C, M, Y, and K values required.
A profile might define several mappings, according
to rendering intent. These mappings allow a
choice between closest possible color matching,
and remapping the entire color range to allow for
different gamut.
The operator will choose in his/her ICC compatible
software, Photoshop for instance, the relevant
rendering intents to achieve the desired translation
when no perfect match can be found. The accuracy
of this “approximation” will depend on the rendering
intent chosen.
The conversion between two profiles is a general
concept, valid for any kind of device. To obtain
the same colors on two monitors for instance, the
numbers will need to be converted from the profile
of the first monitor to that of the second one. There
will be a translation from the RGB coordinates of
the source monitor to L*a*b* then to the L*a*b*
coordinates of the target monitor.
Only the use of an accurate CMS will guarantee a satisfactory color
consistency throughout the workflow. In the next LaCie White Paper, we
will describe in more detail the CMS Profile creation steps.
“Through a combination of cutting-edge technological
engineering and a rich history of unique design aesthetics,
LaCie continues as a firm leader in the color display industry.
Established in the United States, Europe and Japan, LaCie is a
leading worldwide producer of PC and Macintosh compatible
peripherals, including a new generation of color LCD monitors.
By providing top-of-the line tools for multimedia innovation,
LaCie anticipates the needs of creative professionals such as
graphic designers, photographers and filmmakers, who require
genuine, practical solutions for accurate color management.”
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ICC Profiles form the links in your image chain between device-
dependent monitors and the device-independent PCS