RGB-to-CMYK Conversions
veryone out there who’s
had difficulty with output
when converting files from
RGB to CMYK, raise your
hands. I’ll bet a number
of hands are in the air.
Converting RGB to CMYK
for final output is a critical
step that makes or breaks
the quality of a printing
job, and it’s a demand that
more and more clients are
making of photographers
who shoot with digital
cameras or scan their film.
This is both a benefit
and a curse. The curse is
that it’s the photographer
who decides how the file
conversion will be made and upon
whose shoulders rests the responsibility for the quality of the outcome.
The benefit is that now the photographer has full control over the
32 • PEI • FEBRUARY 2000
color from inception to the laying
of the ink on paper. Wise photographers see this critical step as a
new profit center where they can
increase their fees for producing
images—income that used to be
paid to someone else down the
production line.
Let’s examine some of the options
you have for making high-quality
conversions, and how you can avoid
some of the problems inherent to
the process. The first problem with
CMYK files (and RGB to a lesser
extent) is that CMYK is highly device
dependent. You can send a file
you converted from RGB to
CMYK to a dozen CMYK printers
and see a dozen different results in
the output. CMYK should be
optimized for a specific output—
it’s not always possible, but we can
come close.
The most common application
for RGB-to-CMYK conversions is
Adobe Photoshop. You could
simply open an RGB file, go to
Image>Mode>CMYK, and end up
with a separated file. You could do
that, if having a sound conversion
didn’t matter to you. Assuming the
default settings in Photoshop are
going to produce satisfactory
results is like assuming that setting
the f/stop on a lens somewhere in
the middle will produce an
acceptable exposure. First we have
to inform Photoshop how we wish
to accomplish the conversion.
In today’s desktop workflow,
users want to scan once, then use
the digital image file many times;
therefore, it’s appropriate to archive
an RGB file and simply convert to
CMYK whenever necessary for
each kind of printing requirement.
Photoshop 5.0 has at least two
ways in which we can inform the
application how we want to produce
a conversion, but we must begin by
knowing the condition of the printed
output. This critical information
comes from the printer. It’s important
to know the linescreen at which the
job will print and the condition of
the press or (in most cases) the
proofing system—information not
often readily available. The print
shop might tell you, “We print to
SWOP [Specifications for Web Offset
Printing] standards,” which is hardly
specific enough. This generic information is like saying all E-6 films
are the same because of the processing method. SWOP tells us just a
little bit about the supposed method
of printing. For example, SWOP
specifies line screens of 133 lpi. If
you print 150 lpi, it’s not SWOP. Yet
it’s done and it can be made just as
consistent and predictable as 133 lpi.
PEI • FEBRUARY 2000 • 33
Contract Proofs
Many clients expect to receive a
"MatchPrint" from the printer prior
to the printing of their jobs. (I prefer
to use the generic term “contract
proof” because MatchPrint is a
specific product from Imation/3M
—your printer might use Fuji
ColorArt, Kodak Approval, Kodak
Contract, and so on.) It’s important
to have a conversation with the
people providing your film and
contract proofs. Ideally you will
optimize your conversion from RGB
to CMYK for this very proof. It’s
called a contract proof because it’s
a contractual agreement between
you or your client and the people
printing the job. With print jobs
that involve making the separated
film negatives (black-and-white
negatives for all four colors)
that will be used to burn the
plate for the press, the contract
proof will be made with the
actual film, and both client and
printer will accept the proof as a
representation of the pages that
will come off the press.
Filmless proofing and printing
systems such as Kodak Approval
are also used for direct-to-press
jobs, and the contractual concept is
the same. When clients are satisfied
with the contract proof, they signoff on the proof and the film (or
digital file) and the contract proof
will go to the printer. The pressman
will control the press to the best of
his ability to produce a printed page
that matches the contract proof.
Usually the client comes in for a
“press check” to compare the prints
coming off the press to the contract
proof, under controlled lighting. At
this point, the client should be able
to approve the output from the
34 • PEI • FEBRUARY 2000
or separation setting is loaded, this
press, and the entire job will be run.
interface allows the user to save
As you can see, the contract proof
ICC profiles reflecting the current
is a critical element against which
settings when the user clicks on
you should strive to optimize your
the Save button. The Built-In
conversions. Ask your suppliers
mode is what most users are
which proofing system they will be
familiar with.
using so you can tailor your
Notice in Figure 1 that the top of
conversions to it.
the dialog box allows you to select
Armed with knowledge about
the ink options, which I prefer to
the press conditions—or better yet,
call the “ink model.” This is supposed
the contract proof—it’s time to
to describe the color of the inks you
optimize your conversions for this
are using. It’s set to SWOP Coated
process. Photoshop offers a few
here, but you can achieve a variety
ways to do this, depending on your
budget and your
determination to
produce the best
possible output.
Ideally, you will
make custom
conversions for
your output needs.
But when that’s
not possible, you
can aim for providing CMYK
conversions that
are at least close to
the true conditions
under which
they will be
reproduced.
Photoshop has
three main methods
of setting a conversion: Built-in
(also called the
“Classic Engine”).
ICC, which uses
ICC profiles, and
Tables. Tables is
really just a means
of loading the
older style
Figure 1. The three dialog boxes for CMYK Setup:
separation tables
Classic (called Built-in), ICC, and Tables. The Save
into Photoshop.
button in the Tables dialog box allows the setting
to be saved as an ICC profile.
Also, when a table
menu called
a rectangular selection of your file
“Custom.” The corthat’s large enough to measure. To
responding dialog
get exact dot gain values for each
box seems rather
ink, specify each ink color in the
overwhelming and
following percentages: 2, 4, 6, 8, 10,
frightening, as
20, 30, 40, 50, 60, 70, 80, and 90%.
seen in Figure 2.
You should end up with a total of
However, there is
61 patches to measure (nine for
one way in which
solid colors plus paper white, plus
you could create a
13 dot gain patches for each ink).
custom ink model
After printing the test file,
for use in Photomeasure each solid color and
shop. You have to
the white of the paper stock,
run an actual test
and enter the values in the
for the printer,
appropriate fields. ColorShop 2.6
then measure the
has all the tools to do this easily
output with a
and correctly. In the Dot Gain
Figure 2. The Ink Colors dialog opens when you click on
the Custom menu option in the Built-in CMYK Setup.
spectrophotometer
popup menu, select Curves. Enter
You can enter the values for the various inks once they
or
colorimeter,
the values in each field.
have been measured. Do not pick Estimate Overprints.
evaluating eight
This method works best for
of results when you use different
colors, plus the substrate. You
making a custom ink set for the
conversions called SWOP. You will
will need a hardware device and
press that the job will be printed
also have control over dot gain and
software that will
other ways of controlling the black
produce either CIE
generation of the conversion (the
xyY or CIE Lab
black channel, which will create the
values, such as
black plate). Now’s the time to talk
the X-Rite Digital
to the printer so you can enter the
Swatchbook or
correct settings for these options.
ColorTron II
The various ink models built
and the supplied
into Photoshop are really intended
software, Colorfor press conditions. There are no
Shop, which in
models that mimic a contract
Version 2.6 has a
proof, although if the press and
ready-to-print and
contract proof match, you could
measure target.
certainly pick one of these models.
Lacking this
Picking one of these ink models
target, simply
and perhaps tweaking the settings
output the nine
for dot gain and other settings
colors and make a
(such as total ink limit) will get
file yourself.
you in the ballpark—but keep in
Use Photoshop
mind that these settings are vital
to specify the
to the outcome of the printed
colors in CMYK
piece. Being off just a wee bit with
(for example,
Figure 3. This ColorShop 2.6 target can be output and
dot gain can make or break a job.
100C/0M/0Y/0K
then measured using one of the ColorShop hardware
You will also notice that in the
for pure cyan) and
products to provide the numeric values to be entered in
classic engine, you have a pop-up
load that color into
the Custom Ink dialog box (Figure 2).
36 • PEI • FEBRUARY 2000
ships with a group
of excellent
profiles for such
proofing devices
as 3M MatchPrint,
Fuji ColorArt and
several flavors of
SWOP. In addition,
printers and
service bureaus
may have ICC
profiles on hand
that you can use to
make your converFigure 4. You can save this custom ink model as an ICC
sions. And finally,
profile. For most users, profiling the press isn’t a viable opyou can use a
tion, so use this technique for profiling the contract proof.
spectrophotometer
and software to
on. According to Adobe, however,
make your own ICC profile (or use
the Built-in method works with
the method described above to
offset lithography and toner-based
measure the 61 patches and
devices as well as ink-jets. You can
establish an ICC profile). For a
also use this technique for contract
roundup of software packages that
proofing devices. Best bet: Actually
can make ICC profiles, see PEI
run the patches on the press (if
October 1999.
your printer is willing) on the same
If you have the capability to make
stock with the same inks you
your own profiles, it’s likely you’ll
intend to use. That still leaves total
want to profile the contract proofing
ink limit and other black generation
device. This means sending the
issues to work out, but this
shop a file with a few hundred
technique brings you much closer
color patches that will be read into
than using a standard ink model.
a software package to generate a
profile. When that is accomplished
ICC Profiles
and the profile is loaded in the ICC
Now let’s talk about the real
section of the CMYK Set-Up, all
power of conversions. Photoshop 5.0
you need to do is to make a
allows users to create or use existing
standard mode change and the file
ICC profiles for making mode
will be converted using the profile.
changes. You can make your own
Whatever method you use to
or you can look for existing profiles
create a custom conversion, I
that correctly describe the conditions
recommend sending a test to the
of the printer, press, or contract proof.
same shop with a mix of images,
There are some very good profiles
just to confirm the soundness of the
floating around the Web that were
conversions. Most software
created for contract proofing devices
profiling packages will allow the
(visit www.chromix.com/default.shtml).
conversion to be tweaked with
The Radius/Miro line of displays
respect to total ink limits and other
black generation parameters—
again, it never hurts to contact the
printer. Dot gain problems should
be accounted for in the proofing
system, if the contract proof is
sufficiently dialed into the final
print process. When no information
is provided, your safest move for
coated stock is to keep the total ink
limit between 300 and 340 percent;
a light to medium black generation
usually works well. Remember that
in profiling and converting for
contract proofs, the goal is to
produce film or files (if direct to
press) with sufficient latitude for
the pressman to adjust the press to
the desired output and match the
proof. Short of profiling the press—
a very difficult job—the aim point
is the contract proof.
Photoshop, CMYK, & WYSIWYG
Computer displays are RGB
devices, so how on earth do we
view CMYK files? It’s all in the
recipe for CMYK to RGB
conversions onscreen. Photoshop
cannot produce an accurate
preview of CMYK unless it’s
informed about the condition of the
display and the output device.
With this information, Photoshop
can do an on-the-fly conversion
from CMYK to RGB, just to
preview your files. If you expect a
CMYK file to display properly, you
must make a profile of your
display and a profile of the CMYK
conversion method loaded in
Photoshop (if you are not using
ICC profiles, you need to load the
correct separation table).
To see how the output profile
alters the display, simply convert a
file from RGB to CMYK, then go
into the CMYK Set-Up and pick a
PEI • FEBRUARY 2000 • 37
GretagMacbeth ProfileMaker Professional 3.0
I
n my roundup of ICC profiling software packages (PEI
October 1999), I had yet to review ProfileMaker Professional
from GretagMacbeth. I’ve since had the opportunity to test
the newest version. The various modules of ProfileMaker
Professional 3.0 can be purchased as a package or
separately. I used ProfileMaker, the product that creates
ICC profiles, to profile the contract proofing system used at
The Lane Press in Vermont, where PEI magazine is printed.
Because the magazine is produced direct-to-press, sans
film color separations, the company uses an IRIS proofer
for the contract proof.
For my profile, I simply output a TIFF file from
ProfileMaker with 875 color patches and e-mailed it to The
Lane Press for output on the IRIS. Proof in hand, I used
the X-Rite DTP-41UV/T to measure the patches in
ProfileMaker. The software is quite easy to use and has an
especially beautiful interface.
Lane provided specific press requirements, such as total
ink limit and other black-generation data, which I then
entered in ProfileMaker. The application has several really
nice features. For example, with the Measure tool, you can
save the measured data to a separate file to use for
Figure 5. The ProfileMaker Measure tool interface in
ProfileMaker 3.0. The buttons on the left are other functions,
such as profile averaging and comparing profiles.
diagnostics or for updating the data in the future. There’s
also a feature for sending a small patch sample to a printer
or output device, then updating the profile for device
drifting, without re-measuring hundreds of patches. The
Pro package also contains the ProfileMaker module, which
profiles for scanners, monitors, digital cameras, and output
devices, and the ProfileEditor, an easy-to-use module for
38 • PEI • FEBRUARY 2000
altering existing profiles, which I find very useful. With this
module, you can tune a profile using any image you wish
for a visual guide, and alter lightness, contrast, and
saturation, as well as display the gamut of profiles.
Figure 6. The ProfileEditor interface has an especially
nice feature, the Gamut View, which displays the color
gamut of the output device on the CIE charts, such as
profile averaging and comparing profiles.
The only tool missing is one for tuning selective color,
an operation I often use in profile tuning. With the
ColorPicker module, you can specify spot and process
colors to output devices using ICC profiles, then use the
profiles to generate specific recipes for Pantone colors for
any device you’ve profiled. The BatchMatcher module
allows you to create hot folders for applying profiles to
images, and to process TIFF, JPEG, ScitexCT, and PNG
files, with support for EPS and Postscript files. The
package also includes the booklet “Postscriptum on Color
Management,” an excellent and beautifully illustrated
introduction to color management, plus a 4x5 transmissive
target and a 5x7 reflective IT8 target for profiling scanners.
I also used ProfileMaker to profile my Epson 1200
printer for RGB output on a new matte paper from Epson.
The quality of the output was excellent, thanks to the
profile. Neutrals were beautifully gray balanced, tonal range
was dead-on, and color and saturation were superb. It
appears that ProfileMaker is adept with RGB as well as
CMYK devices. The only negative comment I can make is
that the software, like nearly all the CMS products I’ve
tested, requires a Dongle—a necessary but annoying
device. List price for ProfileMaker 3.0, $3,500.
different CMYK profile or table.
The file itself will not change but
the preview will, because
Photoshop now has a different
method for converting CMYK back
to RGB for the screen. For a proper
preview, it’s important to ensure
that whatever method was used to
convert RGB to CMYK is specified
in the Photoshop CMYK Set-Up.
By embedding an ICC profile in
saved files, users can be informed
when their files are out of sync
with Photoshop. In the Profile SetUp, you should set Preferences to
Ask When Opening. Then when
you open a copy of Photoshop that
has 3M MatchPrint loaded in the
CMYK Set-Up, you will get a
Profile Mismatch warning when
you open a CMYK file that was
separated in SWOP Coated CMYK
and embedded as such. At this
point, it’s far better to simply
change the CMYK Set-Up to match
the file you are attempting to open
than to convert the file! Simply
click Don’t Convert and change the
CMYK Set-Up to SWOP Coated to
make Photoshop match the file; the
preview will update correctly.
Once you have a good profile or
separation table for your CMYK
output, you should preview the
results before making the actual
conversion. In Photoshop, select
“CMYK Preview” in the View menu
to bring up a soft proof. Since you
are still working in a wider gamut
RGB file, you could do some
tweaking prior to the actual
conversion. If you have a good
profile of your display and a good
profile of the output device, the soft
proof should be accurate. Although
there are significant differences
between the appearance of the
translucent display and the final
reflective printed piece, you should
get an excellent idea of how the
colors will appear after conversion.
If you do edit the RGB file while
looking at it with the CMYK preview,
save that RGB version apart from
the original RGB file. You’ve
optimized or edited the file for the
intended output and these edits
may not be appropriate if later on
you need to re-separate the file into
CMYK using a different profile.
When the file looks like you
want it to, doing a mode change
from RGB to CMYK will actually
alter the data and convert the file,
without changing the preview at all.
To illustrate how different
conversions can affect the output,
I’ve converted some RGB images
using several different methods.
First I used the default settings in
Photoshop (SWOP Coated)—a
conversion using the “classic
engine.” In the second test, I used
the Radius SWOP Coated ICC
profile, and in the third, the Radius
3M MatchPrint ICC Profile. Finally,
I made a custom ICC profile using
ProfileMaker 3.0 for GretagMacbeth
(see sidebar). This topic is a complex
process that cannot be properly
addressed here, so I highly
recommend the book Real World
Photoshop 5.0, by David Blatner and
Bruce Fraser (Peachpit Press).
[Editor’s note: Real World
Photoshop 5.0 is a 1999 PEI Cool2
Award winner.]
§
Andrew Rodney owns and operates
The Digital Dog in Santa Fe, New
Mexico. You can e-mail him at
[email protected]
More info? PEInfo No. 45
PEI • FEBRUARY 2000 • 39
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