Color
Management in Adobe Photoshop® part 2:
Other Color Settings
Please note: this section covers
technical aspects of Photoshop's color settings that most
users should not change from the North American Prepress 2/
US Prepress defaults. It was written for those who want to
understand all of the implications of the options available
in color settings. Those who just want a general overview
of color management in Photoshop can set their color
settings to North American Prepress 2 and skip straight
to section
3.
Further down on the color settings are the color management
policies. The most important part of this section is the
bottom, where all of the “ask” boxes should be
checked. This forces Photoshop to display a dialog box
every time that a color management decision is made and
allows you to intercede in that decision. The top of the
policies section, regarding whether to preserve embedded
profiles or to convert to the working space, will only
affect which default choice comes up in that dialog box
when opening or pasting from a file using a color space
other than your working space. The other choices in this
section (“convert to working space” and
“do not color manage”) will also show up in the
dialog box, as long as the “ask” boxes are
checked. If they are not checked, the default choice will
be used, possibly causing a color conversion to take place
as the file is opened or pasted. If this happens, some
color information may be lost. You may not realize it
however, because no dialog box shows up and the conversion
takes place before you see the file on your screen. This
topic will be covered in depth in Section 6: Saving
embedding, and Opening Files. For now just make sure that
all of the ask boxes until you are confident enough with
color management to turn one of them off.
Next down the color settings box is the “Conversion
Options” section. The engine or CMM (color management
module) is the “calculator” that performs the
translations between color spaces/profiles/modes. Early on,
some color management software vendors adjusted their
engines to tweak color conversions. As color management
matured a proliferation of generic color management modules
began to ship with imaging software (ACE- Adobe Color
Engine) or operating systems (Apple Colorsync). At that
point it became more realistic to build those tweaks into
the profiles (and sometimes color spaces) themselves.
According to the wonderful color management book
“Real World Color Management”, Kodak is about
the only company that still builds significant proprietary
advantages into their color engines. So, as long as you
don’t use profiles created with Kodak software (Which
would be rare- you would probably know if you were using
Kodak profiles), you should see little difference between
color engines. While it might be a good idea to test all of
your available engines, if you are using Photoshop you have
Adobe ACE for “free”. ACE is a tried and true
conversion engine that is used by many if not most color
management experts, making it a wise choice. The only
drawback to ACE has been that for years it was only
available within Adobe applications. So RIPs and other
third party software were unable to take advantage of it.
In 2007 this changed, as Adobe decided to offer the ACE
engine as a free download that could be configured for use
by non-adobe Applications.
Rendering intent is
your next choice. Very often there are colors in a color
space or input profile that simply do not exist in a
destination color space or output profile. When converting
between them, the out-of-gamut colors must be
“mapped” to some color that is reproducible in
the destination space or profile. Rendering intent
specifies exactly how this is done. There are several
options, and a discussion of the various pros and cons
would be considerably longer than this discussion of
Photoshop's color settings. We will talk about rendering
intent in more detail later in the "Convert to Profile"
section of this series, but here in the color settings we
are just setting the default rendering intent. Any time that you will
be making a critical profile conversion in PhotoShop, you
will have the option of choosing something other than the
default intent. Because of what this default controls,
which I will cover in the next two paragraphs, it is my
opinion that for almost all users the relative colorimetric
intent is the best choice. So to get started you can just
select relative colorimetric and leave the rendering intent
debate for later.
Whatever intent you choose, the most important part of the
choice is understanding exactly what this default rendering
intent in the color settings box actually controls. Most
noticeably, it controls what default choice comes up in all
"convert to profile" dialog boxes. However, the convert to
profile dialog allows you to change from the default to any
other rendering intent at any time via pull-down menu, so
this really isn't a big issue. What is more significant is
that the intent chosen in color settings is used for all
mode changes that do not go through the convert to profile
dialog. So for example, opening an RGB file and using
Image>Mode>CMYK would cause the file to be converted
to CMYK via the default rendering intent. Please note that
you can force a convert to profile dialog by selecting
Image>Mode>convert to profile and choosing a profile
or color space from the mode that you want to switch to.
A more subtle effect of the rendering intent chosen in
color settings is the control of the way that color modes
other than the one used in the current file are represented
in Photoshop's info palette. The info palette's secondary
readout is set to CMYK by default, and experienced color
management users will often set the it to display device
independent Lab values. To display this information,
Photoshop must convert the current color mode to calculate
the second readout. In performing this conversion, some
method of mapping must be used. So for users who count on
this information to be correct, such as prepress operators
who use the info palette to track final CMYK values in an
RGB file, the default intent must match the intent used
when the conversion to CMYK is performed or the info
palette values will be inacurate. A similar dilemma exists
for the small percentage of users who use Photoshop to try
to hit specific Lab colors on output and use the info
palette to track them. For these users absolute accuracy of
this information, rather than a ballpark approximation, is
very important and absolute colorimetric rendering intent
must be chosen as the color setting default.
The final choices under
Conversion options are black point compensation and dither.
Black point compensation maps the darkest point in the
source to the darkest black in the destination (much like
white point shifting in relative colorimetric). Not using
black point compensation can cause problems in the way that
shadows are mapped. For instance, a range of shadows could
correspond to lower L values (Lightness from LAB) than a
particular paper can reproduce. Without black point
compensation, all of the range darker than the measured
paper black would be mapped to paper black, causing shadow
gradation to abruptly stop at a flat black blob. Unless
there is a rare and specific reason to render the source
black absolutely correct by measurement, “use black
point compensation” should always be checked.
Dither introduces a small amount of digital
“noise” to hide the transition banding that may
occur when converting a tonal file between spaces and/or
profiles. Introducing this negligible amount of noise
during the conversion process is beneficial for most tonal
photographic images because it prevents banding in smooth
gradations. In contrast, this noise may be objectionable in
flat spot colors. The general rule is to always use dither
when converting tonal files (which includes all
photographic images) but not to use dither for files
containing only flat, spot, or solid colors.
At the end of our tour of Photoshop’s color settings
is the “advanced controls” box. The first
option is to desaturate monitor colors by a user defined
percentage. After profiling your monitor, the last thing
you want to do is alter it’s color response. This
setting exists primarily for non- color managed workflows.
If you are serious about color accuracy, use color
management and leave the desaturate box unchecked.
Finally we have the “blend RGB colors using
gamma” setting. Photoshop’s description states
that this control is used to modify edge effects of blended
colors, as would happen with a Gaussian blur. While this is
true, the check box also controls the way that blended
colors are calculated. The effects of using this control
can be dramatic. In order to fully discuss this control
option, a basic understanding of the concept of gamma is
necessary.
Gamma is a correction designed to compensate for the
non-linear nature of human vision. In other words, humans
do not perceive equal steps in density as being equal, and
gamma attempts to correct for this. (Think of a darkened
room. Lighting one candle would seem to make a large
increase in the amount of light in the room. The same
candle giving off the same amount of light outdoors in
bright sun would barely be noticeable.) The correct gamma
number necessary to compensate for this phenomenon varies
somewhat based on the situation. Most color spaces use
gammas between 1.8 and 2.2, with a higher gamma number
indicating a more dramatic correction curve and therefore
more compensation. A gamma of 1.0 (often called linear
gamma) has no correction and is the same as not using gamma
at all.
Most applications blend colors using the native gamma of
the images’ color space, and this is what Photoshop
does by default. Checking the blend using gamma box allows
you to enter a different gamma. Linear gamma (a setting of
1.0) is used as an alternate gamma most often because it is
more colorimetrically correct, meaning that colors would
blend in a logical way that closely follows how the colors
would mix in real life. The drawback to linear gamma
blending is that colors that are compliments of each other
mathematically (colors whose coordinates add up to equal
numbers, such as 30,15,0 and 0, 15, 30 whose sum is 30,30,
30 for example) do not blend to a neutral gray as they
should. Blending in native gamma causes mathematically
complimentary colors to blend to a perfect gray, but ranges
in between may pick up hue shifts that do not seem logical.
Considering these factors with the original aim of the
control, to minimize blending artifacts in blurs, there is
not a single setting for this control that is optimal in
all situations. Good arguments could be made to either
leave the default setting of “off”, or to check
the box and use a gamma of 1.0. In images where blends
become critical, you could open the color settings dialog
box on screen and move it so that you could see the blends.
You could then use the settings box’s
“preview” button (on the right side under the
load and save buttons) to evaluate the effects of different
settings. (Change the setting and turn preview on and off.)
In conclusion, to configure Photoshop’s color
settings:
1. Open the color settings dialog box. (Edit> color
settings… (Windows) or Photoshop> color
settings… (Macintosh).
2. Change the settings: pull-down to “U.S. Prepress
Defaults” and check “advanced mode”.
3. Select an RGB working space. If you are unsure of what
to use for RGB, Adobe 98 is a very widely used compromise
between small spaces that restrict gamut and large spaces
that are difficult to use because of the large size of each
color “step”.
Your color settings are now configured. You can save the
settings with the save button to use as your own default in
the future. Remember to close out of Photoshop even if you
save the settings, because Photoshop only records changes
to its preference files when it shuts down. If your system
were to crash, Photoshop would load your previous color
settings from the last time Photoshop closed down properly,
not your new ones. You are now ready to take advantage of
Photoshop’s powerful color management
potential.
Go to part 3: Assign Profile
Versus Convert to Profile
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