Color
Management in Adobe Photoshop® part 3:
Assign
Profile Versus Convert to Profile
Now that we have
optimized our color Adobe Photoshop color settings, we can
focus on how to actually use color management in Photoshop.
Everything that you can do with Photoshop color management
boils down to two categories, “assigning”
profiles and “converting to” profiles. Contrary
to popular belief, the two terms do not mean the same
thing. In fact, confusion over which one to use is by far
the most common cause of image color being wrecked by color
“mismanagement”. Fortunately the two terms are
fairly simple to understand and an awareness of the
difference should keep you and your images out of trouble.
I frequently say that color management is about color
communication. The difference between assigning and
converting profiles can be understood by thinking of
profiles and color spaces as “languages” that
communicate color meaning. Converting to profile is like
translating a book into another language. As long as both
languages contain words that have equivalent meanings, the
book can be converted to the new language with its ideas
unchanged. Converting to profile in Photoshop works the
same way; color information in one profile is translated to
another profile. The image’s color meaning remains as
unchanged as possible within the constraints of the
destination profile’s gamut and the rendering intent
used. This makes convert to profile the correct choice for
most color management transforms, like converting to a
printer profile for printing or moving from one color space
to another.
Convert to
profile translates color meaning from one profile to
another, changing the color mode numbers. This can be seen
in the RGB numbers from the sRGB profile above changing to
different numbers in the ProPhoto destination. Note that
color
appearance, indicated here by the Lab readout, stays the
same.
Convert to profile changes color mode numbers, such as
RGB values, while attempting to keep color meaning the
same.
Most color management
errors in Photoshop come from using assign profile where
one shouldn’t. If convert to profile can be thought
of as translate to, then assign profile is interpret as. To
stretch the book translation analogy a bit further,
assigning a profile is like choosing a language that you
think the book was written in. If you choose the wrong
language; i.e. a different one than the author used to
write it, you cannot accurately translate it’s
meaning. In color management assigning a profile gives
color meaning to the numbers in an image file, which means
that it also changes the file’s color meaning. This
can be a big problem because all profiles and color spaces
of a particular color mode use similar sets of numbers to
denote color. So it is almost never apparent from the
numbers themselves which profile is correct, or was correct
before the current profile was assigned. Unlike the book
analogy, in most cases it is not immediately obvious that
the wrong color language was used to interpret an image
file. This is especially true when the profile is assigned
before the user has a chance to look at the file, which we
will discuss later.
Assign profile
keeps color mode numbers the same but changes
their meaning, as seen above. You can try these
demonstrations
for yourself by creating a new RGB document. Convert the
profile to
sRGB if it isn't already, then fill it with R 82, G 96, B
168. you can then
try out the effects of assigning and converting to
ProPhoto. Use the
flyout menu (in the upper right corner) of the info palette
to set your
secondary readout to show Lab
numbers.
Of course, there are some situations in which assigning a
profile is the correct approach. Probably the most common
is with an input profile, like those used with scanners and
digital cameras. Since the input profile’s purpose is
to characterize the color behavior of the input device, you
have to assign it to a file from the device in order to use
it. Methods for doing this will be discussed later in the
"Assign Profile Dialog Box" section. Other examples where
assigning a profile is appropriate include simulating how a
file might print on a device if no color management is
used, such as sending a file to an offset CMYK press. This
is actually the first step in making many of the contract
proofs for the press industry; the press’s
fingerprint profile is assigned to the file and then it is
converted to the proofer profile (using absolute
colorimetric rendering) to be printed.
Go to part 4: The Convert to
Profile Dialog Box
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