different standards, there is no one hard-and-fast formula.
Rather, channels are mixed together in varying degrees ac-
cording to the recipe outlined in the CMYK profile. So, for
example, the Cyan channel is mostly a duplicate of the Red
channel, but it also mixes in bits of the Green and Blue chan-
nels to account for the idiosyncrasies of a given press.
Looking at the figure below, however, you might find this hard
to believe. The top row shows grayscale versions of the RGB
channels; the middle row shows their CMY complements.
The CMY channels bear a passing resemblance to the ones
above them, but it’s as if the darkest levels have been replaced
with light gray. And that’s precisely what has happened. Pho-
toshop generates the contents of the Black channel by leech-
ing shadows from the Cyan, Magenta, and Yellow channels.
When we add the Black channel, as in the bottom row of the
figure, the CMY channels more closely match the brightness
of the RGB channels in the top row. (Yellow + Black appears
lighter than Blue only because yellow is such a light ink.)
The most popular method for transferring dark pixels to the
Black channel is called gray component replacement, or GCR.
The idea is that paper can absorb only so much ink, after
which point the ink begins to smear. Suppose that a press
subscribes to a total ink limit of 300 percent. This means that
the percentages of cyan, magenta, yellow, and black ink can
add up to no more than 300. GCR makes sure the total ink
limit never goes higher, even in the darkest shadows in the
image. Photoshop steals the shadows from the CMY chan-
nels, puts them in the Black channel, and then darkens the
old CMY shadows until the total ink limit is met. And so it is
that black restores the darkness at the heart of the RGB image.Red Green BlueCyan Magenta YellowCyan + Black Magenta + Black Yellow + BlackWhyWhy (and(and How)How) ThreeThree ChannelsChannels BecomeBecome FourFour 441