Adobe Photoshop CS5 One-on-One

from the image, another just the green,
and a third just the blue. Shine the three
projectors at the same spot on a white
screen and you get the full-color photo-
graph in all its glory.

This is precisely how a digital image
works, except that in place of slides,
you get channels. Each channel con-
tains an independent grayscale image,
as shown on the right. To generate the
full-color composite, Photoshop color-
izes the channels and mixes them to-
gether, as in the bottom-right diagram.

In each channel, white indicates a big
contribution to the full-color compos-
ite image, black means no contribution,
grays contribute something in between.
The background is very bright in the blue
channel and darker in the other two, so
the composite sky is blue. The face is
lightest in the red channel with some
middle grays from the green channel. Red
with a little bit of green make orange.
The lips are dark everywhere except the
red channel, so the lips are red.

Each channel contains 256 luminosity
values—black, white, and the 254 shades
of gray in between. When colored and
mixed together, they produce as many as
16.8 million (256^3 ) unique colors. (For
a less common scenario with even more
colors, see “Exploring High Bit Depths”
on page 302 of Lesson 9.)

You can access both the full-color com-
posite and the individual channels in the
Levels and Curves dialog boxes. To see
what the grayscale channels in an image
look like, choose Window→Channels,
and click Red, Green, or Blue in the
Channels palette. Click RGB to return
to the full-color composite view.

Red channel Green channel Blue channel

Red channel

Green channel

Blue channel

The Nature of Channels 197