Sidebar Title The nature of Channelsgoes Here
To understand how to modify colors in Photo-
shop, you have to know how color is calculated.
And this means coming to terms with the two
fundamental building blocks of color: luminosity
values and color channels. Be forewarned, a little
math is involved. Nothing tough—no calcula-
tor required—just enough to get you grounded.
For starters, let’s consider how things work
without color. When you scan a black-and-white
photo, the scanner converts it to a grayscale image,
so named because it contains not just black and
white but also hundreds of shades of gray. Because
we’re in the digital realm, each pixel in the image
is recorded as a number, called a luminosity value,
or level. A value of 0 means the pixel is black;
the maximum value (typically 255) translates
to white. Other luminosity values from 1 on up
describe incrementally lighter shades of gray.
When you add color to the mix, a single lumi-
nosity value is no longer sufficient. After all, youhave to distinguish not only light pixels from
dark, but also vivid colors from drab, yellow
from purple, and so on.
The solution is to divide color into its root com-
ponents. There are several recipes for color, but
by far the most popular is RGB, short for red,
green, and blue. The RGB color model is based
on the behavior of light. In the illustration below,
we see what happens when you shine three spot-
lights—one brilliant red, another bright green,
and a third deep blue—at a common point.
The three lights overlap to produce the lightest
color that we can see, neutral white. By adjust-
ing the amount of light cast by each spotlight,
you can reproduce nearly all the colors in the
visible spectrum.
Now imagine that instead of shining spotlights,
you have three slide projectors equipped with
slightly different slides. Each slide shows the
same image, but one captures just the red light196 Lesson 6: Adjusting Color and Luminance