Graphics Programs Reference
In-Depth Information
All the chromaticities within the triangle can be reproduced by mixing the three primaries
of the device. Since like some photographers I'm mathematically challenged, getting deeper
into what the values mean is not necessary to utilize the tools and techniques we hope to cover.
What is important to understand is that these values can be used to specify primaries and other
colors and these values are used in defining and plotting colors scientifically and without ambi-
guity. This plot (among others) is often seen in color management products and is useful for
showing how various color spaces compare.
RGB versus CMYK, and What's the Fuss about LAB?
At this point in our discussion of color management, we need to discuss
the use of the various color models. RGB and CMYK were touched upon
briefly. Most devices that capture images initially produce RGB data.
Many printers require a file in CMYK to reproduce a color image. When
I first began to work with Photoshop, it didn't support CMYK files. When
Photoshop was able to produce and edit CMYK images, I was under the
impression that the world of imaging was divided into either RGB or
CMYK and that was about it. Every file was either RGB or CMYK, and
all RGB files and all CMYK files were the same. Photoshop also supports
the CIELAB color model (Photoshop refers to this simply as LAB). LAB
is a synthetic color space used by color management systems. Let's take
a quick look at LAB and similar color spaces, how they are constructed,
and how they can be useful.
The LAB color model is a three-dimensional space shaped like a sphere.
In LAB, the L*axis (pronounced “L-star”) specifies the lightness, or the
range from light to dark. Zero represents the blackest black and 100 is the
whitest white. Think of this as the vertical axis of our sphere. The a and
b axis represent the chromaticity. LAB is derived from CIE XYZ (1931),
which defines human perception; it represents all the colors we can see.
Another color model you may come across is called LCH. Some prod-
ucts use LCH for color adjustments although unfortunately, Adobe Pho-
toshop is not one of them. In LCH, L again specifies the luminance. Zero
represents the blackest black and 100 is the whitest white. The C is
chroma, which is a fancy name for saturation or purity. The scale runs
from zero (no chroma and thus neutral/gray) to 100 (fully saturated).
The H represents hue, which refers to the pure spectral colors of the
rainbow. H runs around the equator of our sphere and is defined in
degrees from zero to 360 (see Fig. 1-8).
Photoshop uses HSB, a color model that is very similar to LCH. HSB
stands for hue, saturation, and brightness. You may now want to go to
the first tutorial in the topic, which discusses several of these color models
available in Adobe Photoshop's Color Picker. In an upcoming chapter dis-
cussing RGB working spaces in Photoshop, I will discuss the pitfalls of
editing in LAB. Photoshop does allow a user to convert a file into LAB,
but LAB is a rather difficult color space in which to edit. There are a few
