ChApter 2. digitAl workFlow
80 For more on textbooks in digitAl photogrAphy, visit http://www.photoCourse.Com
http://www.photocourse.com/itext/CMYK/
http://www.photocourse.com/itext/RGB/
http://www.photocourse.com/itext/colorspace/
Color management–Color models and Color spaces...
As you’ve seen, the image sensor in a digital camera captures just red, green
and blue (RGB) light. RGB is what we call a color model and it’s based on the
way the human eye perceives full-color images by blending various propor-
tions of red, green and blue light. The RGB model used in display devices
creates an image with transmitted red, green, and blue light; and the CMY
model used in printers creates it with cyan, magenta, and yellow inks that
absorb colors so only red, green and blue are reflected.rGB Color
Color models are fairly basic in that all they tell you is what amount of each
color needs to be blended to create a third color.- RGB specifies the amount of each color in units between 0–255.
- CMY specifies the amount of each color as a percentage between 0–100%.
For example, if you start with pure red its RGB values would be R:255 G:0
B:0 indicating that the color’s red component is 256 (remember, we count
from 0, not 1) and both green and blue components are zero. This sounds
like a detailed description of a color but it isn’t because it doesn’t refer to a
specific color as you would perceive it, it just tells a device such as a display
screen or printer to generate all of the red it’s capable of. (One expert calls
these RGB values “input signals”.) The most fully saturated red could be
bright and vibrant on one device and dull and muddy on another. It’s as if a
driver’s manual told you that to reach a specified speed you press the acceler-
ator down 1 inch. However, if you follow this instruction in a Ford, you might
go 35 miles per hour, while following it on a Ferrari would have you going 135
miles per hour. In digital photography what’s needed is a way to make the
values refer to a very specific color and that’s where color spaces come in.
Changing Color spaces
A color space plots each of the millions of possible colors on a three-dimen-
sional chart in such a way that their positions and spacing show how they
relate to one another—often called scaling. Each color can be specified or
located by its coordinates in this space.
One of the key features of a color space is its gamut—the range of colors
it represents. Different color spaces have different gamuts as do different
devices. It’s not at all uncommon to have a color in an image that is within
the gamut of the display but not of the printer and vice versa. When a color
falls outside of a color space’s gamut in this way, it can’t be reproduced by the
device and is called out-of gamut. In the next section you’ll see how a color
management system can bring such colors back into the gamut of a device.
One thing to keep in mind is that a wider gamut doesn’t mean more colors.
The only way to do that is to capture images in the RAW format rather than
JPEG. A wider gamut just spreads out the available colors.
In digital photography you will find references to a variety of RGB color
spaces. Here are the most common:- sRGB has the smallest gamut of the spaces discussed here, but is ideal for
images that will be displayed on a screen or projected. Almost all cameras
assign this as the default space for JPEG images. Almost all browsers and
display devices are set to display this color space most accurately.
Click to explore how
sRGB and Adobe RGB
color spaces compare
when it comes to the
number of colors they
can capture.
Click to explore how
red, green and blue can
create full color images.Click to explore how
cyan, magenta and
yellow can also create
full color images.
Here is the sRGB color
space superimposed
over the larger Adobe
RGB color space (shown
here ghosted). You can
see how much smaller
its gamut is.
If you align the letters
CMY under RGB, you
have a guick guide to
how CMY works. Cyan
creates red (directly
above it) by absorbing
the other two colors,
green and blue;
Magenta creates green
by absorbing red and
blue; and yellow creates
blue by absorbing red
and green.