Page 32 - Enhancing CAD Drawings with Photoshop

4386.book Page 15 Monday, November 15, 2004 3:27 PM

UNDERSTANDING MODES, BITS, AND CHANNELS 15

What this procedure should teach you is that although color is stored in channels, each channel by
itself has only tonal information (light/dark) and can be represented as a grayscale image. Only when
the channel data is reproduced with colored light does the true color image emerge.
A traditional cathode ray tube (CRT) monitor has three electron guns inside—a red gun, a green
gun, and a blue gun. Each gun is fed the corresponding grayscale channel data by Photoshop. When
the colored light emerges on the screen, it combines to form the color image that you perceive.

Color Bit Depth
Traditionally, each channel in a color image is an 8-bit grayscale image. Therefore, an RGB image has
a bit depth of 24, or 8 times 3 (see Table 1.1). If you do the math by raising 2 to the 24th power, you
will get more than 16 million possibilities of color for each pixel in the image.
That sounds like a lot, and it is a lot because our human ability to perceive differences in tonality
fails in the range of a few million possibilities per pixel. In other words, 24-bit images have more gra-
dations of color than our eyes can perceive.
For a long time in the history of digital imaging, 8-bits/channel has been sufficient for all but the
most discerning professionals. However, when you manipulate images you lose some of the data in
the process (see Chapter 3, “Retouching Photos,” for more on this subject). The differences in tonality
post-manipulation often fall below our perceptual threshold—in other words, we can see problems
with heavily manipulated images.
The solution some photographers are adopting is to shoot in 16 bits per channel. This is currently
possible only on prosumer or high-end professional digital cameras. Photos with 16 bits per channel
have 65,536 possibilities per pixel per channel (refer to Table 1.1), and in RGB color mode this equates
to 48-bit images (16×3). The benefit to shooting in 16 bit is that you can heavily retouch photos without
being able to perceive banding (imperfections) in the result.
WARNING The downside to shooting in 16 bit/channel is the huge volume of data that your sys-
tem then has to handle. Much larger images require expensive digital cameras with huge storage
space available. In addition, your computer must have more than a gigabyte of memory, huge hard
drives, and fast processors to reasonably manipulate the data sets.
You can check an image’s bit depth by choosing Image Mode and seeing whether 8 Bits/Chan-
nel or 16 Bits/Channel is selected.
To arrive at the total bit depth stored in an image, see how many channels appear in the Channels
palette and multiply this number by your bits per channel. For example, if your image has 4 channels
(CMYK) and it stores 16 bits/channel, multiply 4 by 16 to arrive at a composite 64-bit image.
NOTE Photoshop CS now offers greater ability to work with images containing 16 bits per channel.

Additive versus Subtractive Color
RGB color mode was designed with light in mind. It is a fact of physics that when you shine three
beams of red, green, and blue light together, they combine into white light. RGB color is an additive
color system because when these components are added together, they yield white. Therefore, RGB
is an ideal way to represent color on a computer monitor, which shines light directly into your eyes.
When you look at printed matter, the color you see comes from the illumination in the space where
you are. This light reflects off the surface of the page before entering your eyes. This situation is phys-
ically quite different compared with directly viewing colored light on a computer monitor.