Adobe PHOTOSHOP 5.0, User Manual, Page: 76 / 400

CHAPTER 4 68
Choosing a Color Mode
This understanding also will help you choose a file
format for your images. For more information, see
Chapter 14, Saving and Exporting Images.
About bit depth
Bit depth—also called pixel depth or color depth—
measures how much color information is in the
image to display or print pixels. Greater bit depth
(more bits of information per pixel) means more
available colors and more accurate color represen-
tation in the digital image. For example, a pixel
with a bit depth of 1 has two possible values: black
and white. A pixel with a bit depth of 8 has 2
8
, or
256, possible values. And a pixel with a bit depth of
24 has 2
24
, or roughly 16 million, possible values.
Common values for bit depth range from 1 to
64 bits per pixel.
Photoshop supports up to 16 bits per pixel for each
channel in an image. A 24-bit RGB image, for
example, would have 8 bits per pixel for each of the
red, green, and blue channels.
For a color illustration of bit depth as
displayed on monitors, see figure 4-6
on page 224.
Color display options for 8-bit color
displays
Each image mode in Adobe Photoshop uses a color
lookup table, or color palette, to store the colors
used in the image. When you’re working with a
display system that supports 8-bit color (or fewer
colors), the monitor displays only 256 different
colors at a time. For example, a 24-bit RGB image
can display 16.7 million colors in any image at a
time. However, if the monitor can display only 256
of the 16.7 million colors, Adobe Photoshop uses a
technique called dithering to mix pixels of available
colors and thus simulate colors not currently
available.
By default, Adobe Photoshop uses pattern
dithering, which can result in a distinctive pattern
of darker or lighter areas in the image. In contrast,
diffusion dithering eliminates this distinctive
patterning by using the surrounding pixels in the
mix of pixel color. But diffusion dithering can
cause visual inconsistencies when only part of a
screen is redrawn as you scroll, edit, or paint. Keep
in mind that dithering effects only appear on-
screen, not in print.
To select a color display option:
1 Choose File > Preferences > Display & Cursors.
2
Choose one or both of these display options:
•
System Palette to use the system’s standard
256-color palette.
•
Diffusion Dither to minimize dither patterns
produced by dithering.