Channels And Bit Depth (Photoshop) - Adobe PHOTOSHOP 6.0 Manual

CHAPTER 3
116
Working with Color
To adjust RGB color display to match Photoshop color
display (ImageReady):
Choose View > Preview > Use Embedded
Color Profile.
Note: In order to use the Use Embedded Color Profile
command in ImageReady, you must save the
original image, with color profile embedded,
in Photoshop.
Channels and bit depth
(Photoshop)
A working knowledge of color channels and bit
depth is key to understanding how Photoshop
stores and displays color information in images.
About color channels
Every Adobe Photoshop image has one or more
channels, each storing information about color
elements in the image. The number of default
color channels in an image depends on its color
mode. For example, a CMYK image has at least
four channels, one each for cyan, magenta, yellow,
and black information. Think of a channel as
analogous to a plate in the printing process, with a
separate plate applying each layer of color.
In addition to these default color channels, extra
channels, called alpha channels, can be added to an
image for storing and editing selections as masks,
and spot color channels can be added to add spot
color plates for printing. (See "Storing masks in
alpha channels" on page 199 and "Adding spot
colors (Photoshop)" on page 191.)
An image can have up to 24 channels. By default,
Bitmap-mode, grayscale, duotone, and indexed-
color images have one channel; RGB and Lab
images have three; and CMYK images have four.
You can add channels to all image types except
Bitmap-mode images.
About bit depth
Bit depth—also called pixel depth or color depth—
measures how much color information is available
to display or print each pixel in an image.
Greater bit depth (more bits of information per
pixel) means more available colors and more
accurate color representation 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 , or 256, possible values. And a
8
pixel with a bit depth of 24 has 2
16 million, possible values. Common values for bit
depth range from 1 to 64 bits per pixel.
In most cases, Lab, RGB, grayscale, and
CMYK images contain 8 bits of data per color
channel. This translates to a 24-bit Lab bit depth
(8 bits x 3 channels); a 24-bit RGB bit depth (8 bits
x 3 channels); an 8-bit grayscale bit depth (8 bits x
1 channel); and a 32-bit CMYK bit depth (8 bits x
4 channels). Photoshop can also read and import
Lab, RGB, CMYK, and grayscale images that
contain 16 bits of data per color channel.
, or roughly
24