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Adobe Photoshop Help
Working with Color
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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 280
and
“Adding spot colors (Photoshop)” on page 272
.)
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 infor-
mation 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
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.
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.
Converting between bit depths
A 16-bit-per-channel image provides finer distinctions in color, but it can have twice the
file size of an 8-bit-per-channel image. In addition, only the following Photoshop tools and
commands are available for 16-bit-per-channel images:
•
The marquee, lasso, crop, measure, zoom, hand, pen, eyedropper, history brush, slice,
color sampler, clone stamp tools, healing brush tool, and patch tool, as well as the pen
and shape tools (for drawing work paths only).
•
The Duplicate, Feather, Modify, Levels, Auto Levels, Auto Contrast, Auto Color, Curves,
Histogram, Hue/Saturation, Brightness/Contrast, Color Balance, Equalize, Invert,
Channel Mixer, Gradient Map, Image Size, Canvas Size, Transform Selection, and Rotate
Canvas commands, and a limited set of filters. (See
“Using filters” on page 321
.)
To take full advantage of Photoshop features, you can convert a 16-bit-per-channel image
to an 8-bit-per-channel image.
