User Guide

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Chapter 10: Using Channels and Masks
n Adobe Photoshop, channels store an
image’s color information. In addition,
channels and masks store selections that let
you manipulate and control specific parts of an
image in sophisticated ways.
About channels
Color information channels are created automati-
cally when you open a new image. The image’s
color mode (and not its number of layers) deter-
mines the number of color channels created. For
example, an RGB image has three default
channels: one for each of the red, green, and blue
colors plus a composite channel used for editing
the image.
Image (RGB), and channels in Channels palette
Alpha channels store selections as 8-bit grayscale
images and are added to the color channels in an
image. You use alpha channels to create and store
masks, which let you manipulate, isolate, and
protect specific parts of an image. An image can
have up to 24 channels, including all color and
alpha channels.
In addition, spot color channels can be added to an
image to let you specify additional plates for
printing with spot color inks.
About channels and file size
The file size required for a channel depends on the
pixel information in the channel. For example, if
the image has no alpha channels, duplicating a
color channel in an RGB image increases the file
size by about one-third, in a CMYK image by
about one-fourth. Each alpha channel and spot
color channel also adds to the file size.
Certain file formats compress channel infor-
mation and can save space, including the TIFF and
Photoshop formats (see “Saving and managing
channels” on page 243). The size of a file,
including alpha channels and layers, appears as the
second (right) value in the lower left corner of the
window when the Document Size command is
chosen. (See “About image size and resolution” on
page 42.)
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