Using Channels And Masks; About Channels; Using The Channels Palette (Photoshop) - Adobe PHOTOSHOP 6.0 Manual

Chapter 7: Using Channels and Masks
E ach image in Adobe Photoshop has
channels that store information about the
image's color. You can create additional
channels to store spot colors for printing with
special inks and to store masks for sophisticated
selections.

About channels

Photoshop uses special grayscale channels to store
an image's color information and information
about spot colors. If an image has multiple layers,
each layer has its own set of color channels.
Color information channels are created
automatically when you open a new image.
The image's color mode (not its number of layers)
determines the number of color channels created.
For example, an RGB image has four default
channels: one for each of the red, green, and blue
colors plus a composite channel used for editing
the image.
You can create alpha channels to store selections as
8-bit grayscale images. You use alpha channels to
create and store masks, which let you manipulate,
isolate, and protect specific parts of an image.
Saved selections in alpha channels can be loaded in
ImageReady. (See "Loading a selection into an
image" on page 201.)
In addition, you can create spot color channels to
specify additional plates for printing with spot
color inks. An image can have up to 24 channels,
including all color and alpha channels.
The file size required for a channel depends on
the pixel information in the channel. (See "About
image size and resolution" on page 92.) Certain
file formats, including TIFF and Photoshop
formats, compress channel information and
can save space. (See "Managing channels
(Photoshop)" on page 187.) The uncompressed
size of a file, including alpha channels and layers,
appears as the rightmost value in the status bar at
the bottom of the window when Document Sizes
is chosen from the pop-up menu.
Note: As long as you save a file in a format
supporting the image's color mode, the color
channels are preserved. Alpha channels are preserved
only when you save a file in Adobe Photoshop, PDF,
PICT, TIFF, or Raw formats. DCS 2.0 format only
preserves spot channels. Saving in other formats may
cause channel information to be discarded.
(See "About file formats" on page 364.)
Using the Channels palette
(Photoshop)
The Channels palette lets you create and manage
channels and monitor the effects of editing.
The palette lists all channels in the image—
composite channel first (for RGB, CMYK, and Lab
images), then individual color channels, spot color
channels, and finally alpha channels. A thumbnail
of the channel's contents appears to the left of the
channel name; the thumbnail automatically
updates as you edit the channel.
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