264 ABSOLUTE BEGINNER’S GUIDE TODIGITAL PHOTOGRAPHY
FIGURE 17.10
Different types
of alpha chan-
nels: color,
adjustment
layer, alpha,
quick mask.■ Alpha channels. The channel named “sky” in the preceding image is a
mask that was created to lighten the dark tones in the clouds.
■ Quick Mask. There is a second type of alpha channel on this palette, a Quick
Mask. Quick Masks are convenient, temporary alpha channels in Adobe
Photoshop. They are created by clicking the Quick Mask Mode icon on the tool bar
(not shown). You can convert a Quick Mask into
an alpha channel by loading and saving it, but
you can have only one Quick Mask at a time.
The Quick Mask is the red-tinted area cover-
ing the image. It is visible in the image win-
dow because its eye icon is switched on.
Note that the eye icons of the adjustment
layer mask and the alpha channel are
switched off. Masks whose eye icons are not
visible cannot be seen in the image window.Alpha channels are a part of the image’s file; they
may remain when the image is saved. One disad-
vantage is that they increase the file’s size. For
example, an RGB image file that is three
megabytes can expand to four megabytes with one
alpha channel.
When your image editing is complete, you may
have no reason to keep extra channels, so you canWhen you discard
alpha channels, be careful
not to unintentionally discard
masks attached to layers. Masks
with names that appear in italics
in the Channels palette are not
alpha channels. These masks are
used by adjustment layers.