We can change the opacity of all the pixels in an image layer by changing the
Opacity value on the Layers panel. This opacity information is stored in
an alpha channel. However, opacity alpha channels are not what we see in
the Photoshop Channels panel. The alpha channels we see in the Channels
panel are used to store masks and selections. Therefore, for purposes of
the scope of this web site, we will focus on using alpha channels for the
storing of selections.
Figure 1 is an example of the Photoshop Channels panel. For an RGB
image, we will see four layers. The topmost layer is the composite color
channel. The next three layers are the individual
color channels. The topmost layer. the composite color channel, is Photoshop's composite rendering
of the three individual color channels. The composite channel is not
physically stored in the image file. The individual color channels are.
Figure 1. Channels Panel
We can also see two other channels in Figure 1. The fifth layer is a
typical alpha channel. We can have more than one alpha channel. The
bottommost layer is a special Photoshop alpha channel called Quick Mask.
We can have only one Quick Mask layer.
Alpha channels are used to store selections. A selection is stored in an
alpha channel as a black and white mask. By storing the selection, we can recall
the selection as many times as we need. Because it is stored with the
image, the selection will not disappear when the image file is closed.
Quick Mask is a convenient feature for creating a selection in a special
alpha channel. However, even though an image can have many alpha channels,
it can only have one Quick Mask layer.
You can learn how to store selections as alpha channels
and how to use Quick Mask on the
Saving Selections page.