Like an alpha channel in a paint object, a channel mask is basically a
grayscale image. Channel masks can be edited using image-editing
tools and techniques, similar to alpha channels.
You can think of a channel mask as a template for transparency. The
channel mask is the same size as the masked object and is aligned
with it. In the case of a paint object, the channel mask has the same
resolution and number of pixels as the paint object.
Channel masks, like grayscale images, contain pixels that are
assigned 256 possible lightness levels, or luminance values. Lumi-
nance values in a channel can range from 0 (black) to 255 (white).
In an alpha channel, luminance corresponds to selection intensity. In
a channel mask, luminance values correspond to 256 levels of trans-
parency, from 100% to 0% transparency, in the masked object. Black
pixels (0 luminance) produce 100% transparency, while white pixels
(255 luminance) produce 0% transparency.
Therefore, when you paint in a channel mask, painting with black
produces clear areas and painting with white produces opaque areas
in the masked object. Painting with gray produce partial transparency
relative to the gray value, with darker grays producing greater trans-
parency than lighter grays.
Keep in mind that an objects appearance can be affected by its trans-
fer mode, scope setting, and opacity, in addition to a channel mask.
Changing the transfer mode can completely change the appearance of
an object that has a channel mask. See Using transfer modes on
page 20.374.
Black in a channel mask
(left) produces 100% trans-
parency
A feathered edge produc-
es partial transparency
Channel mask and paint object
Masked object