Graphics Programs Reference
In-Depth Information
About masks and masking
Alpha channels, channel masks, clipping masks, layer masks,
vector masks—what's the difference? In some cases, they're in-
terchangeable: A channel mask can be converted to a layer mask,
a layer mask can be converted to a vector mask, and vice versa.
Here's a brief description to help you keep them all straight. What
they have in common is that they all store selections, and they all
let you edit an image nondestructively, so you can return at any
time to your original.
•
An
alpha channel
—also called a
mask
or
selection
—is an extra
channel added to an image; it stores selections as grayscale im-
ages. You can add alpha channels to create and store masks.
•
A
layer mask
is like an alpha channel, but it's attached to a specif-
ic layer. A layer mask controls which part of a layer is revealed or
hidden. It appears as a blank thumbnail next to the layer thumbnail
in the Layers panel until you add content to it; a black outline in-
dicates that it's selected.
•
A
vector mask
is essentially a layer mask made up of vectors, not
pixels. Resolution-independent, vector masks have crisp edges and
are created with the pen or shape tools. They don't support trans-
parency, so their edges can't be feathered. Their thumbnails appear
the same as layer mask thumbnails.
•
A
clipping mask
applies to a layer. It confines the influence of an
effect to specific layers, rather than to everything below the layer
in the layer stack. Using a clipping mask clips layers to a base lay-
er; only that base layer is affected. Thumbnails of a clipped layer
are indented with a right-angle arrow pointing to the layer below.
The name of the clipped base layer is underlined.
•
A
channel mask
restricts editing to a specific channel (for ex-
ample, a Cyan channel in a CMYK image). Channel masks are
useful for making intricate, fringed, or wispy-edged selections.
You can create a channel mask based on a dominant color in an
image or a pronounced contrast in an isolated channel, for ex-
ample, between the subject and the background.
