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Selections and masks are used to isolate
parts of an image for editing. Masks offer far more flexibility
and creative power than simple selections.
In a nutshell, a mask is a channel (an Alpha
channel) in the image, with the full 256-levels of 8-bit
grayscale available. The various levels of gray represent what
parts of the image are within a selection – and to what degree.
In a simple selection (without feathering or
anti-aliasing), a pixel is either inside the selection or
outside the selection. If a filter is applied, it will have an
effect on pixels inside the selection, but not on those outside
the selection. By using a mask, you can assign up to 256
different levels of how much that filter will be applied.
In the figure below, a circular select was made in the center of
the image and the filter Texture> Mosaic Tiles was applied.
Pixels within the selection were changed; pixels outside the
selection were not changed.
In the next figure, a mask was created with a
radial gradient and the same filter with the same settings was
applied. Note how the filter fades out as it moves farther from
the center of the image. In the corners of the image the filter
is not applied at all.
When we look at the Alpha channel from which
this selection was made, we can see that the white area allows
the filter to be completely applied, and the darker areas
restrict the filter. In the areas that are black in the mask,
the filter is not applied at all, the image is completely
protected.
With the image also visible, the mask is shown
as a red overlay. As you can tell from comparing the image below
to the image above, the red areas of the overlay represent the
areas of the image that are protected.
Looking at the Channels palette, you'll see that
the mask channel (Alpha 1) is both visible and active, and the
composite channel (RGB) is visible but not active.
So, now we know what a mask is (and Alpha
channel) and how it functions (protects and exposes parts of an
image based on a grayscale channel). Let's look at the mechanics
of creating and editing a mask.
The easiest way to make a mask, in most cases,
is to start with a selection. Use any of the selection tools and
commands. Once you've got a selection, you can use either of two
techniques: Quick Mask mode or the command Select> Save
Selection (which will be discussed below).
With a selection active in the image, you can
press Q on the keyboard or click on the Quick Mask button near
the bottom of the Toolbox.
Note in the Channels palette that entering Quick
Mask mode creates a channel with an easily-identifiable name.
(Keep in mind that Quick Mask channels in the Channels palette
are like Work Paths in the Paths palette: They go away
automatically if you don't save them.)
With the Quick Mask channel active and visible,
we can edit the mask using any of the painting and selection
tools. When using painting tools, such as the Paintbrush,
Pencil, Paint Bucket, and Gradient tools, remember that black
protects the image, white exposes the image, and shades of gray
leave the image partially exposed. With the composite channel
visible as well as the mask, the red of the overlay represents
the black areas of the mask. Painting with black adds to the
masked area (which will show as red).
Using the Eraser tool or painting with white
will remove red areas from the mask overlay, exposing parts of
the image.
You can even use filters on the mask, just as
you can on any channel. (Applying a filter to a mask can lead to
very creative results, especially if a filter is then applied
after exiting Quick Mask mode.) In this case, the Magic Wand was
used to select the non-red (unmasked) area and the command
Select> Modify> Border was used. With a border of 36 pixels, a
radial gradient (black to white) was dragged from the center of
the image.
Once the mask is ready, pressing Q again (or
using the button to the left of the Quick Mask button) will
return you to Standard Mode. You'll see a selection border that
may or may not match the edges of your mask. Keep in mind that
the "marching ants" selection border shows the extent to which
pixels are at least 50% selected. In the mask channel, that
includes all pixels with a grayscale value of greater than 128.
Note that the selection border shown below does
not match that shown above. The preceding selection border was
used only to modify the channel itself. The following selection
border will be used to modify the image (the composite channel).
It represents the extent of the mask.
In the next image, the command Select>Inverse
has been used to reverse the mask, and then the filter Noise>
Add Noise was applied.
Note that the area around the bear's head, which
had been modified in the mask with the selection border, has
less noise than other areas. Making the mask visible once again,
we can see why.
Once you've created and edited you mask in Quick
Mask mode, you may want to save it. You can do that by exiting
Quick Mask mode and using the command Select> Save Selection
(which opens the dialog box seen below).
You can also save a mask within Quick Mask mode
by dragging the mask to the New Channel button at the bottom of
the Channels palette. That creates an Alpha channel called
"Quick Mask Copy." Note in the figure below that the copy's name
is not in italics. That indicates that it is a regular Alpha
channel and not a quick mask.
As mentioned above, when you have made your
initial selection, you can use the command Select> Save
Selection. This allows you to by-pass Quick Mask mode and simply
create an Alpha channel directly from the selection. That Alpha
channel can be edited just as a quick mask is edited, with the
full range of tools and filters. |