Masks and Selections (Part 2 of 3)
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.
In Part 3 we'll work with a practical
exercise in creating and editing masks.
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