Tutorials
Save Time Tweaking
04/06/2004
Sprucing up a landscape scene only takes a few seconds. Select
the blue sky. Easy! Put some cloud formations in it. You think
you're done until a friend or client says, "Hmm, I think
I want a different sky." Not so easy to select the sky now.
If you had made an Alpha channel when you selected the original
sky, you wouldn't be in this predicament. On today's 'Savers,
I'll show you how to create one and make reselecting a snap.
Channeling colors
Channels containing color information make up every image, and
you can see them on the Channels palette. Open an RGB file and
you'll see Red, Blue, and Green channels as well as an RGB composite.
Open a CMYK file and you'll see Cyan, Magenta, Yellow, and Black
channels as well a composite. Open a Grayscale image and you'll
see a single channel.
The Alpha dog
You can add a lot of additional channels to a Photoshop file,
called Alpha Channels, which contain no color information.
You can use
these purely grayscale channels as specialized, reusable selections
or masks.
Select part of an image with any of the selection tools, such
as the Lasso, Rectangular Marquee, or Magic Wand. You can also
make
selections based on color by choosing Color Range from the Select
menu. You can select by sampled colors, specific colors, highlights,
mid-tones, shadows, or colors out of the Gamut.
When you select an area of an image, any modification to the
image will apply exclusively to that area. You can colorize
it, filter
it, scale it, or even duplicate it. Anything you wish to do will
be done to that area while leaving the unselected areas of the
image untouched.
If you deselect it, the selection is gone. If you save the selection
to an Alpha channel, you can recall it at any time. To save
a selection, choose Save Selection from the Select menu to
store
it to an Alpha
channel.
Mess with gray
As a grayscale channel, the Alpha channel contains 256 levels
of gray, which can be very useful.
Anything white in an Alpha channel exposes the image to an alteration
just as the normal selection process does. Black protects the
image from any alteration just as the unselected portions of
an image
are protected.
You can apply gradients with the Gradient Tool, paint with the
Paintbrush, or use any of the other tools to select areas of
your image. (No, they aren't in color.)
Pesky file formats
When you change the format of your image for output to something
else, you'll want to discard the Alpha channels. Some file formats,
such as EPS, will automatically strip them away. Always keep
a PSD (Photoshop native format) file of your work in your archive
so you have all your Paths, Layers, and Alpha channels if you
need
them later.
Expose more of your subject
When you apply a Layer Mask to a layer, either from the layer
palette or the Layer menu, you allow portions of a layer to
be seen. In
the Layer Mask, you use any of your tools to apply tones from
white to black with two 254 additional grays in between, just
like in
the Alpha Channel.
Where the mask is white, you can see the contents of that layer.
Where the mask is black, the contents of the layer are hidden.
Where the mask is gray, you see a combination of the masked layer
and the layers underneath it. The lighter the gray, the more
you see the contents of the layer. The darker the gray, the
less you
see of the layer.
Alpha Channel takes on the Layer Mask
The bottom line: the Alpha Channel and Layer Mask operate in
similar ways with different end results. White in the Alpha
Channel exposes
an image to an effect. White in the Layer Mask exposes how much
of an image you see. Black in the Alpha Mask protects an image
from change. Black in the Layer Mask hides an image from view.
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