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Creating
an Alpha Channel and other Basic Selection
Tricks
A teacher once told me,
Photoshop's real strength is not in
any of it's editing tools, but rather
in it's robust selection engine. In
truth, if you can't select something
to manipulate, then how do you expect
to manipulate it?
Alpha Channels
are saved selections within Photoshop,
classically used to separate a background
from a foreground. Some file formats(.tga,
.psd) naturally may incorporate an alpha
channel, or the ability to save an alpha
channel into their compression, but
it's always good to know how to create
one yourself. This tutorial will explain
a few different selection techniques
while creating an alpha channel.
Here's the image we're going to use:
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Before
doing the tutorial: In
your 3d program. Set your background
color as something drastically different
from any color in your image, if possible.
The "hot magenta" or "green
screen" colors work well.* Render
out your 3d piece to a bitmap format,
preferably not a targa file (.tga)
so you're not cheating, since usually
targas will prompt you if you want
an alpha channel, and that takes all
the fun away.
First things first, you need to decide
what you need to select, and what
you're going to do with it. In the
case of an Alpha Channel, we want
to select the background so that we
can separate it from the rest of the
image, and easily manipulate it at
any time. The background in this picture
is all the lightlightlight blue.
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*The
reason my background color is not drastically
different is because of two reasons:
1: I'm no fun and used a Targa file
format to skip all this (this is an
offshoot of the
Post tutorial)
2: I'm a stupidhead.
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The
EASY way to select the background:
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There's
always an easy way out, and this is
it:
Go to Select>Color Range
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This
will allow you to select all the pixels
of a certain color range in your piece.
A:
Use this tool, similar looking to the
standard eyedropper to sample the color
you want to select.
B: This is a good sampling area.
C: Adjust the fuzziness slider
to widen or narrow the selection set.
Adjust this until only the background,
but as much of the background as possible,
shows up as 'white' in the preview window.
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Click
"OK" when you're ready and
you'll see the selection on your piece,
just waiting to be saved.
Open your Channels Palette (found under
Window>Show Channels, if it's not
already open)
and click on the "New Channel"
Icon (
). This creates a new channel, called
"Alpha 1" by default. Your
image will also turn black, by default,
but you'll still see your selection
marquee.
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Now
all you have to do is click your paint
bucket tool and fill the selection with
white, or go to Edit>Fill and select
white, 100% opacity. Click on the RGB
Channel, and you're image will pop back
up again.
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When
you save your image, you should save
in the native Photoshop format (.psd)
to preserve the alpha channel, and
any other saved selections you might
have.
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More
EASY ways to select stuff:
In a situation like the one above, it
might be best use other methods of selection.
All of the selection methods, no matter
how easy or advanced, have their goods
and bads, so choose carefully.
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Magic
Wand
The most common
method of selecting large areas, the
Magic Wand can do easily as good as
a job as the "Color Range"
method, if you tweak the tolerance
enough. Otherwise you'll end up with
a "fringe" along the outside
of your selection, as shown in the
picture. The Magic Wand also falls
short on smaller, more precise selections.
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Lasso,
Magnetic Lasso,
and Polygonal Lasso
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The
Lasso Tool is effective when used
with the SHIFT and ALT buttons, but
is time consuming, and aggravating if
you're impatient.
The
Polygonal Lasso Tool is handy for
a very geometric shape, or a shape with
geometric elements. Holding ALT while
using this switches temporarily to the
regular lasso.
The Magnetic Lasso Tool is effective
when working on an image with high contrasting
edges. Selecting the background out
of the image above is easy since the
background is so light, and the lego
fort so dark.
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Like
the Magic Wand, if you're reading this,
you're probably already familiar with
the Lasso toolest.
The Lasso tools are the most basic selection
tools aside from the "shape selection
toolset". Each one is used for
a little different purpose.
The downside of all of them is that
they are time consuming, and also can't
partially select pixels.
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Similar
By
selecting a small area only of the color
you want, you can then go to select>similar
to add all areas of similar color to
the selection.
"Similar", like most of Photoshop's
tools, works off of the basis of luminosity.
This means you can easily pick up other
colors too if you're not careful. In
this case, using similar leads to the
Legoman's shirt being partially selected.
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There
are plenty of other ways to select stuff,
some much more advanced than the ones
listed above, but those are for another
tutorial.
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