 Understanding
Alpha Channels
What Every Video Editor Needs to
Know
by Pat Bailey
Every video editor that does any compositing at all (and who doesn't?)
needs to be familiar with two terms: alpha channels and variable
transparency. Understanding what these terms mean unlocks some of
the most powerful video tools you can use.
What's an Alpha?
Alpha channels are digital bits of information that allow you to
use variable transparency to composite multiple layers of video
and animation, which is a fundamental part of build eye-catching
images. Alpha channels allow you to use variable transparency, which
provides a range of transparency between values of 0 and 255.
What makes this possible is the 32-bit color depth found in certain
formats. This includes the 24-bit color depth required for RGB true-color
(3 channels x 8-bits = Millions of Colors), plus an extra 8-bits
of information that includes the alpha channel information (Millions
of Colors+). Alpha channels require at least 32-bit color depth.
It's easy to understand the RGB parts of an image and, indeed, you
can separate these out and view only the red, the green or the blue
components if you want. The alpha channel is invisible in a composite,
but mask tools can let you see it and it looks like a gray scale
image (Figure 1). In fact, you can export an alpha channel to a
separate file, which is basically just a gray-scale image mask.
Figure 1
Can I use Alphas?
Not all codecs or editing software support 32-bit color depth and
alpha channels. The best way to find out if you're equipped to handle
alphas is to check your hardware or software documentation or visit
the manufacturer's Web site. Two prominent still-image formats that
support alpha transparency are the animation format TGA and the
popular PNG format. Digital Juice products often use PNG alpha transparency
and you can output QuickTime® movies using PNG encoding with alpha
channels as well.
If you've rendered using a 32-bit codec, then the next step is to
import the content into your video editing software. Some software
requires you to import alpha clips to specific layers on the timeline.
Sometimes this is done by applying the alpha directly to a video
clip or on a layer located above the video layer. The main thing
to remember when you're ready to edit with alphas is that you'll
have to key out the alpha information.
Keying Alphas
The method for keying out an alpha is specific to your editing system,
but you'll always want to do a true alpha key and not a chromakey
or lumakey. Although those techniques will work, what you'll wind
up with is holes in your video clip where they shouldn't be or jagged
edges where the alpha was removed. Some, but not all, programs detect
and key out alphas automatically once they've been imported to the
timeline. Alpha transparency is often indicated with a checkbox
pattern in the background of the preview window (Figure 2). Depending
on your editing application, you may have to key them manually.
Figure 2
See Through
Once you've keyed out the alpha information, you'll see the clean
transparent areas with your video clip underneath the alpha layer.
This sort of technique is ideal for music video projects, but is
also great for adding overlays for sports footage and any other
project that needs a professional, cutting-edge visual look. Many
of the lower thirds and overlays in your Editor's Toolkits employ
alphas (Figure 3).
Figure 3
There you have it. Understanding alpha channels isn't as complicated
as you might have thought.
Pat Bailey is a freelance writer and video producer. |
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