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 QuickTimeŽ 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 commercially available 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.
