Alpha Channels and Transparency 1 (Back to the Basics series)

By Alex on August 18, 2008 3:00 PM | Permalink | TrackBacks (0)

Whether you're using titles, creating supers, or doing any other sort of compositing, you're trying to tell the computer to show some parts of an image but to hide other parts.

I've been surprised by the number of folks who were never really taught how that process works from the computer's point of view. After the jump, an easy little primer that may help you understand --

The Very, Very Basics

Often, when we're editing video or creating effects, we'll use a process called compositing. It's as simple as combining more than one video or image resource into a single resource. Creating a dissolve between two pieces of video, for example, means that you're mixing frames from each of your source videos in order to make the output. Superimposing a title on a piece of video, likewise, means that you're telling the computer to "cut out" around the title and let the video show through.

Remember those old overhead transparencies in high school? They were see-through sheets of plastic, and you'd draw on them with colored markers. You could tell the difference between a nice, bold, new marker and an old marker that was running out, because the old one would draw lines that were more see-through. And if you stacked several transparencies - say, a map and some kind of key, or a piece of paper - you the student would see the combined (let's call it "composite") result on-screen. Perhaps you see where I'm going with this.

So let's say one of your slides is, I don't know, a screenshot from your latest film. And you've got another transparency that you're scrawling the title on. To show the class, you put your screenshot on the projector, then stack your title on top of it - just like you might do in, say, Final Cut or Avid. In essence, then, what we need to tell the computer about our title is (1) where the marker has scrawled and (2) how see-through each different part of the title is.