Alpha Channels and Transparency 2 (Back to the Basics series)

By Alex on August 19, 2008 1:49 PM | Permalink | TrackBacks (0)

Alpha Channels

As we first start thinking about transparency in a computer sense, let's go back to the beginning of how computers think about our NTSC broadcast images. You might remember that, for our purposes, a computer thinks about videos as a sequence of many individual images (frames). It thinks about each frame as a big rectangular "quilt" of pixels, or individual dots of color - for our purposes, 720 dots wide and 480 dots tall. And it thinks of each of those dots of color as the amount of red, green, and blue in the color.

Each primary-color component of a single color has a value. For example, the dark green we picked in the above image corresponds to these three values mixed together:

Notice how each number value corresponds to somewhere along a scale of an individual primary color. Each of these scales is called a "channel" - and the computer thinks of transparency in exactly the same way.

So if we call the red channel "Red," the green channel "Green," and the blue channel "Blue," what shall we call the transparency channel? Well, how about the "Alpha" channel**?

Now, then, if our video has an alpha channel (A) as well as its color information (RGB), the computer will know exactly how much of the image to show and how much to "see through" to other images.