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Avid: August 2008 Archives

Masks vs. Alphas

Alpha channels are our most sophisticated way to handle transparency, but they're by no means the only way.  We commonly use the term "Mask" to refer to something similar to the alpha channel above - that is to say, a full range of "see-through-ness" for each point.  Photoshop, for example, uses the term "Mask" to mean just that.  But more primitive incarnations of "image masks" in applications like DVDs and some still image files like GIFs use a different approach.

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.

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 --


As you know, fonts normally come in a single color. Essentially, each letter is a shape - and the computer needs to describe your logo as a shape as well. Therefore, you need to reduce your logo into one or more black-and-white images that correspond to the regions you want the computer to fill when you're using your font.

Incidentally, the very basic techniques we'll use in Photoshop might help you with other matte and compositing work as well.

Installing the free software you need to create your fonts can be a little bit tricky.  Read on to find out how to set up the font software and the auto-tracing plugin on Mac or Windows.


Most modern titling software allows you to import logos or graphics to incorporate with your text.  Avid's Marquee Title Tool does not, at least in any sophisticated way - which is a shame.  Wouldn't it be nice to be able to do the easy 3D manipulations to logos just like you can to letters?

That's what motivated this little tutorial series, but, depending on your workflow, the implications may be even broader.  If you have a set of many logos or shapes that you use often, for example, you can put all of them just a keystroke away.  You can use them as an easy way to make mattes, as some of our other tutorials describe.  For that matter, you could create characters (glyphs) for things like your signature.

Also, the kinds of fonts that we'll be using are vector-based - meaning that they're a set of equations that completely describe each character.  In English, that means that you can zoom the characters up to the size of a skyscraper without worrying about the ugly, fuzzy, pixelated edges that a normal image would show.


* - If you're just doing one-off work with logos (or anything else for that matter), the process we're about to go into is probably overkill.  Interested in a tutorial on vectorizing these more "expendable" images?  Drop us a line, and we'll get one in the pipeline ...

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This page is a archive of entries in the Avid category from August 2008.

Avid: June 2008 is the previous archive.

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