Alpha Channels and Transparency 3 (Back to the Basics series)
Masks vs. Alphas
Alpha channels are our most sophisticated way to handle transparency, but they're by no means the only way. Â 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. Â Photoshop, for example, uses the term "Mask" to mean just that. Â But more primitive incarnations of "image masks" in applications like DVDs and some still image files like GIFs use a different approach.
In these image masks, every individual point on our image's "quilt" is either a color or completely, 100% transparent.  Ever wondered why your funny-shaped DVD buttons have jagged edges on their highlights?  Well, this is why. Â
Consider these same two images with transparency. Â Both are just a black square, with an alpha channel that ranges from opaque (0%) to completely transparent (100%), but one uses a web format that has a full alpha channel (PNG-24) and the other uses a web format with only the on/off image mask (GIF). Â Remember that your browser is compositing each image with the white background of this page - so the image should show a nice, clean fade from black to white.
Wow - a stark difference. Â Notice especially how jagged the diagonal is in the GIF. Â This is probably the most important difference between formats with full alpha channels and ones with on/off image masks: on/off image masks cannot accommodate antialiasing. Â I'll write up a bit about antialiasing in another series, but for now, let's just say that's the process that makes jagged edges smooth on screen.
For your convenience, here's a quick list of which formats and software use which kind of transparency:
Full Alpha Channel:
- Final Cut suite
- Marquee
- Photoshop
- PNG-24
- Flash Video (for an example, see Ms. Dewey)
- TIFF
- TGA
- Video codecs with "Millions Of Colors +" listed as an option - the "+" is important.
Limited, On/Off Image Masking:
- GIFs
- DVD Buttons
- PNG-8
No Transparency
- JPEG
- Most output-oriented video codecs
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