[Definition] Alpha Channel

[Doug Nelson]

Most color modes ("indexed" being the only exception that comes to mind) are made up of "color channels". As counter-intuitive as it may seem, each of these color channels is actually an internal grayscale file that represents the dispersal and intensity of its color. By "internal file" I mean embedded in the overall image file. So, in RGB mode you have 3 grayscale channels, one each for Red, Green, and Blue. In CMYK you have 4, and so on.

You can, however, have additional channels. Just like the color channels, these are simply internal grayscale files. Different programs use them for different purposes, but they can usually be used for selections, masking, transparency, spot colors, or other purposes specific to a given program.

Any of these channels, apart from the color channels, are called "Alpha Channels".

(Note: not to get confusing here, but some older programs, and even some newer ones made for specialty use, store alpha channels as external grayscale files. They are still called alpha channels, even if they're external. By 'external', I mean a separate, discrete file with its own filename.)

[Emjai]

What is the difference between an adjustment layer and alpha channels?

Malcolm

[Doug Nelson]

An adjustment layer is a distinct layer that offers various methods of adjusting the output of the layers below it. By itself, it stores no image information, just adjustment information.

An alpha channel is an actual grayscale image, stored as a channel, visible and editable via the channels palette. Alpha channels are stored selections (in fact, if you make a selection and select 'save selection', an alpha channel is created).

[christo]

Doug,
I just read over your post on Alpha Channels. What is the purpose of having or making an alpha channel?

[Racc Iria]

Alpha channels are used quite a bit in the video industry. If you see graphics on top of a video image (such as the name and description of someone speaking -- called a lower third in the TV bizz), chances are it uses an alpha channel.

Just as the RGB channels are grayscale images used to indicate color imformation, the grayscale alpha channel is used to indicate transparency information. In most cases, white is opaque and black is transparent and shades of gray are semi-transparent. In Photoshop an alpha channel is just a stored selection and an image can have any number of "alpha channels." However, for video, there can be only one alpha channel and sometimes needs to be external, although it is much rarer these days.

One reason an alpha channel would need to be external is if the images have been recorded to tape. Videotape does not support 32 bit imagery. In other words, there is no way to embed the transparency information into the video signal itself, so it needs to be separate. So, the RGB image or video would be recorded and then the grayscale alpha information would be recorded separately later, and then recombined in post-production to key (overlay) the image onto other video. Using an alpha channel is the cleanest way to overlay one image onto another.

Another use (and perhaps the most common one) for an external alpha channel would be during the switching of a live TV program. A switcher is a complex device that controls cuts and dissolves between cameras and other video sources as well as special effects such as keying one image over another. Hence the need for an alpha channel for a clean key.

But as mentioned above the video signal does not allow embedding of transparency information. So when graphics are fed to the switcher, two video signals are sent... one being the RGB color information and the second being the alpha channel information. The switcher recombines these in real time to create a mask (just like a selection or layer mask in Photoshop) to carve out transparent and semi-transparent areas of the RGB signal to reveal other video underneath.

And since the alpha channel signal is separate, this opens up other possibilities such as changing the video source supplying the RGB information which can lead to some very creative treatments.

Sorry for such a long explanation, but the subject of alpha channels and their uses can get very complex and have many many many uses of which I only described two.

If you'd like to know more, please don't hesitate to ask.

--Racc