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Help needed with alpha channel
Could someone be kind enough to point me to a link that does a decent job explaining alpha channel? It’s just one of those things I keep reading about, but can’t quite make it sink in. I tried wiki’s definition, and it helps a little, but I’m having difficulty applying their definition to practical scenarios.
Perhaps a good, basic real-world example might help? Thanks a bunch.
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5 Replies
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Hi, an alpha channel is a black and white image, which, when applied to another image, controls the transparency of the image to which it is applied. Areas of the alpha channel that are white remain visible after the alpha channel is applied. Areas that are black become transparent. For example if you have an image of a rose against a sky, and another image which shows a white silhouette of the same rose against a black background, if you apply the silhouette image as an alpha channel (in a program that understands alpha channels, like After Effects) to the colour image of the rose, the result is a rose with transparency all around it behind which you can add a new background.
And that’s it
Peteroguekeyframe.com
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Thanks, Peter. I’ve got a little graphic that was created by someone else. It’s nothing more than a little bug that sits in the lower right corner of the screen, and it’s completely surrounded by black. (I guess the author of this graphic intended to have the black keyed out, so that it could be used on top of other stuff.) I’ve tried using AE’s Keylight to get rid of the black, but it won’t let me. It seems as though I lack the other image with the graphic being totally white. Is this correct? Where does this other white image come from? Is this simply something I can create myself? (I suppose that would be simple enough, just make a duplicate and crank the brightness all the way up) How do I incorporate this new white image into my original so I can key out that black? (I’m really sorry for such a basic question…I feel like I was out sick the day everyone else learned this)
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Hi Paul.
You do not need to key anything out. To take advantage of the alpha channel you have to put it on top of the footage you want it to affect and set the footage to use it as a matte.
Like this:
Hope it was what you were looking for.
Cheers.
ninguem.
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Thanks for the help. I’ll give this a whirl later.
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The “About alpha channels and mattes” section of After Effects Help contains an explanation and a visual aid, plus a link to a video overview here on the COW by Aharon Rabinowitz.
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Todd Kopriva, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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