Premultiplied Alpha vs. Straight Alpha
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Every wonder what is the difference between a straight or premultiplied alpha channel? Well here goes a little explanation as well as an example when “guess” does not work.
Straight - In a straight alpha channel each of the RGB channels are filled with color and the object only resides in the alpha channel
Example 1:
Premultiplied – In a premultiplied alpha channel the object resides in all channels and the background is usually black or white but it could be any color.
Now when not to you guess in After Effects.
I have imported example 2 into After Effects and hit guess. After Effects and hit guess. After Effects guessed it as straight which was not right. See the white halo around the drop shadow in the image below.
Premultiplied alpha imported as straight, below.
Below, the same image imported as premultiplied with a white background. No more halo.
Damon Ledet Damon@Ledet.com
AE Instructor www.damonledet.com