Get your alpha straight
20. February 2017
In this post, I’ll show you why it’s better to use straight alpha channels and what happens if you don’t.
Two different kinds of alpha channels
In After Effects, there are basically two different ways to create files with alpha channels: Premultiplied and straight.
The difference between those two is how they save their transparency information. Adobe says this:
“With straight (or unmatted) channels, transparency information is stored only in the alpha channel, not in any of the visible color channels. With straight channels, the results of transparency aren’t visible until the image is displayed in an application that supports straight channels.”
“With premultiplied (or matted) channels, transparency information is stored in the alpha channel and also in the visible RGB channels, which are multiplied with a background color. Premultiplied channels are sometimes said to be matted with color. The colors of semitransparent areas, such as feathered edges, are shifted toward the background color in proportion to their degree of transparency.”
Check out this link and look for premultiplied or straight: https://helpx.adobe.com/after-effects/using/importing-interpreting-footage-items.html#WS3878526689cb91655866c1103906c6dea-7f7ea
That can cause ugly black lines and halo effects on your footage. Also, some applications can’t even handle premultiplied alpha channels, such as Avid Media Composer.
Here is a simple text I have created in After Effects and rendered as a TIFF sequence. One time with a premultiplied alpha channel and on time with a straight alpha channel. This is what happens when you bring the footage into Premiere Pro or Avid Media Composer (it’s the same result in both applications):
The premultiplied alpha channel onshows black outlines around the white text, but the straight alpha has a perfectly clean edge.
Here is another example with a feathered background and feathered objects:
In this example you can see very clearly how the premultiplied alpha channel creates an ugly black halo around the feathered parts of the image. The straight alpha shows our picture exactly as it’s supposed to look like.
In Premiere Pro, you can change the way your alpha channel is interpreted by right-clicking on your footage and choose Modify –> Interpret footage. In the new window that pops up, you choose to conform your alpha premultiplication to premultiplied alpha.
Click ok, and your footage will look perfectly fine, just as the one with the straight alpha channel. I don’t know why Premiere can’t detect the premultiplied alpha channel on it’s own, so you have to do that extra step.
In Media Composer, this option does not exist. Avid Media Composer can not handle premultiplied alpha channels correctly at all.
So in my opinion, there’s no scenario in which it makes sense to use a premultiplied alpha, especially if you work with Avid Media Composer. I can’t speak for Final Cut or 3D applications like Cinema 4D or Maya, but between After Effects, Premiere and Media Composer, there is no reason to use a premultiplied alpha channel. The straight alpha channel will always give you better results with less effort.
I hope this post is helpful to some of you guys out there. If there are any questions left, or you know a situation where premultiplied should be used instead of straight alpha, please let us know in the comments.