straight vs pre-multiplied

#1

What is the difference between straight vs pre-multiplied alpha?

In practice, when using AE, when should i use one or the other, and what is the workflow?

Generally, I render elements with an alpha channel, from my 3D package. Then I use “pre-multiplied alpha interpretation” upon import to AE, because this removes fringing, ie the background color that bleeds into the anti-aliased edge of the rendered element. Seems to work, but still I’m not sure this is the best prctice, or exacly what is happening.

thanks for reading!

#2

See this page:

“Alpha channel interpretation”

#3

Very helpful! thank you.

#4

I always use “straight” because if you have effects etc you don’t necessarily want the added “fringe” from the background color. Straight matte is best in my opinion. You only really would use “Pre-multiply” if the comp you’re pulling it into has the same background as what you rendered from but even that is a little dicey.

#5

hmmm, it is still confusing.
In after effects, the interpret footage dialogue allows you to “interpret footage as” pre-multiplied or straight. with footage created with the default settings in my 3-D app, which is very commonly use, I need to use “interpret as pre-multiplied” in order to get the results you describe. Otherwise I get fringing. What I do is interpret as pre-multiplied, then I select the background color, the color that is rendered into negative space, generally black, as the pre-multiplied color. The results are perfect, as far as I can tell. But then again everybody keeps saying use straight, not pre-multiplied, so I’m just wondering what information I am not understanding.

#6

The 3D application would have to export the render as straight to make the difference. If the 3D application can export straight, you might give it a try. It should fix the problem when you import with Straight interpretation.

Theres also a good video that explains the whole straight vs. premultiplied thing at:

http://library.creativecow.net/articles/rabinowitz_aharon/straight_vs_premult/video-tutorial.php

I like the way it’s summed up. Straight is better, but scares clients. Premultiplied looks great, but can cause issues. :wink:

#7

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