View Full Version : Alpha Channel confusion

05-26-2011, 03:25 PM
Yes this is another "Straight" v. "Premultiplied" question. I've read lots about the subject. For years I thought that I understood the difference between the two, but have been doing some playing around lately and discovered that maybe I don't fully have a grasp on this.

I rendered out two identical images from 3D software, but with different alpha channel types. One is a TIFF with "Straight" alpha channel and the other is a TIFF with a "Premultiplied" alpha. I imported both images into After Effects, and then interpreted the alpha channels correctly for each ("premultiplied" for the premultiplied alpha image, and "straight" for the straight alpha image).

Now, I thought that these two images would look exactly the same since they were interpreted correctly, but they actually look different. The premultipled image looks lighter. Again, I thought in theory these image should look exactly the same, yet now I can't seem to figure out why they look different.

Can anyone explain this to me?

05-26-2011, 08:16 PM
When in doubt, I generally assume that straight is the most color-correct way to move images between programs. With premultiplied you have to worry about how each program exports and interprets the image respectively, and it can cause headaches. (Have you tried switching between black and white BG colors for premultiplied)

The best description for alpha channels i've seen is here:

I think it's pretty accurate to break it down to "Straight can be more accurate, but scares clients, Premultiplied can look just fine, but can cause issues.

05-27-2011, 09:17 AM
It is said that premultiplied alpha images are more "inaccurate" when it comes to maintaining colours, this has never been a problem for me since i think the difference in colour is very small.

However it's in the semi transparent areas you should look for the difference.
I'm actually about to post a thread about an issue i've found concerning the premultiplied alpha interpretation in AE. This issue might give reason to some confusion here so let me post the thread and see if there's actually some sort of bug going on with the premult setting.

05-27-2011, 10:32 AM
Have a look in this thread

05-27-2011, 11:49 AM
Thanks for the replies. Yep the tests I did involved a largely semitransparent object. Glad it's not just in my head, thought I was going crazy.

Swahn, the first tests I did were with images that had been rendered with "linear" workflow. I did some more tests with the same exact renders but without linear workflow. EDIT: Posting the results in your other thread.

05-27-2011, 12:10 PM
Actually, never mind my post. I think my "issue" had nothing to do with yours ;)

05-27-2011, 12:13 PM
Actually, never mind my post. I think my "issue" had nothing to do with yours ;)

Actually I think it solved my problem. Like I said, I thought I was going crazy as the two images should look the same. Well, they do look the same in AFX if linear workflow isn't used.

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05-27-2011, 12:13 PM
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