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Premultiplied vs. Straight Alpha Import

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Max Jackson
Premultiplied vs. Straight Alpha Import
on Nov 2, 2010 at 5:39:00 am

Hi CC,

Okay, I found this post about what the difference is:

http://toolfarm.com/tutorials/alpha.html

From what I can tell is that the importing process makes an alpha of each color channel before importing it vs. an alpha of each object solid or with a matte.

In using Adobe Flash for web purposes this seems a bit similar to exporting as PNG-24 (hi-rez) vs. JPG. The reasoning in Flash animation is commonly due to whether the designer wants an image with aliased alpha or a square matte image.

Physically for the image, is that sort of the case when deciding to premultiply an image with transparency? That is to say, the decision is based on whether it's matte like a JPG or transparent like a PNG with alpha?

I was wondering what the rationale for either would be? I looked at Mark Christiansen's example and like the bottom (incorrect) one so I was wondering what I was looking for or wanting for the import.

Thanks for any feedback. :)


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Todd Kopriva
Re: Premultiplied vs. Straight Alpha Import
on Nov 2, 2010 at 6:37:38 am

> From what I can tell is that the importing process makes an alpha of each color channel before importing it vs. an alpha of each object solid or with a matte.


That's not a correct description.

The gist of the difference is this:
Premultiplied color channels include some amount of the background (matte) color in areas of partial transparency. Straight channels don't.


The oversimplified answer about what to use is that straight channels are better but premultiplied channels are compatible with more software.

There's much more information here. I recommend reading it. I'd type it here, but since I wrote the material that I'm pointing you to, I'd just be repeating myself.

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Todd Kopriva, Adobe Systems Incorporated
Technical Support for professional video software
After Effects Help & Support
Premiere Pro Help & Support
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Max Jackson
Re: Premultiplied vs. Straight Alpha Import
on Nov 2, 2010 at 8:51:26 am

(After reading)

Ahhh, I think I get more from this now. So premultiplied is kind-of a technical word for matte-transparency. Which in plain English makes very little sense.

The matte color in the example you gave is black, hence the premultiplied aliasing is black. If it were white or red, it would thusly preserve an alias of that color instead, correct?

As for use, it seems like it's mostly an importing protocol that prevents a halo affect. Right? That's how one might tell they've picked the wrong import setting. I'm still a little fuzzy on the purpose on one versus the other though.


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Todd Kopriva
Re: Premultiplied vs. Straight Alpha Import
on Nov 2, 2010 at 3:27:16 pm

> As for use, it seems like it's mostly an importing protocol that prevents a halo affect. Right?


Yep.

I'm still a little fuzzy on the purpose on one versus the other though.


Some software doesn't understand straight channels. When exporting for such software, use premultiplied channels on export. But, if the software that will be making use of your output can understand straight channels, use those; they're better at preserving color and transparency details.

---------------------------------------------------------------------------------------------------------
Todd Kopriva, Adobe Systems Incorporated
Technical Support for professional video software
After Effects Help & Support
Premiere Pro Help & Support
---------------------------------------------------------------------------------------------------------


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Max Jackson
Re: Premultiplied vs. Straight Alpha Import
on Nov 2, 2010 at 4:54:38 pm

I see, so the guts of it is watching for an undesired halo effect based on the importing/exporting between software because not all software can export transparent images with straight unmultiplied aliasing.

Wow, that's a long one.

I got it, right? Is it mostly a Quicktime thing to look for?


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