View Full Version : UN-premultiplying footage w/alpha? How?

05 May 2003, 10:16 AM
Hi all!

Given that it should be mathematically easy to make a premultiplied clip UN-premultiplied, based on its alpha and the background color, I'm sure there has to be a way to do this in most professional compositing packages, right?

So..... how? :)

I'm using After Effects (but posted here, hoping the package-independent forum would increase chances of a solution).

Any advice would be greatly appreciated!

- Jonas

05 May 2003, 01:03 PM
AFX as you know have tool for unpremultiply (in interpret footage ...), same as many other compositing apps. If there is no such tool, you can simulate operation by dividing color chanels by matte, where:

red = red/matte
green = green/matte
blue = blue/matte

In this case "semi-transparent" areas wont be completly corrupted (unpremultiply is not perfect proces) during for example - CC.
Result then should be remultiplied by original matte chanel after CC or whatever (in Discreet FFI this is done automaticaly).

For same purposes, you can use Knoll "unmult" plug-in, Sapphire MathOps etc.



05 May 2003, 07:37 PM
Afx only lets you set if you're using premultiplied footage or not.. I cant find any way to unpremult footage.. :shrug: for example in df and shake you can easily create it with a custom tool/colorX. I'd have to set XSI (read: the 3d app of your choise ;) ) to not render premultiplied images..

05 May 2003, 09:31 PM
You can use Knoll "unmult" plugin, or "Remove Color Matting" to remove color fringes from layers that are premultiplied with a color. Well, I am not AFX expressions expert but AFX 5.5 has 4D vector containing (r,g,b,a) and DIVIDE math-op., so I believe it can be done true expressions too, maybe ...


05 May 2003, 09:50 PM
Thanks for your replies, guys! :thumbsup: Gawd, I love this place!

I think I'll have a go at that expression thing, then...

- jonas

05 May 2003, 10:08 PM
Maybe you should try
it seams they have premultiply/unpremultiply plugin. And there is free demo too.
If you achieve something, let us know.


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