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RichH
03-17-2006, 11:30 AM
Hi All, I've got a cgi image from a third party which is premultiplied against black - i've also got an alpha channel. I would like to CC the image and was wondering if it's possible to UN-Premultiply (i.e divide r/g/b by alpha) either directly in AE or by gettin a plug-in from somewhere. I'm sure this is a common problem, any help would be greatly appreciated.

p.s I'm turning the alpha into greyscale and using it to matte out portions of the background in multiply mode.

Cheers, Rich

beenyweenies
03-17-2006, 06:45 PM
Hi All, I've got a cgi image from a third party which is premultiplied against black - i've also got an alpha channel. I would like to CC the image and was wondering if it's possible to UN-Premultiply (i.e divide r/g/b by alpha) either directly in AE or by gettin a plug-in from somewhere. I'm sure this is a common problem, any help would be greatly appreciated.

p.s I'm turning the alpha into greyscale and using it to matte out portions of the background in multiply mode.

Cheers, Rich

Not sure if this is what you mean, but if you right-click the footage in your project window, select "Interpret Footage - Main" you can select Premultiplied.

beaker
03-17-2006, 09:08 PM
need a plugin called "unmult" and luckily it is free :)
http://www.redgiantsoftware.com/unmult.html

beenyweenies
03-18-2006, 06:27 PM
need a plugin called "unmult" and luckily it is free :)
http://www.redgiantsoftware.com/unmult.html

Duh, I didn't even read the whole question... just saw "premultiply"

Vympel
03-18-2006, 07:32 PM
Duh, I didn't even read the whole question... just saw "premultiply"

"as wondering if it's possible to UN-Premultiply (i.e divide r/g/b by alpha) either directly in AE or by gettin a plug-in from somewhere. I'm sure this is a common problem, any help would be greatly appreciated."


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You can make this with AE tools, not?

avinashlobo
03-19-2006, 09:10 AM
Sounds like you need to place the matte as a layer above the image layer, and then choose Alpha Matte from the Track Matting options on the image layer. Provided the alpha is accurate, you should get clean transparency.

RichH
03-20-2006, 07:44 AM
Thanks beaker, will give it a try

Kind regards

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