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pre multiplied with alpha

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Nick Chamberspre multiplied with alpha
by on Sep 12, 2011 at 2:03:08 pm


Creating some stock footage and have an diffuse image sequence with the background matching to colour of the footage to stop any edge 'bleeding' and an alpha pass.

I am trying to create a pre - multiplied video format combining the both so I have footage with the RGB channel showing the colour background as well as the footage but also having an alpha channel.

Was wondering if anyone had any tips on getting this?

Using Track Mattes I keep loosing the colour background in the RGB / diffuse channel.


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Kevin CampRe: pre multiplied with alpha
by on Sep 12, 2011 at 2:57:44 pm

hmmm.... i'm not sure if this is posting or not, i don't see it being added to your thread, sorry if you are seeing duplicates of this

it's probably made with a particle system, or at leaset that's where i'd start.

if you are new to particle systems, try cc particle systems 2 (it's bundled with any full version of ae for the last several versions).

the fist thing you'll want to do is open the particle properties and set the particle type from line to shaded sphere, set the birth and death colors to white and the transfer mode to add. i'd also try setting the opacity map to fade in and out and adjust the death size to be close to the same as the birth size.

in the physics properties, i'd set the velocity low (around .1) and set gravity to zero.

i'd also reduce the birth rate (the number of particles that are created) to around 1, and the reduce the longevity to around .5

in the producer properties you can set the size of the producer, as you adjust that, you amy also want to adjust the size of the particles and birth rate.

Kevin Camp
Senior Designer

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Nick ChambersRe: pre multiplied with alpha
by on Sep 12, 2011 at 3:06:02 pm

That has nothing to do with my question. Think you might have the wrong thread mate.

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Dave LaRondeRe: pre multiplied with alpha
by on Sep 12, 2011 at 5:40:59 pm

In order to create transparency, AE can't have any kind of background, so you can't do what you want in AE.

I absolutely stink at Photoshop, but I believe you could do it there.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA

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Darby EdelenRe: pre multiplied with alpha
by on Sep 12, 2011 at 11:26:38 pm

The background color that your RGB channels will be pre-multiplied with is set in your composition settings it's the "Background Color" at the bottom of the composition settings dialog.

If I'm misunderstanding the original post I apologize, hard to tell exactly what you're trying to do.

Darby Edelen

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