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Keying out black background in AE

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Joshua Hughes
Keying out black background in AE
on Jul 5, 2012 at 4:25:13 pm

I have a few clips of things like blood splats and such against a black background and I want to key it out without putting it as Add or Screen as they are semi-transparent.
I do not have the option to premultiply the black :( Even when I go into Interpret Footage the option for the Alpha is grayed out.
Please help!

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John Cuevas
Re: Keying out black background in AE
on Jul 5, 2012 at 4:33:21 pm

You could use the Luma Key effect and set it to key out darker and set the threshold to a low number 16.

Knoll also has a free plugin called Unmult that does a pretty good job of keying out black. Sometimes I might have to throw a levels effect on and adjust the black/white in the alpha channel to make it a little crisper.

Unmult Link

Johnny Cuevas, Editor

"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.

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Dave LaRonde
Re: Keying out black background in AE
on Jul 5, 2012 at 9:06:55 pm

....and just to show you there are typically a couple of ways to skin a cat in AE, I like doing this kind of thing using the Extract effect.

But if there are semi-transparent areas in your graphic, you'll want to get one with NO background on it, as in something with an alpha channel.

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

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Kevin Reiner
Re: Keying out black background in AE
on Jul 5, 2012 at 9:33:48 pm

This was similar to a request that was posed before. Couple options here. Maybe try out Andrew Somers suggestion. I haven't had an opportunity to try it out myself.

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Andrew Somers
Re: Keying out black background in AE
on Jul 5, 2012 at 11:59:03 pm

To add to the link Kevin listed to my earlier posts, I just want to mention that there is a free plugin available at Red Giant that can be helpful and can eliminate some of the steps I indicated in that earlier post especially for transparent items like smoke.

It's called UNMULT

It's free and I highly recommend keeping it in your arsenal of tools.

I'll use Unmult to pull an alpha by unmultiplying an object against black (though I will still use the gamma/blur techniques I discussed in the link Kevin posted to adjust the alpha edge). But Unmult really needs a *solid* black - if your black is not at 0, then you may still need to do some of the tricks indicated on that link.

Black screen is best for:

Objects that are transparent such as smoke or water.
Objects that are well lit and have very little shadow in them.

For instance, a blonde with backlit hair may be a good candidate for black screen, but on the other hand, something like a wine glass may not - while we think of a wine glass as being "transparent", the caustics of the glass actually cause quite a lot of internal shadow and this may make them a bit difficult against black screen.

Things that are *dark* and should be *opaque* are bad candidates for black screen (though in some cases, that kind of object might be good in white screen).

Getting to the OP's question: Blood Splats.

Blood is so dark and you usually want it fairly opaque, I don't think I like the idea of shooting it against black screen.

If you are using something from a library, there *should* be a version with an alpha built-in.

If you are shooting this yourself, and you want to use blackscreen techniques, then don't shoot "dark red blood" INSTEAD, try shooting milk or cream, or flour mixed with a lot of water, then use the TINT plugin to tint it in AE to the desired color. Light it so that you have the appropriate specular highlights and shadow modeling. Keep the *shadow* at least two stops brighter than your black screen, and get the black of the black screen as close as possible to the clipping point.

ALSO: The techniques I discuss here and in the previous thread essentially require that you be working in 32 bit LINEAR colorspaces. IMO working on a gamma encoded space gets in the way.

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Andrew Somers
Re: Keying out black background in AE
on Jul 6, 2012 at 12:20:52 am

I may have skipped over a bit above:

When using the UNMULT plugin, in combination with the gamma/blur technique, You would place the UNMULT plugin at the top of the stack, then CHANNEL BLUR and blur only the alpha channel, and then LEVELS set to ALPHA, and adjust the Alpha's GAMMA.

Use the amount of blur and the alpha's gamma to adjust the edge (i.e. choke/softedge etc). Adjusting the alpha's input black and white point will additionally adjust the density/opacity.

This would generally be for a layer using NORMAL transfer mode.

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