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Key just part of an image

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Wladimir BonsegnoriKey just part of an image
by on Feb 2, 2010 at 4:19:20 pm

Hi!

I have the following problem: I want to key a white background behind a person. So I don't have green or blue, but just white. The same white that I have on the background, I have also on the person's face. So is it possible to mask out the skin of the person for the key effect (applying the key to the whole image except for the skin)?


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Todd KoprivaRe: Key just part of an image
by on Feb 2, 2010 at 5:01:11 pm

You can use a hold-out matte.

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Todd Kopriva, Adobe Systems Incorporated
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Dave LaRondeRe: Key just part of an image
by on Feb 2, 2010 at 5:12:41 pm

Todd mentions the use of a Hold Out Matte. In the example he cites, the subject doesn't move.

I bet your person moves, so the job gets much harder... and you're not going to like this.

First, try to isolate the subject on a duplicated layer through a rough, quickly-drawn animated mask around the face, called a garbage matte. Then use some combination of the levels effects to make the face black on a white background. You can then use this as an inverted luma matte for the face.

If that doesn't work you can also Rotoscope the face, animating an extremely detailed mask around the face, almost frame by frame. It is difficult, painstaking and time-consuming.



I have no idea why people think, "I don't have a green or blue screen, so I guess I'll just use a white background. That's good enough, isn't it? Sure it is!"

Dave's Stock Answer #2:

When you're out on a shoot, and you say, "we'll fix this in post" without knowing PRECISELY HOW you're going to fix it in post, don't shoot it! You'll only end up shooting it over again.

Since post typically costs three times the cost of production, fixing something in post is not a way to save money, but rather a way to spend more of it.

And, before you say "well fix it in post," always consider who's doing the work, especially if you're the one doing the work.


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


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Wladimir BonsegnoriRe: Key just part of an image
by on Feb 2, 2010 at 9:54:34 pm

Hi Dave,

the best way to learn is making mistakes. So If you don't know how to do it in post, shoot it and then you'll have a lot of trouble in the post and then you'll never forget what to do next time.
That's the best school, in my opinion.
Anyway I didn't shoot it white to key it later. I had the idea to key in the post, so that's why I'm asking here if it's possible to do so.

I'll try out your technique and see if I manage to key the background. Both persons have lots of hair, so maybe it won't be that difficult to separate the background from them because of the high contrast between the hair and the white.





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Dave LaRondeRe: Key just part of an image
by on Feb 3, 2010 at 5:19:31 pm

[Wladimir Bonsegnori] "Anyway I didn't shoot it white to key it later. I had the idea to key in the post, so that's why I'm asking here if it's possible to do so. "

It MIGHT be possible. If the footage isn't critical to your project, it's a good learning experience. However, if it is indeed critical, you may have shot yourself in the foot.


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[Wladimir Bonsegnori] "...maybe it won't be that difficult to separate the background from them because of the high contrast between the hair and the white. "

Depending on the type of footage shot, don't expect miracles. Even a full head of hair has hard-to-matte wisps in it. I presume you know enough to use AE's various channel effects to look for the most contrasty channel, apply levels, invert it, change it to an alpha channel, perhaps even add a bit of matte choker and blur.





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


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