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Using alpha channel with keylight

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Esther Casas
Using alpha channel with keylight
on Nov 12, 2009 at 7:58:35 pm

Hi everyone,
I have a simple shot with bluescreen but I am not an expert with keylight and like t know what is the best thing to do:

the shot: people moving in a park with the background being a bluescreen behing trees.

problem: i can't keylight all the shot as some people is wearing blue clothes

suggestion I got: someone told me to create an aplha channel and then pre-comp...

what i did: first keylight the bluescreen on footage.. Then duplicate layer, and then create from this layer the alpha channel: the problem is still the same, the blue form teh clothes is still keylighted...

question: at the beginning i started doing a mask around thebackground and keylighted that mask, but as the mask is not very easy to do in this shot, someone suggested to do the pre-comp with adding an alpha layer.So, what is the step-by step to do alpha channel correctly and pre-comp?
thank you so much!
e.


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Dave LaRonde
Re: Using alpha channel with keylight
on Nov 12, 2009 at 9:14:31 pm

You may be better off just getting the shot over again. If that isn't possible, you may be in for a lot of work.

If the key looks good except for people in blue clothes, you may have to animate masks around those people on a duplicated layer. This is a process known as rotoscoping. It is very painstaking, tedious work, and it takes a long time.

Rotoscoping is the one sure method for solving your problem. There may be others, but we can't see the footage, so we don't know for sure if there would be any better way.

To be honest, I have no idea of any benefit your friend was thinking about by saying, "create an alpha channel and then precomp".


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 editing.


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


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Esther Casas
Re: Using alpha channel with keylight
on Nov 12, 2009 at 11:57:40 pm

Hi, well, actually this was thought very well,so that is not the issue here.
I just wanted to know how to do an alpha channel, as it was one suggestion i like to follow.
does anyone can tell me how to do it?
the shot is just a sky to keylight, and like to know how an alpha channel will work together with keylight and why?
thank you!
E.


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Dave LaRonde
Re: Using alpha channel with keylight
on Nov 13, 2009 at 3:59:05 pm

[Esther Casas] "the shot is just a sky to keylight, and like to know how an alpha channel will work together with keylight and why? "

You got bad advice. You don't want to do that.

You are losing portions of the shot because people wore blue clothes. Keylight doesn't do a good job of keying blue things against a blue background. So you must somehow put those lost portions of the shot back.

An alpha channel creates TRANSPARENCY. You don't need transparency, you need opacity. So you must do it by animating masks on duplicated layers. It will put the people with blue clothes back into the shot.

Your description of the shot makes it sound like the chroma key background is there for sky replacement, and the blue-clothed people don't interact with the background. You may be able to get by with easy-to animate garbage mattes around the problem people.

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


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