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Greenscreen problem in After Effects

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Philip Helander
Greenscreen problem in After Effects
on Feb 15, 2017 at 7:44:48 pm

Hello!

I have a problem using after effects to fix my greenscreen, the edges of my characters closest to the greenscreen have very rough edges of green around them, which I have problem getting rid of. I've mostly used keylight or color range, selective color effects to help me with this but it does not work.

I don't really know what the problem is. I'm aware of that the greenscreen is not as even lit as it probably should be and that one of the characters has a dark brown/green color.



Is it the shadows around the characters? If that's the case, how do I solve this?
I've had this problem before and it would be great if someone could give me some tips and tricks so I can finally move on.

Thanks.


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Simon Ubsdell
Re: Greenscreen problem in After Effects
on Feb 15, 2017 at 8:31:53 pm

There is a general consensus among DPs that you should expose the green screen somewhere between a stop and half a stop below the foreground.

This is really, really unhelpful and in my view flat out wrong.

I don't know if this is the advice you have been following but your green screen is very seriously underexposed and is giving you problems as a result.

Because your green screen value is so low there is minimal differential between the green value and the other channels (specifically the red) and it is the differential which creates the key.

You should aim to at least expose the green screen to around 50 IRE. Your backing reads about 30 IRE (very approximately).

It does not make any sense to underexpose the backing, except where it is giving you excessive spill problems, but most modern keyers can handle spill pretty well.

There is a lot of misinformation about exposing green screens.

Simon Ubsdell
tokyo productions
hawaiki


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Dave LaRonde
Re: Greenscreen problem in After Effects
on Feb 15, 2017 at 9:14:58 pm

Some additional info:

To give yourself the best chance at an easy key, move that green screen away from the tabletop to allow for separate lighting of the subject & background (FLAT background lighting -- no hot or dark spots), eliminate troublesome shadows on the background and to avoid unnecessary spill. Think in terms of 6 feet to two meters. More distance than that wouldn't hurt, but it's not deal-killer.


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Simon Ubsdell
Re: Greenscreen problem in After Effects
on Feb 15, 2017 at 9:19:24 pm

[Dave LaRonde] "Some additional info:"

These are all good points.

But underexposure is the single biggest issue facing most green screen shoots that I have seen - and I have seen a lot. Adequate exposure of the backing would counteract any other issues in a situation like this. It can't be stressed enough how important this is.

And yet there are hordes of DPs lining up to tell you the wrong thing about exposure.

Simon Ubsdell
tokyo productions
hawaiki


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Dave LaRonde
Re: Greenscreen problem in After Effects
on Feb 15, 2017 at 9:28:37 pm

When I used to do a lot of chroma key work, I lit my backgrounds to about 65 IRE... assuming the hottest part of the subject was 100 IRE. No problems except for crummy codecs occasionally.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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