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Green screen lighting- edge lights w/ + magenta?


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#1 Jason Fobart

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Posted 29 September 2010 - 10:41 PM

I shot my first green screen band performance a few weeks back, unfortunately on a less than shoestring budget and we had to make a lot of compromises. Unfortunately it bit us in the butt and we've all learned a few lessons (including we needed more budget to do the green right! :-)

But as part of diagnosing our problem with the keying in post, the footage was sent to a colorist for his opinion on if he could do anything to help the editor make a better key-out. The colorist came back with some suggestions for lighting changes on the reshoot, but one thing that caught my ear was "use + magenta on all your edge/backlights"

Really? Anybody have any thoughts or tips on this?


Basically overall the footage is unusable because we tried to wing it, and we failed. Oh well, I'm learning my lesson. But where can I do some research and learn more about how to light and rig a good green screen setup for a 4 piece band performance (ie, not just a talking head against a backdrop which is all I've been able to find people talking about on the internet). I need to learn how to do a whole sound stage, not just a 5'x5' wall. Any ideas on where I can learn this? I know that practice and experience are the mothers of all knowledge, but I need some starting points to work from because obviously I thought I knew how to sort of make it work (which I actually did), but the lack of budget didn't allow me to utilize a bigger space (for more room to work & light with) or provide enough sources for the different things I needed to do (background illumination, key lights, edge/back lights, etc)- I was using sources for multiple purposes and it only worked 1/2 the time.


In another thread someone had suggested fxphd.com, but I'm not sure that'll be useful. It seems like that's more about learning to do effects yoruself in the computer, etc.... I just want to learn about how to light a stage versus a wall for chroma-key work.

Thanks for your help guys, I really do appreciate it and any suggestions you can offer!
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#2 Aaron Solomon

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Posted 02 October 2010 - 04:52 PM

If you look at a color wheel, magenta is opposite green. By using hair lights with magenta on them, you make it easier to differentiate the people from the background, and not have greenspill on them....That's what I think anyway.
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#3 Andre LeBlanc

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Posted 02 October 2010 - 06:28 PM

Adding magenta can negate *part* of one problem, and add a new problem. Yes, for areas with green spill, you negate it. However, just as it's difficult to control spill, it's also difficult to control exactly where the magenta hits your subject. So areas that don't have green spill to begin with now have magenta in them. This creates the unintended compositing problem of having an additional color that you didn't want to begin with. Perhaps the colorist you spoke with has some ideas around this issue, but just a heads up that it can be a problem.
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#4 Jeremy M Lundborg

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Posted 02 October 2010 - 07:39 PM

Adding magenta can negate *part* of one problem, and add a new problem. Yes, for areas with green spill, you negate it. However, just as it's difficult to control spill, it's also difficult to control exactly where the magenta hits your subject. So areas that don't have green spill to begin with now have magenta in them. This creates the unintended compositing problem of having an additional color that you didn't want to begin with. Perhaps the colorist you spoke with has some ideas around this issue, but just a heads up that it can be a problem.



I second this. You end up having issues with both colors instead of just one. I used to hear this from a lot of old school gaffers who worked before digi green/blue screens and digital keying. The best advice I can give, is to use a big enough screen to cover your action and at the same time allow enough distance between the screen and your subjects to lessen spill. If you don't have the money to do that, seemingly the issue here, any sort of backlight/kick light that registers will also help a great deal. That is of course if it works within your lighting scheme appropriately. Basically it defines the subject further and also helps negate some of the green spill by simply being present in it's place.
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#5 Jason Fobart

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Posted 02 October 2010 - 08:00 PM

Thanks guys, I appreciate the advice.

In the past few days the footage has been sent to a second editor who's having MUCH better results- so maybe the first guy just isn't great at keying?? We'll see, he's diving in to the footage this weekend and hopefully I'll see a first pass Monday.

re: magenta
Yeah, I thought it made sense a bit, but if you don't have spill, then you've introduced MORE color issues in the other direction. Something to think about the next time I'm on a green set that isn't too big.

And yes, I was working on a set that was too small on not properly lit- but when you've only got Y dollars to work and need to have X, you do what you can.


Thanks again guys, and I'll be happy to take any other input.
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#6 Scott Fritzshall

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Posted 06 October 2010 - 01:55 PM

The best way to light a subject in front of a greenscreen is to replicate as closely as possible the lighting that they would be in if they were actually in the environment you're comping them into. If you're putting them on a spaceship or a beach or a skyscraper, would there be magenta light on them? If not, don't use it. Even with a perfect key, your composite will suffer terribly from being improperly lit.
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#7 Jim Menkol

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Posted 22 October 2010 - 10:48 AM

As Scott said, priority is getting the light right. Know what your background image will be before you go into photography--without the right lighting your key will look like crap.

As far as the minus green goes, avoid it at all costs since it will give you another problem to work with in post; adding a backlight to separate the subject from the background may not fit in with the scene. Instead, place your subject as far away from the screen as possible--not only does this reduce the possibility of spill on the subject, but it blurs the screen and allows for a more even key.

You can also make a number of adjustments to your mask after primary keying, such as choke, shrink, pre blur, white/black max/min, etc... They're not a perfect solution to a bad key, but they help. What are you using to key?
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