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Essential elements of setting up a workable greenscreen key
Repost of original by Bill Davis on Jun 4, 2009 at 12:33:56 am
[Help in] …understanding the essential elements of setting up a workable key.
If I had to put a single aspect at the top of the list of what makes a good key easy to pull, it would probably be the SHOT GEOMETRY.
Let me explain. You have three key surfaces to deal with. The key screen at the back. The subject in the middle. And the imaging CCD behind the camera lens.
What you’re going to discover is that one HUGE factor in pulling a clean key is how the distance between these three planes interact. Let’s say you have 10 feet of usable room between the CCD and the wall. You’d expect to put the talent and the 5 foot mark for some separation from the key and you’ll be fine. But you won’t. Not ONLY will you likely get spill from the green screen in 5 feet – but since if the talent is at 5 feet from the lens, about the ONLY shot you’ll get without a super wide angle lens adaptor is a medium head and shoulders frame. And if you do that – your idea about turning the camera on it’s side will result in you giving the talent almost NO arm room, so you’ll typically constrict the talent’s ability to gesture.
So you move the camera back to 15 feet. And get yourself 7.5 feet fore and aft the subject. But suddenly the geometry of the shot means that the key screen you thought was large enough at 10 x 10 shrinks down to the point where an arm gesture, once again, will fall OFF the key screen.
Suddenly you start to understand why full length green screens are typically VERY big – and are placed in VERY deep studios where there’s often 30 or 40 feet of depth available.
That’s what makes the geometry of a good green screen studio work well – and also why most of them don’t have a few light fixtures illuminating them, but rather bank after bank of broad, soft color balanced sources.
The geometry of good key shots is relentless. Distance from camera to subject is critical to allow you to get as much of head to toe shot as you need. And further distance from the talent to the screen is important to avoid spill.
And trying to somehow overcome these lighting and shot geometry issues with ideas like gelling lights and turning cameras on their side are looking at solving issues that aren’t the primary ones you really have to solve to pull a good key.
My advice is to FIRST set up your space and your shot as best you can. Learn the LIMITATIONS of your lens verses the geometry of your shot. Accept that all you may be able to do is get a head, or head and shoulders shot with the space you have. Then concentrate on lighting the actual SHOT – rather than wasting too much time on the tricky stuff like double gelling thisaway or turning the camera thataway.
My 2 cents anyway.
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17 Replies
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Well I just wanted to add to this something most people already know; that you only need enough green or blue wall to have a few inches of clear color around the widest part of the person; if they don’t gesticulate wildly with arms far out to the sides, you can make do with a very narrow backdrop, because you can mask and matte out the rest of the room.
I get where Ian is coming from, that if the room is tight for space, your talent is just standing there without a lot of movement, a landscape frame composition throws away more of the effective area of the screen to the left and right, than if you shoot “on edge” in a portrait style composition, with the camera on its side. You get more pixels applied to the essential part of the frame, the person, so in post you can zoom in a little tighter if needed and retain a good picture… However, if your shot is already the “right size” to fit the eventual composite, I’m not sure it is worth the extra effort to shoot it sideways and play with scaling in post. It certainly won’t hurt, but it may only improve things by… say, ten percent, just to pull a number out of the air. While costing a lot more fuss to do. And you get a little more work in post preparing this footage. Be sure the extra work and result you get is worth the budget and time you spend to get it, is all I have to say about it.
As to the colored gel thing, I wonder if that’s really more trouble than it is worth. The wall or backdrop is already green. Now you want to wash it with more green using gels on regular lighting. The gels significantly reduce the output of the lights they are on, so now you need more lights, or more diffusion of the existing lights, or closer placement of the lights, to make that green wash even across the entire width and height of the green paper. Frankly, I think lighting it with white light is challenge enough for me…
Now you have unevenly-lit paper with two, subtly different, shades of green, instead of just the one green shade, with a differnce in the brightness or contrast. Haven’t you just added a third variable, in a project where we’re trying to *remove* variables? This gel deal may be one of those ideas that works better on paper than in practice, is all I’m saying.
Where I would do this gel thing is, if I was shooting in the field, say, an office , and sudenly got an idea on the spot that needed a chromakey shot of a person, and I didn’t have a chroma backdrop with me, (and if you saw my gig bag, you’d be surprised if I DIDN’T have one somewhere in that Bag Of Holding) I would shoot a white office wall with the green gels washing it from two sides, then light my actor in front of that, and then I’d take that footage back to the shop and see if I could pull a key from it.
If I’m building a permanent chroma cove, I would pick long tubes that already have a a green spike in their output (cheaper ones do this) and use those in banks to wash the backdrop from the top and sides. The green spike does something like your gel idea, Ian, but not nearly to the same degree.
When I set up chromakey shots, I keep my actor 1.5 times their height away from the backdrop to reduce spill as well as lose any wall shadows he might cast. This also helps the lights I use for the actor’s key and fill to keep from contaminating the green wall in the other direction. Light the green first, turn it off, place the actor and their lights, then turn everything on together at the end, and look for any problems, is how I do it.
I have done shoots in small rooms where the camera has been out in a hallway or adjoining room, shooting in thru a doorway, just to get the right DOF or framing I needed.
If the room for this keying is really small like that, but you’re stuck with it, it may be better to spend the money for a chromafx lightring type system, even though they are muy expensive. Perhaps you can rent one…
The other alternative might be to light the screen from behind: make a large frame to hold diffusion material taut, and put gelled tube lights or green PAR bulbs in banks *behind* that, like making an enormous wall-sized light box. You can keep it very close to the wall and free up more room in front. Matching a green floor to the backlit wall could be tricky, though. You have to run the numbers and decide how much effort and resources you need to throw at this to get the needed result. Everybody appreciates the pursuit of excellence, but there is such a thing as going overboard, spending 90 percent of the effort to get that last ten percent of result. Weigh these things carefully.
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Hi Mark
Thanks for the helpful and considered response. A few things:
-I’ve already been informed that I should cut my key in my compositor anyway, and we are shooting greenscreen, so compositing will be part of the workflow one way or the other, so I’m not sure how much additional work it will really take. My camera’s chips generate about 1400 x 810 pixels, so rotating on it’s side makes for a potential vertical resolution increase of more than 40%, nothing to sneeze at and only costing unused horizontal space. And remember that increased resolution is in 4:2:2, so in theory there is good payoff for cutting the key.
-The gels I’ve seen, and on recommendations from The Cow, purchased, come in a variety of densities the lightest of which I can’t see ‘significantly’ reducing the output.
-I also haven’t had anyone address what seems to me a potentially common dilemna for entry level / low budget shooters: those using tungsten for key and backlight, and fluorescent for backdrop. A light green gel would remove any potential for blue light spilling on to the subject. Right? Rest assured, I will be diligent in preventing my backlights from spilling, but this is a potential fix to a potential problem, is it not?
-Greenscreen is all about color, supposedly. And the best way to enhance an object’s color is to hit it with light of the same color. I’m not sure a light green gel will require additional light. I should be overpowered anyway, and I’m just subtracting light I don’t want or need in the first place. Reducing white spectrum light from bouncing off the greenscreen should improve the color going into the camera. The nice part is that the gels were cheap, and I can fine tune with the camera’s built in waveform, and see what works best. I have to say I like the peace of mind of not worrying about blue wash contaminating my subject.
-I’m not sure where you get the idea there would be different shades of green or uneven lighting. I have two identical lights, and each would have identical gels on them, they’d be placed symmetrically on the screen, and my key light has eggcrates and would be placed to avoid the backdrop. I’m busting my ass to get this to work in an environment with serious limitations of which I’m aware and recognize.
-I researched and ruled out a reflecmedia type system a while back. Grey halo seems an issue, as does the eyelight (I may want my subject in glasses), and the stability / quality of the lightring mount. Getting the lightring as close to the lens as possible is somewhat prohibited by a non spherical lens, and alignment is crucial and I’m not confident in plastic mounts for this quality. The cost is high, not enough people / companies are looking in to putting the green light behind the subject’s head, etc, etc. I think the technology for this will be much more mature / proven five years down the road. Maybe by then I’ll have an EX3 or HPX300, and be able to put a wide selection of spherical lenses on them. And afford the cloth which will be sold less expensively from China or India.
-Actually, the idea of lighting the backdrop from behind is very appealing. My studio has natural places for such lights, and my material is simply paper. Even if too thick to be practical, potent gelled lights behind the right material might be a perfect way to go. Thanks very much for the suggestion.
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-I also haven’t had anyone address what seems to me a potentially common dilemna for entry level / low budget shooters: those using tungsten for key and backlight, and fluorescent for backdrop. A light green gel would remove any potential for blue light spilling on to the subject. …..but this is a potential fix to a potential problem, is it not?
…….I have to say I like the peace of mind of not worrying about blue wash contaminating my subject.”It is not.
Don’t use blue light, regardless of the source –whether it be fluorescent or tungsten — on your greenscreen when you’re using ‘amber’ light on your subject.DS
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“-I’m not sure where you get the idea there would be …. uneven lighting. I have two identical lights, and each would have identical gels on them, they’d be placed symmetrically on the screen….”
Your choice and quantity of instrumentation will result in uneven lighting.
DS
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“-Actually, the idea of lighting the backdrop from behind is very appealing. My studio has natural places for such lights, and my material is simply paper. Even if too thick to be practical, potent gelled lights behind the right material might be a perfect way to go.”
You have neither the money, nor the skill, to effectively utilize this method.
DS
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It is not.
Don’t use blue light, regardless of the source –whether it be fluorescent or tungsten — on your greenscreen when you’re using ‘amber’ light on your subject.Please explain what a blue tungsten source light is. My impression is that you are in contradiction.
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“-I’m not sure where you get the idea there would be …. uneven lighting. I have two identical lights, and each would have identical gels on them, they’d be placed symmetrically on the screen….”
Your choice and quantity of instrumentation will result in uneven lighting.
??? You are making nonsensical statements, not backing them up, and what’s up with the boldface? You seem unaware of the danger of damaging your own credibility by pissing all over someone who is in no way looking for a conflict.
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“…potent gelled lights behind the right material might be a perfect way to go.”
You have neither the money, nor the skill, to effectively utilize this method.
Well, that’s kind of why I came to the Creative Cow. To get expert advice on getting equipment and the job done at less cost than well financed industrial productions using professional gear. See, for example, Michael Palmer’s excellent suggestions for inexpensive DIY fluorescent lights.
I didn’t really post to be curtly dismissed. Since I have been, however, can you confirm you have experience with rear screen projection chroma keying? How many times / different ways have you tried to do this? With which modern prosumer HD cameras?
I think it’s an interesting possibility. Maybe I’ll post a new topic. A custom-made 9′ x 9′ rear projection screen can be made for under $600. https://tinyurl.com/ofx8p4 Who knows what kind of inexpensive / used materials might be found out there for even less cost?
Here’s an excerpt from Tim Wilson’s recent editorial page: “Thanks, too, for helping us play to
win online, as 1.3 million monthly unique visitors help each other get to the next level of the game!”Thanks, you’ve really been a help.
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Let him go, Dennis: he’s going to do what he wants to do. FWIW, I mostly agree with you.
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A tungsten source that is colored blue.
DS
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