green screen spill

clyde villegas · July 15, 2008

Preventing spills from a green screen is as simple as moving the actors away from the green screen background. But how about the green colored floor; how do I keep it from from reflecting on my actors? Thanks and God bless.

Christopher Santucci · July 15, 2008

Preventing spills from a green screen is as simple as moving the actors away from the green screen background. But how about the green colored floor; how do I keep it from from reflecting on my actors? Thanks and God bless.

Drown out the green by using some strong side/rear white light. And, you can have the subjects not wear light colored clothing.

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clyde villegas · July 17, 2008

Thanks Christopher. But since the lights will be coming low from the side, they could spill directly to the floor, changing it's (the floor's) brightness. Is placing those "leg lights" closer to the legs the right solution? What else can I do so that the lights will only hit the legs and avoid the floor?

I am very new to green screen. I have read in one thread that the "luminance" of the foreground should be different from the luminance of the background (the green screen). What does that mean? How do I make sure they're different? Thanks and God bless.

Christopher Santucci · July 18, 2008

Thanks Christopher. But since the lights will be coming low from the side, they could spill directly to the floor, changing it's (the floor's) brightness. Is placing those "leg lights" closer to the legs the right solution? What else can I do so that the lights will only hit the legs and avoid the floor?

I am very new to green screen. I have read in one thread that the "luminance" of the foreground should be different from the luminance of the background (the green screen). What does that mean? How do I make sure they're different? Thanks and God bless.

If you really need to put the talent ON the green, you'll have to tailor your lighting and wardrobe to minimizing green spill, and/or deal with it in post with spill suppression, etc. Traditionally, greenscreen footage is approached by use of space lights to get an overall flat and even light for the green, AND sources are used on the side rear of each side of the talent to drown out the green reflection and provide a white rim on the border of the talent with generally a frontal key.

The "luminance" difference refers to having the green behind the talent less bright (generally by a stop to 2 stops) than the light level for the talent so as to minimize reflection on the talent while still having a suitable level of green to key with.

Since you want to place talent ON the green, there's only so much you can do in terms of light falling on the green that the talent is standing on. Most greenscreen lighting is very flat which minimizes shadows on the talent that may become lit by the green.

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Walter Graff · July 18, 2008

You only need about 30-50 foot candles of light to make a green screen work properly for keying. As was suggested before, space lights, or soft lighting that is no directional works best. There is a prevalent fable that firing light from behind someone prevents green screen spill but this as I have found in my 25 years of matting is often useless and actually does more harm than good. Many folks who shoot but have little editing experience with keying believe this and that makes sense from a shooting perspective but I can tell you that the best guys in the business that I work with who do the post side say this 'technique' is often met with more problems than solutions. And my editing experience has shown that to be true to.

I am coming out with a new green screen instructional DVD POD next month for those on my mailing list where I show that this 'back lighting technique' is useless 99% of the time. In fact I show exactly this set up you discuss with all the variables and show the post process and prove that most of the stuff folks preach about back lights and green screen often hurts more than helps. Basically it's a DVD that explores all the folklore of green screen and shows with example after example that most of what folks are taught about green screen is based on little fact. The key is that I not only show the set up but the result when one tries to key and that is what most folks do not see/or understand how to do well. Especially in this situation as you have green both behind and in front of the talent so any hard back lighting would do little but make a very unflattering edge on a persons legs which makes them look worse when matted on a back plate and makes the green screen harder to cut in post as it would make shadows forward of the talent on the floor which is the last thing you want. Colored back lights (straw, etc) are just as useless most of the time with today's software that has so much fine tuning as to make situations where I hadn't planned on keying yet had a blue background key as well as if I spent a day lighting.

And every professional keying software today has spill filters that allow you to isolate spill very well so most of the time if you light a screen at forty foot candles you'll never have a problem keying. The other consideration is what your talent wears. Many people do not know that keying also involves luminance differences even though it is a color key so you would want to also consider what your talent is wearing and whether blue or green would be a better choice as a result. For instance, if your person is wearing black pants, blue would be the worser choice compared to green as the luminance difference would be less as today's keyers also use luminance for fine tuning keys. Yellow clothing would work best with blue as the luminance difference would be greater than it would be with green. If you took a vectorscope on a set and lit your green 'dot' so it is in the target of green on a scope not only will you have perfect green, but will find it keys easier than you've ever done before.

The give and take of full body green screen is that you can not often make the same subtle lighting tweaks with your talent that you can when just the area behind the person has screen so you must usually end up using softer overall light unless you spend lots of time with lenses fixtures and flags and create lighting that is more dramatic (if called for by the back plate). Bottom line, green screen spill can be a problem if you light your set too hot (most do), use green when blue would have been better, and you know little about the post process and the capabilities. That's why the first thing I teach folks is to learn the post process so they see exactly what happens after they shoot. If you never shot film, I would never just give you a meter and a camera and say go shoot. I'd do the same but let you watch the result and learn how the process of exposure works from seeing what you did and how it can be resolved. Why many folks today light green screen having little experience on the other side is a mystery to me. It's simply a waste not to know how the entire process works and many of these mythical shooting techniques only validate that.

clyde villegas · July 20, 2008

You only need about 30-50 foot candles of light to make a green screen work properly for keying. As was suggested before, space lights, or soft lighting that is no directional works best.

Sorry, I'm very new to this. How do I measure it. Haven't used a light meter. Will a light meter provide me with those readings?

I am coming out with a new green screen instructional DVD POD next month for those on my mailing list where I show that this 'back lighting technique' is useless 99% of the time.

How can I get a copy of that DVD? How much is it? Thanks!

Christopher Santucci · July 20, 2008

There is a prevalent fable that firing light from behind someone prevents green screen spill but this as I have found in my 25 years of matting is often useless and actually does more harm than good.

I think it's meant to overpower any green spill that may occur, not "prevent" it. I myself, do not use this technique, but I've argued at length with an old-school gaffer who swore that you just cannot shoot greenscreen without the rim lighting (with some magenta gel, haha.) Of course if he'd spent hours and hours actually shooting greenscreen stuff and pulling mattes like I had, he'd understand.

I think if you lack the distance from the talent to the back wall, this technique might be more valid, but there still are slews of people in the industry that think the rim lighting is a necessity for greenscreen.

This music video I shot and posted has no such rim lighting and was shot on HDV. It's not 100% perfect, but hey NO rim lighting.

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Edited July 20, 2008 by Christopher Santucci