Rim light and hard edges for greenscreen

[Tim Fabrizio]

So the battle continues...when shooting greenscreen does it create a problem in post to have your actors have hard edgelights? Doing a greenscreen where they want a more of a silhouette look similar to this music video: http://www.youtube.com/watch?v=9nz6Rq1Pvh0...&playnext=1

It would be nice to pick up some skin tone as they turn in and out of a hard backlight or edge.

Thoughts?

[David Mullen ASC]

Edge lights tend to be good for people against greenscreens because they wash-out green spill.

 

But obviously in that video you linked to, they just used silhouettes, which is a very different effect than a backlit / edgelit person.

[Walter Graff]

Edge lights can help in limited situations. You really want to more light your person for the bakcplate they are going ot be placed into. Most all times edge lights don't help that senario. If you light a green scren for hue and luminence you need no more than 40-50 footcandles with video so spill is often not an issue. And even if it is, today's edit systems have all sorts of spill plug ins that make any spill non existant.

[Scott Fritzshall]

We should have an FAQ about this, because it gets asked rather frequently. Light your subject the way they would be lit if they were in their intended environment. You don't need to anything special on your subjects in order to key it properly. Improper lighting is just as bad for greenscreen shots as poor keying.

 

For the video in question, those guys were wearing really high-contrast makeup and that look was basically put together through compositing. You're asking for skin tone, though, which that video really doesn't have, so not exactly sure about what you're going for.

[Tim Fabrizio]

Yes there is BIG difference between the two. I was just curious about problems in the edit. I was going to shoot them in silhouette with little separation and like I said to pick up little elements (in closeup) of some eyes, hands, etc with some backlight. But I guess if I have good green screen to subject distance then the spill is minimized. Would going with the softest light source possible for the green screen help in reducing the spill?

Thank you, David.

I was reading a post on here that it may wash out green spill but may cause problems in the edit room. Here is a quote (sorry for the length) from a post:

 

 

"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.

[Tim Fabrizio]

Edge lights can help in limited situations. You really want to more light your person for the bakcplate they are going ot be placed into. Most all times edge lights don't help that senario. If you light a green scren for hue and luminence you need no more than 40-50 footcandles with video so spill is often not an issue. And even if it is, today's edit systems have all sorts of spill plug ins that make any spill non existant.

 

I just realized that I quoted you, Walter, in my last post. HA. Thanks for the info.

[Walter Graff]

I'd do it differently. Dress your talent in black body suits over white background. If you want the glow in the shirt etc, you can stitch white material where you want it on the black body suit. Buttons, the "V" of a v-neck, various glowing points, etc. Do an eye cut out to get the whites of the eyes and hit the face with light. Want teeth glow? Do what they used to do on the show MAX HEADROOM which was literally paint his teeth with whiteout. Now in post the high con will cut real well. Using hicon edge softeners and chokes you can make really nice edges. Much easier than color in a scenario where you don't want color. You can even downstream key the white to be any color or potions of the white to be any colors you want to choose from. And you can still hicon key the white background with whatever picture you want to insert. Really easier to do than color which in the end you end up cutting a hi con from anyway. And the bonus in this situation is that with a bit of edge light (more like 45 degree to camera kickers) on your talent, degrees of reflection off the material make for reflection glows that make nice senses of 3d when they turn.

[Peter Moretti]

I've heard a rule of thumb that the green screen should be uniformly lit about one stop darker than the subject. Is that apocryphal or accurate?

 

I ask because many of the green screen setups I've seen have screens that appear brighter than the subject :huh: . And the thirty to fifty footcandle recommendation mentioned above is an absolute measurement.

 

Perhaps ~ forty fc works well when lghting "normal" activities, but let's say a couple is eating a romantic dinner (no salad, LOL). So they're lit below even thirty fc, in which case should the green screen be lit even darker than they are?

 

Thanks very much!

[Will Earl]

I've heard a rule of thumb that the green screen should be uniformly lit about one stop darker than the subject. Is that apocryphal or accurate?

 

It generally depends on the format and the compositor as to the best stop at which to light a green/blue screen and is something you need to test for yourself.

 

The last keying I had to do were a few film plates that were shot with the greenscreen that was about a stop brighter than the shooting stop - although the FG was about 3-4 stops below so it was a reasonably easy key.

[Peter Moretti]

Will, if understand correctly, the green screen was four to five stops brighter than the subject. Wouldn't that cause a fair amount of bleed through?

 

I'll be using a variety of HD formats from the dreaded HDV (Canon XH-A1) to HDCAM (F900). Avid MC with a full set of Boris plugins will be used to create the mattes.

[Will Earl]

Well yes it was a fair few stops brighter, but relative of course to a subject that was in shadow. The subject was lit for the 'environment' they were suppose to be in - driving along in a pitch black tunnel so they were a silhouette lit only by headlights and the occasional overhead lamp. The stage they lit it on had the greenscreen a fair enough distance away so spill wasn't a major problem, reflected green was the only real problem - which can be fixed by procedural keying, but is often easier to roto the problem area.

Edited February 17, 2009 by Will Earl

[Peter Moretti]

Will, sorry to sound like such a moron with all this, but one last clarification if possible :).

 

The rule of thumb of lighting the green screen about one stop under is, is that one stop under relative to the shooting stop or relative to the reading off the subject?

 

I realize that this is "rule" is often times not followed, but I'd like to know how to interpret it properly. THANKS MUCH.

[Will Earl]

Relative to the shooting stop. I think the general rule of thumb that tends to often get quoted is 'at-or-under' for film and 'at-or-over' for digital. The 'rule' also tends to vary depending on whether you choose blue or green.

 

Most of the greenscreen/bluescreen tests I've seen done for films involved doing wedges of various combinations of exposures for both the subject and the screen - so for example what happens if you underexpose the subject by one stop and light the screen at the shooting stop or what happens if you overexpose the subject and underexpose the screen. They'll also test the effect of colour temperature on the lights hitting the screen - for example what effect using supergreen or superblue kinos will have. Blonde wigs and colourful plant-life are often used as test subjects.

[Walter Graff]

Relative to the shooting stop. I think the general rule of thumb that tends to often get quoted is 'at-or-under' for film and 'at-or-over' for digital. The 'rule' also tends to vary depending on whether you choose blue or green.

 

Most of the greenscreen/bluescreen tests I've seen done for films involved doing wedges of various combinations of exposures for both the subject and the screen - so for example what happens if you underexpose the subject by one stop and light the screen at the shooting stop or what happens if you overexpose the subject and underexpose the screen. They'll also test the effect of colour temperature on the lights hitting the screen - for example what effect using supergreen or superblue kinos will have. Blonde wigs and colourful plant-life are often used as test subjects.

 

I think you said it best Will. Here's what confuses people. In one regard your screen has little to do with your foreground, but in another way the level you choose to light your screen at can effect not only your ability at a clean key, but what you can do later with your talent. In general with film and blue you want to light for saturation and not go too far under exposure with that blue as if you have to bring that level up later the noise in the blue channel can become a factor. In green it's easier as it generally takes less light, and noise is far less a factor than it is with blue. In video you generally want more one to one or slightly more exposure for your screen because later you can always crush levels easier than having to bring them up (introducing noise) in post. But then again you have to know what medium you are working with. HD/HDV for instance has the ability at making far more noise if you have to increase the levels later so if you were shooting blue, I'd over expose one stop at least as a general rule if you felt you might have to make screen level adjustments later. But if it was green with HD/HDV I'd go for a one to one or slightly less exposure as the green saturation will be easier to cut than blue. And then you have to know which color would work best for which backplate. If you are shooting a guy over green and lighting him for a nighttime scene you could potentially have more problems with that green leaking, causing edges, etc than you would if you used blue as in the night scene blue would be so much more acceptable if it showed up in a key, as something you could not as easily work out of a shot. Bottom line, your screen and your foreground have nothing to do with each other in the sense that you are lighting the screen for even illumination and the talent for how they will show up in the backplate. But since they are married in the sense that both exposures need to be workable together you can only have so much exposure difference between front and back before you get in trouble. In general what many do not account for is that once you cut your key in post, you can now do a vectorscope anything color correction wise to your talent and you will not effect your key. So in post they really are separate entities from shoot to finish. If it's video the simplest way is to light your screen using a. Put your blue or green color inside the appropriate box on a vector corresponding to the color and pretty much nothing else you do can be wrong. It really needs far more in depth explanation than I am giving here in my random thought, so sorry for the shortness of the answers. Having lit screens extensively for 25 years, it just becomes something you learn to do with your eyes closed rather than working from rules as if you feel the colors more than wonder about exposure. Sort of like when you work with alight meter you start to learn exposure numbers by eye. In other words, keying is best learned by experience, and with that making mistakes. I can tell you about $100k mistakes I made. In my green screen classes now I spend as much teaching the post production side as I do the lighting side, as lighting with green screen is 40% of what you need to know to do keying well. So I’d suggest folks at least learn how it is done from the post side and see what lighting does to know what works and what is myth about keying. That is pretty much the only way to really understand keying. It’s far more complicated than one stop over or under.