Down and Dirty Green Screen
by Walter Graff

NOTE: Your computer monitor is not a TV set. The gamma settings of your computer monitor are usually different than a TV set, hence some of the photos may look dark or not as good as they should. Use these photos only as a reference.

NOTE ADDED 11/2003: I have received many emails asking why folks just can't seem to do what I am doing here. All tell me they are trying this with less than stellar DV cameras. I will preface this article by saying if you are using an inexpensive, small CCD DV camera to do a chromakey, you may be at a disadvantage. Larger 2/3 inch CCD systems have easier times with chromakeys as do formats that are more robust in terms of signal quality. It's not that you can't do it with DV, it's just that it can be difficult to do more intricate keys with a consumer DV recorder. And the digitalness of a DV signal in post adds to the problem as it creates small boxes that represent pictures which means when trying to cut digital DV keys you can get what are called jagged edges that don't look smooth. On top of that the software and hardware configurations you use may also affect quality. There are a number of software plug-ins to help smooth out this problem.

Chromakeying for television is one of the simplest effects one can do to enhance a production. I recently shot a direct marketing piece and needed to do a simple green screen for the spot.

This was to be a shot of the talent in front of a screen in a medium close-up speaking to camera. The first thing one needs for a chromakey is a colored background material. I use green and blue as my two colors of choice for background. Green will present your talent as sharper when you look at them in-studio than blue as our eyes have a difficult time focusing on blue. That might be a factor when you are doing intricate rear screen work and you want to be able to distinguish things easier as you look at them in front of the screen, but it has little to do with the post-production process as either of the two colors will key well.

The material I am using in the still above was purchased for about $100 from a scenic supply shop. It is a pure hue of green. You don't have to spend as much money though. A trip to a local sewing materials/fabric shop will offer you all sorts of blue and green solid fabrics that you can purchase to create the same effect.

It's more knowing how the post works than the actual shoot!

The most important thing about shooting a chromakey hasn't anything you do with what you do in the studio, but everything you do after the shoot. Hence, to understand how to shoot chromakeys properly, you must understand the process of cutting a key. I'm not talking about what a textbook says on the subject, rather I'm talking about actually knowing the equipment used to cut the key and how it works. You can only learn this from doing it. Today most of you do your work on a non-linear editing system. Hence if you are shooting a chromakey and you haven't played with the chroma key cutting abilities of your editing system first hand, then you are at a great disadvantage. Very simply knowing how to drive a car and how to get somewhere are two different things. If you have been shooting chromakeys and have yet to see the final process and how it is accomplished, I strongly suggest you try it before you shoot your next key. Doing so will open up a world of understanding that all the articles in the world can not explain. Chromakeys are about subtleties and cutting a perfect key can only be completely understood after you've played with the chromakey cutting controls yourself.

The first thing I consider when doing a chromakey is how much area of a colored background I need. In this case I was shooting a small area so I stretched a green material with auto-poles and some spring clamps enough to cover that area. There is no need lighting a 20 foot chromakey if you're only using 5 feet. I make sure that any wrinkles near the area where the key is to take place are at a minimum. I don't need to have a pristine ironed background but one that is relatively clean of wrinkles. The only way I know what is acceptable in this case is because as I just mentioned, I have actually done the post-process and I know the intricacies of keys and what I can get always with.

The Green Monster!

I consider two elements when lighting a screen. One of the most important considerations is maintaining the value of the color. In other words, taking a green background and lighting it with a whole lot of light effectively making it look like a washed out green is not the way to light a screen. Rather all I am trying to do is take the color and enhance it with light so it reads evenly on camera. That means I actually don't need much light at all. In this case, all I needed to successful light my screen was two 600 watt open-face fixtures with 216 diffusion clipped to the front for better light dispersion. The fixtures were placed equal distance from either side of the background. In this case they were about 15 feet from the screen.

The second consideration is smooth, even illumination. There are a number of ways of determining this. Looking through a darkened piece of glass (gaffer glass) or dark gel will help you see variables slightly better than your naked eye. A light meter will allow you to see variables across the front of a screen. A spot meter will do it too. A waveform monitor will show you how even your lighting is. The simplest method and the one I used was to rely on the exposure zebras in the view finder. Irising up and down caused the zebras to glow giving me a sense of what part of the screen was brighter than the other. Once again, knowing what I can get way with from having first-hand knowledge of the keying process allowed me to realize how even my screen had to be. Was it 100% even? No but I know what level of evenness would cut properly in post from having experienced it in the past. Notice in the photo below that my screen isn't exactly even in terms of illumination, rather it's a bit darker over the talents left shoulder and gets a bit darker going down that side of his body. I knew that, but also knew from having experienced trying to cut many keys in the past that it's perfectly acceptable in this case.

So I've got my screen where it's going to do the trick and now I need the talent. Lighting green screens is actually about lighting two elements as if they had little to do with each other. In other words, when I light a green screen (could be blue too) I light the screen making sure it's properly illuminated, and then I light the talent, not for how he looks in front of the screen, but how he is going to look in the imaginary setting I'm going to put behind him. In order to do that properly, after I light the screen sufficiently, I turn it off and light the talent by him/herself.

I always try to make sure my talent is far enough from the screen so as not to allow any color from the screen to spill onto the talents skin potentially causing problems later in post. In hits case the talent is about 15 feet in front of the screen giving me plenty of separation.

Isn't that easier on the eyes?

Look at the photo above and the photo above that. Notice the difference? It's much easier to see how the talent is lit when the green screen is turned off. And since the two elements (screen and talent) have nothing to do with each other, I say turn off the screen so you can concentrate on your talent.

Key light for the talent

If I was to light my talent just to make him look good for the moment I'd be missing out on what is most important; the element that will become my background once the key is made. In other words I have to use my imagination now and pretend that the background element has been inserted and my talent is being seen in that 'set'. So imagine that I am going to have my talent in front of an exterior shot of a bridge on a bright hazy day. First I need to know what the background element looks like. In this case, I notice the sun coming from the distance to the right of the background element. So my first consideration for my talent is where the back light (if any is needed) will go. Did I actually say 'if any is needed'? Yes. Sometime long ago in a far away galaxy someone created the myth that in order for a green screen to work, you need to surround your talent with lots of back light. And through the years all sorts of other mystical stories were added such as 'lighting with orange gel', 'never using colors for back light., etc. While lighting with a pool of back light. in years past may have made cutting keys easier in the days when chromakey abilities weren't what they were today, but as far as I am concerned, the only determination as to what I use as a back light. is what the final composite is going to be and what pleases my eye. And as for colors? As long as I don't use a green light on my talent (in this case because I have a green background), I can use any color I want.

Example of a final composite

Just a note about the term ‘ratios’; I hear all sorts of folks talking about ratios of foreground to background with video chromakeys. I think they are trying to make a science out of something that isn't that difficult. The bottom line is that if your background and your foreground fall comfortably within the range of acceptable exposure (they will naturally if you light your green screen for hue and your talent as explained above), I don't think there are any rules to this ratio 'thing'. Remember, effectively your background and foreground are two different things. I may have a final product that will be my talent standing in front of a star field. What does a foreground background ratio have to do with that? Nothing! My background needs to be illuminated so as to be even and represent the true hue of the color material I am using (blue or green), and my talent needs to be lit not for any relationship to the background color but to the background plate that is to be inserted later. If it was a star-field, I might light my talent a bit more from one side and depending on the darkness of the background, I might give him a kicker type back light. from one side or a 'down the middle' back light. It's all about personal taste, but simply surrounding a person with back light. is one of the first clues that he is not part of the background element in many keys I've seen that didn't look good.

In the composite shot above, I used a bridge shot (the East River Arch over Hellgate in Queens, NY) for this example although in the spot that was not the actual plate we used, but it works for the purposes of this demonstration. In fact, something interesting is demonstrated. Notice the sense of sun coming from the right side of the background image (the bridge shot). See how the back light on the same side of the talent helps to make him look more appealing when placed in front of the picture as if he is really standing in front of a bridge with the sun hitting his shoulder as it does the bridge?

 

Lighting diagram

So to recap what I said

1. Light your screen for it's hue and not just to make it bright.
   
   
2. Light your screen as evenly as possible. (first hand experience trying to cut a key in post will make that skill more of a skill and less of a guess).
   
   
3. Make sure your talent is far enough from the screen as not to have any reflections on his cloths or face from the screen.
   
   
4. After lighting the screen, turn it off and light your talent.
   
   
5. Light your talent not to make him look good in the moment but as if he was standing in whatever the background plate is going to be. If he was standing in front of an underwater background, light for that. Example of fun effect: you could try taking a dish of water, aiming a light at it from a low side angle, making waves with your hands, and allowing the ripples to move across your talents face creating a water motif to his shot that might work well with the background.
   
   
6. Back lights are dependent on nothing more than taste and how they work into your background. In that water example you might have streams of light coming down from on side of the background plate as if we could see the beams of sunlight in the water. Whatever side that light is coming from, I might put a back light. from that angle. You can use gels for backlights as long as they are not too similar to the color of your background. Sometimes hair is shiny to the point where the background color might reflect a bit. Try a color to help eliminate the color cast from the background. For example, a straw gel on blonde hair might help eliminate some of the green tinge if it's a problem.
   
   
7. Color can be what you want it to be as long as it's not near the hue of the background color.
   
   
8. All these things mean so much less if you haven't experienced creating the actual effect at least once. If you have your own edit system, take some time to play with the chroma key controls and see what it takes to cut a clean key. That will give you a better understanding of the intricacies of shooting a chroma key. If not, visit your edit room and ask the editor to show you how it all works.

This article appears courtesy of Hellgate Pictures, Inc. http://www.film-and-video.com