Now very often we are just putting a person (like a newscaster) against a random image of something quite unrelated to the foreground. If that’s the case it doesn’t really matter what the background looks like or how you light the foreground as long as the greenscreen is exposed correctly (maybe a half stop under from key) making it possible to pull a good matte.
Most of the time I do greenscreen or bluescreen matting, I am trying to create the illusion that the foreground and background, when composited, are a single image shot in one location. This requires that the background and foreground have matching lighting.
What does this mean and how do we go about it?
We have to start with one image or the other. It is usually easiest control the lighting of the foreground, therefore we need to choose (or shoot) the background “Plate” before we shoot the foreground “FG.”
In shooting or choosing a Plate we should make sure the camera is not moving. While it is possible to track an FG to a Plate, it is not easy or cheap. This is especially critical when we are looking at the FG actor’s feet contacting the ground in the Plate. The one exception to this is when we shoot a Plate from a moving vehicle to show our FG character to be riding in a car or an airplane.
Very often when we shoot a Plate, it has it own intrinsic lighting. (ie. The sunlight coming from, say, the right side. Skylight coming from overhead.) We can tell this by looking at the Plate or keeping notes when we shoot it. Sometimes we need to choose Plates from a stock footage library. Get these Plates ahead of time, before you shoot the FG so that you know how to light it.
Having analyzed the lighting in the Plate, try to reproduce that lighting in the FG. In our example, that would mean having a hard light coming the right side at the same angle as it was coming from in the Plate.
Then create a soft overhead source simulating the cool skylight. The sunlight will probably be warm (perhaps a tungsten incandescent hard light). The overhead a blue light (an HMI or and incandescent with a blue gel) bounced into a large 4’x8’ white card or maybe bounced into the white ceiling of the space you are shooting in. Make sure the lighting ratio between the “sun” and the “sky” are the same in the Plate as they are in the FG.
My pet peeve in lighting greenscreen is when people put in a hard backlight for no good reason. “It will help separate the FG subject,” they say. We are actually trying to get them to blend. That’s the whole point. A well-exposed greenscreen is all you need to separate your FG. The only thing that backlight will do is to is to make it look fake and create what looks like a matte-line even if you pull a perfect matte. Keep your lighting natural.
Now we are ready to light the greenscreen. There is a tendency to over-light greenscreens and that makes it very difficult to pull the matte, as the green will start to eat into the FG subject matter if it is too bright. Also being too bright, it will start to spill onto the FG subject. I try not to light digital greenscreens at all. In our example, you might find that the spill from the soft overhead sky light is enough to light the green. When I’m outside, I almost never light the green, but I do try to keep it out of direct sunlight.
In any case, try to light the green with as few lights as possible, to save time and multiple shadows. I hate lighting greenscreens with overhead lights: they take too long to rig and usually create hot spots. If I’m on a stage and I do need to light the greenscreen, I usually light it with two studio floor lights, one each side with a little diffusion to soften out any wrinkles in the rag.
Have the greenscreen as far back from the FG subject as possible. This will often mean you will need a much bigger greenscreen and more studio space that you think. Especially if you are using wide lenses. A large greenscreen as it get further away from a wide lens becomes the apparent size of a postage stamp very quickly. Long lenses, not so much.
It is always preferable to match the focal length of the camera, which recorded the Plate to the one you are using to record the FG. This is not always possible and the process can be quite forgiving as long as there are no architectural objects in the Plate and/or FG that give away the cheat.
What you want to strive for, is to have the greenscreen only just big enough to cover the action in the FG. If it is any bigger it will act as a soft backlight spilling green light onto the FG subject. This spill light will make it difficult to pull the matte and you will have to get rid of it with a spill suppression plug-in during post. It is best to move the greenscreen as far back as possible if you have the room. If not, flag the top and sides. Or just roll them up.
A word about bluescreen. Digital bluescreen is actually my favorite matting material. For one thing if there is any spill light on the FG actor, blue spill looks a lot more natural than green. Also I have found in tests that the motion-blur artifacts are a lot more pleasing in a matte pulled from blue than they are from green. Obviously if the FG actor is wearing a bright green shirt or there are plant elements in the FG, you will want to use bluescreen. But if you are using bluescreen don’t under-expose it too much, because in digital cameras the blue record is the weakest and has the most noise.
That’s about it for now. When I come back to this subject, I will be talking about the post pipeline for pulling mattes and compositing good greenscreen.
About Bruce Logan ASC
Bruce shot the opening sequence in 2001 A Space Odyssey.
He blew up the Death Star in Star Wars.
Made the tail-fin prowl through the clouds in Airplane.
Shot the movie’s first journey into cyberspace when Tron fights for the users.
Pushed Lily Tomlin, The Incredible Shrinking Woman, down her own garbage disposal.
Set LA on fire in the opening scene of Blade Runner.
He has shot commercial films for most of the major companies: Pepsi, GE, Visa, Chevrolet, Pontiac, DuPont, Contac, Sprint, Amtrak, Suzuki, Sunlight, and Armstrong. And—he has applied his talents to making music videos for such high profile performers as Prince, Madonna, Rod Stewart, Aerosmith, Glenn Frey, The Go-Gos, Karyn White, Tevin Campbell, Hank Williams, Jr., and Michael Cooper.
Combining his love of directing and creating visual effects, Bruce has developed a new Virtual Set Process with the Entertainment Design Workshop. He is keeping his hand in directing—having just completed several episodes of The Book of Pooh for Disney and the new PBS show It’s a Big, Big World.