Greenscreen Chroma-Key Matting Tips & Tricks
Simply put, greenscreen or bluescreen matting is for combining two different images, shot at different times, and make them appear to be one shot.
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 (BG) looks like or how you light the foreground as long as the greenscreen is exposed correctly (maybe a half stop under from the key lighting exposure) making it possible to pull a good matte.
Most of the time when 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?
Choosing Or Shooting A Background Plate
We have to start with one image or the other. It is usually easiest to 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 locked off. While it is possible to track a FG image to it’s Moving Plate, it is not easy or cheap. This is especially critical when we are seeing the FG actor’s feet appearing to contact 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.
When we shoot a Plate, it has it own intrinsic lighting (i.e. the sunlight coming from, say, the right side or 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.
BTS Footage From Bruce Logan’s Greenscreen Master Class – BTS Shot On An iPhone.
Lighting The Greenscreen
Having analyzed the lighting in the Plate, try to reproduce that samelighting in the FG. In example I just described, that would mean having a hard light coming the right side at the same angle as it was coming from in the Plate to represent the sunlight.
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 an incandescent with a blue gel) bounced into a large 4’x8’ white card or maybe bounced into the ceiling, if the space you are shooting in has a white paint job. Make sure the lighting ratio between the “sun” and the “sky” are the same in the Plate as they are in the FG that you are lighting.
Greenscreen Lighting Problems To Avoid
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 from the BG,” 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 and BG. 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.
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 or chroma-key screens 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.
Less Is More
Try to light the green with as few lights as possible, to save time and to prevent multiple shadows. I hate lightinggreenscreens with overhead lights: they take too long to rig and usually create hot spots. If I’m on a stage and the greenscreen does need to be lit, I usually light it with two studio floor lights, one each side with a little diffusion to soften out any wrinkles in the rag.
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. This always compromises the matte.
Final Greenscreen Tips
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 than 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.
It is best to move the greenscreen as far back as possible if you have the room. If not, “flag” the top and sides with black cloth to keep the light of the FG subject. Or just roll up the greenscreen to make it smalle.
Greenscreen’s Cousin, The Bluescreen
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 underexpose it too much, because in digital cameras the blue record is the weakest and has the most noise.
You can start editing here.