Green Screen Filming
By Eric Huyton (Lacock Films Ltd)
What is Green Screen?
Shooting a subject against a green screen background allows the compositor to key a different background onto the green area. It is essentially a Chroma Key process whereby the computer is told to recognise the green chroma value and replace it with the chosen background.
For best results, and to give the compositor an easier time, the green screen should be very evenly lit to give a good saturated green with no shadows or lighter and darker areas.
In an ideal world, you'll be in a fairly large studio with an infinity cove and a green floor. This means that you can keep your subject and foreground objects well away from the green background which avoids shadows falling on to the background and also reduces any green coloured reflected light bouncing back into the foreground.
There will be times when you have to shoot in less than ideal conditions and you simply can't achieve perfect flat even lighting on the green screen. This could be because of the size of the studio, the size of the green screen or just because you have got enough time in your schedule to get it spot on. You will frequently have to compromise and it's then a question of knowing what you can get away with.
The compositor's view
Rogerio Alves: Freelance Visual Effects Supervisor
"A perfect, evenly lit, well saturated green screen is ideal as it makes the keying process very straightforward. I load the shots into a compositing suite, pick the colour range and within 30 seconds or so, I've got the background in place. This leaves me plenty of time to work on the creative side and make the whole composited image more believable instead of correcting errors and hand painting frame by frame which is really time consuming."
"However, cameramen and producer's can get too hung up on getting it perfect. Some moderate differences in lighting levels or wrinkles in the green background aren't necessarily a problem. The computer is looking for a particular range of chroma values on which to key the background and I can adjust this range to include some differences in brightness. So, within reason, if it's a light green or a dark green, it doesn't matter - so long as it has a good amount of green value. Obviously, if an area of green is so much in shadow that it's almost black, then there's nothing I can do except replace it frame by frame which is messy and time consuming."