A green screen allows you to create a digital background. A live subject can be filmed and a virtual background added after, giving you the freedom to create any world you can imagine.
In order to isolate one area from the rest, the background color must be distinctly different. Bright green beats blue, partially because it is not a color commonly worn by talent. Any clothing that matches the background too closely will also key out, punching a hole in your subject’s body, or making him invisible altogether.
What is Keying?
Keying is the process of isolating a single color or brightness value in an electronic image and using software to make that value transparent, allowing another image to show through the affected areas. This is essentially the special effects/post-production technique for creating that digital background.
Before you take one step on the green screen, it’s important to plan out where your background will be, how you plan to shoot it and how you plan to light it. Continuity is key if you want your project to look professional. We’ve all seen bad green screen before, and that’s mainly due to the lack of planning before filming.
The trick is to shoot your subject in a way that blends well with the background. In most green screen scenes the digital backdrops will be a still, stable computer generated image or animation, or a clip or video shot from a locked down camera. It’s important to replicate all variables on the green screen that you used in shooting your background. Such variables include, but are not limited to, the distance between your subject and the camera, angle, movement and settings of the camera, such as focal length and color balance, lens type, matching the light sources location and temperature, etc.
Light it Smoothly
Even a professional-quality cyc wall won’t key well without proper lighting. If there is a secret technique to getting good keys, it is in lighting the wall. The goal is to light the set as evenly as possible using soft light. Any hot spots or shadows will make the eventual keying process much more difficult.
Light it Separately
Another important, but often overlooked, essential is lighting your subject independently of your set. This is important for two main reasons: shadows and reflections. Part of keeping your wall evenly lit is keeping your subject’s shadow from falling across it. To do this, you need to position the talent at a distance of at least a few feet from the screen and light him separately using three-point lighting.
If you do not have a lot of distance to work with, position your key & fill lights slightly to the sides, not straight on, so any resulting shadows will fall outside the visible frame. Another advantage of moving your subject away from the wall is the reduction of reflected green spill light on your talent. Reflected spill light can rim your subject in a tinted halo that can be difficult to discern with the naked eye, but if your actor is too close to your wall, it will be there, and any green bouncing off your actor will disturb the cleanliness of your key.