Keying Compositing

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. Luminance keying, or lumakeying, is the process of keying out a brightness value or range, like black or white. Luminance keys are often used for applying mattes. Color keying, or chromakeying, identifies a specific color to remove.

Another important, but often overlooked, essential is lighting your subject independently of your set. This is important for two “key” 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 mess up the cleanliness of your key. You can wash away a fair amount of reflected green using a bright backlight, but you will find that distance is your best friend.