What is green screening? Green screening or chroma keying is the ability to alter a plain colored background to any other digital background. Often the subject will be filmed in front of a blue or green background and in post production the blue or green screen will be replaced with an appropriate piece of footage or photograph.
Why do it? Often cost is a factor. It may not be feasible to move an entire film crew to another location. With green screening, the subject can be magically transported to the location without the travel, cost, possible language issues and environmental factors such as weather and light changes. Also you may wish to place your subject in an environment that is impossible, such as outer space, underwater or a virtual reality set.
How is it done? Firstly your background must be even lit and your subject separated from the background by appropriate backlighting. Using a green screen or chroma keying application you instruct the program to remove the color of the background. If the screen is unevenly lit the color temperature of the background will vary. In this case there will be a number of 'greens' or 'blues' to remove instead of just one color. This can make the end result messy as certain parts of the screen may not be replaced. Secondly you instruct the program to now insert the footage or photograph in place of the existing background. How this occurs will depend on the program. It may be a simple as dragging the new background into a timeline or inputting the background as an asset.
Why blue and green screen ? If you had to, you could use any color background as long as it is even lit. Blue and green are used because they contain no red. Red is one of the primary colors in skin pigmentation. The last thing you want is to remove the subject that you are so carefully incorporating into a new environment.
Any traps for new players? Do not have your subject wear any clothing, makeup or artefact that contains the same elements as the background. The program will not distinguish between what you want in or out of a shot; it will just follow instructions and remove the color you identify.