Blue versus Green Screen

There are no hard and fast rules regarding screen color choice, but there are some known issues and guidelines you need to consider before choosing between a blue or green screen. Obviously, you don't want to put a person with a blue shirt in front of a blue screen unless you are purposely trying to key out the shirt and allow their head and hands to float freely!

Blue is normally reserved for film and high-end professional HD cameras (not the prosumer models that have compression in the DV codec) and can handle dimly lit scenes. On rare occasions, red or even magenta can be used in filming effects scenes that might require a great deal of foreground blue in the subjects. For most DV compositions, however, green is the key color of choice. DV stores luma information (the brightness and contrast) at up to four times the color information, so you want as much contrast between the background and foreground exposures as possible. Plus, the green channel in DV (the "G" in RGB) is where the luma values come from, and it is sampled more than the blue channel, which maintains less color information. This is the same reason why you will find less noise in the green channel compared to either the 54 red or blue channels when they are viewed separately. In addition, a green background z requires less light to illuminate it.

g Although green is used most often by far, there is an exception to using green. If

~ the subject(s) in an outdoor scene must get close to the screen, casting shadows on it or

S coming in direct contact with it, you should use blue because the color spill of the blue g background is more naturally occurring in an outdoor setting (especially at night, or

2 out of direct sunlight) than that of a green light spill. However, several keying plug-ins

ยง for After Effects handle any kind of spill quite nicely, especially Ultimatte. Chapter 9, ijJ

"Matte and Keying Plug-ins," contains several comparisons that show you how different keying plug-ins work in various situations.

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