The Green Screen Technique

23Feb09

The green screen effect is a type of chroma key. The idea is to create a pure green background which is then replaced with whatever background image you want. Also known as the Green or Blue Screen technique, it can help you create truly startling illusions.

Basically, in this technique a subject is shot in front of a green or blue screen. Video editing program then removes this color from the video clip, and fills in the empty space with the picture from another video clip, or a still picture.

Once you have shot the footage in front of the green screen, it’s time to composite it, via your editing program. How tight the compositing looks will depend on the quality and design of the program you use. For example, while Adobe After Effects has a somewhat more cumbersome graphical user interface than Adobe Premiere Pro, After Effects has a matte choker function that can dramatically tame matte bleed. This helps to reduce or eliminate that green-screen halo effect that causes viewers so many flashbacks to those 1970s-era weathermen. And like most compositing programs, they both allow for the creation of large blocky “garbage masks” to block out background objects (such as lighting rigs) in a quick-and-dirty fashion before beginning the fine tuning.

The end result is that you can position the talent in front of a small green screen, record via a standard Mini DV camcorder on a stationary tripod and proceed to composite in all sorts of virtual sets, along with some amazingly slick camera moves created inside the compositing program. These include helicopter shots zooming into science fiction backgrounds and other impossible shots. A less complex backing may be more appropriate for some productions; obviously, experimentation is the key.

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Tags: blue screen, chroma key, chroma keying, green screen, green screen technique