Friday, July 10, 2009

The Rise of Green Screen Technology

by Julia Smith

Did you ever wonder why blue screens seem to have turned into green screens for film effects?

The reason is the simple march of time - and technology.

Blue screen continues to be used in film, as we can see in these composite shots from 300.

"Until the the 1990s most blue screen for films was done optically, and all television composites were done using analog real time hardware." - Bob Kertesz

Optical effects are expensive, exacting and have to be done in post-production, which is the time used by the production after the principal photography is completed. This is the time when music is recorded for the soundtrack, dialogue is replaced through looping, and sound and visual effects are created.

Before the advance of digital technology, the only way to get a realistic-looking visual effect was through blue screen optical matting, which replaced a background component - the blue screen - with a second visual component - usually a risky location such as a cliff.

The process of traditional travelling matte optics is time consuming. Each frame of the actor has to be made into a negative. The blue background is replaced by the new location footage, then photographed again with the negative of the actor superimposed, then the images photographed once again replacing the negative of the actor with the actual actor.

Graphic by Tech On

The television industry has always opted for green screen technology, due to the way TV cameras work to a basic three-color spectrum: Blue, Green and Red. Often the superimposed images are created on the spot in the studio at the mixing console by the video switcher.

"Chroma-Key is a television process only, based on the luminance key. Video cameras are usually most sensitive in the green channel, and often have the best resolution and detail in that channel. Green paint has greater reflectance than blue paint, which can make matting easier." - Bob Kertesz

Because cameras have gradually moved over to digital, even for full-length motion pictures, the effects have moved with them. The green background works best in digital format and has the added benefit of not interfering with actors' blue eyes or blue clothing.

The trick with green screen is knowing how your other colors will behave when you shoot them. Notice how on-set for Sin City, Marv's bandages are red, but in the finished effect shot, the red becomes white. Also notice how a slight blue tinge on Marv's white T-shirt turns the shirt into a truer white in the effect shot.

Actors today can be assured of having to act in front of either a blue or a green screen at some point in their careers. A major challenge for them, but green screen has changed the way film and television is shot. I personally love this technology, especially when it brings me back through time to ancient Rome or to Middle Earth.

Graphic from Make Movies or Die