The Keys To Chromakey:
How To Use A Green Screen

Ed Driscoll
September 2007

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Green Screens and blue screens were the keys to chromakey in the 1970s, the use of blue or green screens as backgrounds for use with chromakey became widespread amongst local TV news programs, conjuring up images of cheesy weathermen in garish plaid polyester jackets standing in front of superimposed maps. The goal of that new technology was to shoot the weatherman in the studio against a blue or green screen, then delete the background color and insert various images, from maps to pictures. But very often, the key would be imprecise, causing said weatherman in his garish plaid polyester jacket to appear to be dematerializing into his map, usually somewhere around the Great Lakes or Sheboygan.

But that was long ago. These days, chromakey and related technologies, such as blue and green screen effects, have gotten so exact, they're the key to Hollywood blockbusters ranging from the recent trilogy of Star Wars prequels to such quasi-comic book films as Sky Captain, Sin City and 300. One reason these effects have taken off in the past few years is that compositing via PC has become incredibly sophisticated. And that technology has recently trickled down rapidly to the consumer level.

While Hollywood films still have multi-million-dollar budgets, they have saved considerable sums with green-screen effects, as sets are built in a computer, not by draftsmen and riggers. Similarly, because compositing via PC is now inexpensive enough that the average serious hobbyist can afford it, green-screen effects allow one-man video podcasts to have an extremely slick look, even if they're shot in a garage or a basement. So let's look at some of the elements involved in producing a successful key.

In the past, the main color for chromakeying was blue. Beginning in the late 1970s, there was a slow industry flip-over to green-colored screens for chroma. That's because of the detail in the green color channel that digital cameras retain. Additionally, green screens typically require less light to properly illuminate. However, both of these colors share a similar trait: unless you're videotaping an Andorian or an Orion, flesh tones don't contain blue or green. So you can remove the screen color without causing the talent's face to dematerialize.

However, clothes can certainly contain either color. Nowhere was the disparity between costuming and chromakeying more of a significant factor than in the first Superman movies starring Christopher Reeve. For obvious reasons, Superman's very blue suit required that special effects technicians film Reeve in front of one of the first green screen backdrops, rather than the then-common blue screen.

Be sure to advise your on-air talent to dress appropriately, to ensure that his or her clothing doesn't interfere with the key. This phenomenon is one reason that companies such as Photoflex (www.photoflex.com) make portable backdrops with both green and blue sides, so that the videographer can quickly swap out the color when necessary.

Many of today's editing programs now have chroma effects built into them. Whichever program you use for green screen, you'll find that the process will be much smoother when you plan ahead. This will help you avoid errors while videotaping the talent and hours sitting in front of the compositing program.

If you'll pardon the pun, there are two primary keys to successful chroma. The first is to make sure to light the background as evenly as possible, with no hot spots. Many programs use a tool with an eyedropper icon (similar to those in digital photography programs such as Photoshop) to select the shade of green or blue in the backdrop and key it out. This is time-consuming, so keeping the color as uniformly lit as possible on the set makes the post-production work much easier.

Second, it helps to stand the subject as far away from the backdrop as possible to separate the two. This helps to reduce spill from the lights illuminating the talent into the lighting on the green screen. It helps to blur the backdrop, which keeps wrinkling and other blemishes from affecting the key.

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.

Additionally, both programs (and many others) can create a shadow effect between the talent and whatever background you insert. Ironically, it's probably a more exaggerated shadow than you'd want were the talent properly lit and standing on a real set. But it can go far in providing that suspension of disbelief that causes the viewer to accept that a chroma shot is working.