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The Keys To Chromakey: How To Use A Green Screen

The Keys To Chromakey: How To Use A Green Screen

How to use a Green Screen to create the background you want in your video.

The Keys To Chromakey

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.

Blue or Green Screen?

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.

Choke Your Matte - Not Your Talent!

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.

Replace the Green Screen with a Convincing Digital Background

In the past, a television production required a dedicated studio and lots of expensive, cumbersome overhead lighting to make the scene appear evenly lit. These days, with a couple of grand for a lighting kit and a digital backing, it's possible to "build" a virtual set that looks like it costs a lot more than it actually does. Digital Juice (www.digitaljuice.com) is one of several companies that sell slick-looking video backing tracks which you can loop to form a digital background, so you can green-screen the talent in front of it. Combining these clips with a DV camera, a tripod, lights and compositing software makes it possible to turn virtually any garage or basement into a video studio.

What's the future for do-it-yourself green screening? Serious Magic (www.seriousmagic.com), which became a division of Adobe recently, points the way towards one scenario. Its Ultra 2 program (now available bundled as part of Adobe's new Creative Suite 3 Production Premium kit) combines a fairly big collection of incredibly slick-looking virtual sets along with their compositing program.

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 that Gene Roddenberry would have given his eyeteeth for during the heyday of Star Trek. What was science fiction a few decades ago is now science reality.

Of course, these virtual sets aren't for everyone: they may be too overwhelming or "arch" for a production that will ultimately end up as a five-minute clip on YouTube. A less complex backing may be more appropriate for some productions; obviously, experimentation is the key.

Chromakey Green Screens Can Make for a Slick "Vent"

As a case study in how green screen and chroma can make a small operation look network-slick, it's worth studying the production techniques employed by Bryan Preston. He's the producer of the five- to ten-minute daily video clips for Hot Air's (www.hotair.com) daily Vent vidcasts. Preston extensively uses chroma to generate digital backdrops behind the on-air talent, such as Fox News panelist Michelle Malkin. Preston recently told me, "We don't have a studio per se, so we're using a Lowel Tota-Light kit. Basically, the way I set things up is that I light my talent with four lights. I point two Rifas - a large Rifa and a smaller Rifa - at the talent. And then I have two Lowel Pros, little 500-watters, as my rim lights. I light the green screen itself with two umbrella lights, Lowel V-Lights."

Preston says that the nature of green screen requires lighting it separately from the talent. "The trick, of course, with any green screen, is to get enough light on the green screen so that the green hits the right tones" for keying, he adds. "But you also don't want to get so much that it bounces onto the talent. So I've played with distances to get Michelle far enough away from the screen, but close enough to it, because the green screen itself is a five-by-eight foot portable green screen." To cut down on spill, Preston eventually ended up placing his talent about six feet from his Botero collapsible fabric green screen.

This article only scratches the surface of what chromakey and green screen can do. You might not create the next Sin City or Star Wars, but to elevate the quality of your next YouTube clip, green screen can go far towards creating champagne-quality video on a Budweiser budget.

Ed Driscoll is a freelance journalist covering home theater and the media.

Tags:  September 2007
Ed
Driscoll
Sat, 09/01/2007 - 12:00am