The Keys To Chromakey: How To Use A Green Screen

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 chromakey became widespread amongst local TV news programs. That resulted in conjuring up the 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 they would delete the background color and insert various images, from maps to pictures. But very often, the key would be imprecise. That ended up 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 been greatly improved. 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.

While Hollywood films still have multi-million-dollar budgets, they have saved considerable sums with green-screen effects. 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 filming 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.

Christopher Reeve's Superman
Christopher Reeve’s Superman. Image courtesy SupermanIV.com.

Be sure to advise your on-air talent to dress appropriately. 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. With those, 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.

First primary key to successful chorma

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 primary key to successful chorma

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 helps the viewer 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. It also required lots of expensive, cumbersome overhead lighting to make the scene appear evenly lit. These days, a couple of grand for a lighting kit and a digital backing, it’s possible to “build” virtual set. Digital Juice (www.digitaljuice.com) is one of several companies that sell slick-looking video backing tracks. You can loop them 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.

The end result is that you can position the talent in front of a small green screen. You then can record via a standard DV camera on a stationary tripod. After, you 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.

Sometimes, a simple green backdrop is best for the video you are shooting
Sometimes, a simple green backdrop is best for the video you are shooting. Image courtesy Authority Marketing Films

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.

Pulling the key

Using green screen is both a useful and valuable tool when you want to make professional video fast and save a few bucks There’s many different steps you can take when keying. A lot of it boils down to your skill level and the amount of time you have. Let’s cover some of the beginning steps to help you get started.

Step one: Do your preliminary color grade

To start off, make sure your image looks clear. You want your green screen to be as clear and vibrant as possible. Just don’t over do it. You want to still keep all your subject’s image details. Over-correcting could result in the loss of those fine details.

Step two: Define color you would like to remove

Define color you would like to remove, which would be green for green screens
Define color you would like to remove

Now you’re actually removing the green screen from the scene. To do this, you select which color you want to remove in your keying software – which would be green. After the green is removed, adjustments may be needed to remove any patches of green left over. You can move the color picker and adjust sliders like highlight. Again, just don’t overdue it.

If you find there’s some green color bounce on your subject, you can feather the edge to get rid of that effect.

Step three: Apply a garbage matte

If you’re able to crop out areas that aren’t being keyed out, you can always crop them out. As long as the crop doesn’t affect your subject, you can opt to crop.

Step four: Position the subject accordingly

Place your subject where it looks most realistic
Place your subject where it looks most realistic

You’re almost done! After importing the background you want your subject to be in front of, it’s time to adjust. You need to adjust your subject to really sell they’re in the environment you’re putting him or her in front of.

Step five: Add a color grade for a cohesive look

For here on out, you need to color grade both the background and the subject together. They’ll look like they’re both cohesive. It’ll really sell the person is there and not in front of a green screen.

If you want a more in depth look at all these steps, watch our keying tutorial video.

Elevate your videos with chormakey

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.

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