How Does Green Screen Work?

 

How do you move characters around in a world that lives only in your imagination? By following this easy step-by-step guide to going green.

     Through the magic of video effects and technology, you can superimpose your subjects onto virtual backgrounds, place them over animated digital backdrops or transport them to a desert oasis. You can shrink down a full-grown man so he can stand on a tabletop, use visual effects to make him fly through the sky like a superhero or simply simulate your own TV weather report. But to do it right, you’re going to need a lot of green. No, we’re not talking about money. The green we mean comes in the form of a green screen. The secret to pulling your subject out of the real world and placing him or her into a digital domain is CHROMAKEY , and that means going green. In this article we’ll cover all the essentials you need to know to pull off keen, clean keys and composite digital backdrops and virtual backgrounds into your edits.

 Why Green?

      Many people use the terms chroma keying and green screening interchangeably, but the principle that powers chrominance keying is not limited to the green parts of the spectrum. In the visual effects world of Hollywood, blue screens are far more common than green. In fact, you can key out any color; red, yellow, purple or pink, blue and yes, green. So why is that odd and ugly shade of green the hue of choice for television and video? The biggest factor is contrast. In order to isolate one area from the rest, the background color must be distinctly different. Bright green beats blue partially because it is not a color commonly worn by talent. Any clothing that matches the background too closely will also key out, punching a hole in your subject’s body, or making him invisible altogether. We narrowly escaped a chromakey crisis a few years ago when I was working at a Northern California TV station. One St. Patrick’s Day our wacky weatherman showed up to work dressed head-to-toe in a bright green leprechaun suit. Fortunately, we quickly pointed out the flaw in his logic and he was able to find a change of clothes before it was time to go live.

What do I Need?

You can erect a simple chromakey setup almost anywhere with just a few basic tools. In order to shoot footage that will key cleanly, you will need a green background, a source of bright, even lighting, and a tripod to lock your camcorder in place. We will cover each of these elements in detail

 1 - Buy or Build a Background. The most obvious need is for the background itself. Fortunately, there are many options, and many of them are inexpensive. In short, anything that’s bright green will work, and anything that works is valid. I have produced professional green screen scenes using giant dedicated cyclone sets, large professional fabric chromakey backdrops, sheets of material purchased by the yard from a fabric store, smooth walls or pieces of paneling painted with a gallon of dinosaur-green paint from the kids section of the hardware store, even sheets of green poster board taped together. (NOTE: Be sure to avoid textured walls if you’re painting it green. Texture causes shadows.) The only requirements are that your background be large enough to fill your screen, smooth enough to take light evenly without showing wrinkles or casting shadows, and bright enough to contrast well with your subject. If you intend to chromakey a sock puppet your backdrop may be relatively small. If you need to chromakey a full-body shot of an adult actor, you will need a much larger background. If you are experimenting with chromakeying for the first time, you can test the process on a small scale before you build a big set. You can quickly create a functional miniature table-top set with an action figure and a sheet of posterboard. It doesn’t take much more effort, however, to stretch a sheet of green fabric between two step ladders or to paint a small.

2 - Light it Smoothly. Even a professional-quality cyc wall won’t key well without proper lighting. If there is a secret technique to getting good keys, it is in lighting the wall. The goal is to light the set as evenly as possible using soft light. Any variation in lighting will read as gradient coloring and will complicate your key in post. Achieving even lighting can be more difficult than you might think. Hard light sources cast narrow and focused beams that create circular hot spots in their center, then fall off rapidly leaving outer portions gradually darker. It helps to move small lights farther away so they cast broader, soft light beams. If you have access to soft boxes, they are excellent options for casting evenly spread light. I have had great success lighting green screens with long tubular fluorescent fixtures along the top and sides of the backdrop. You can get a six-foot garage-style fixture from your local home hardware store for less than lunch. These fixtures cast a soft light that is quite appealing for your green screen. You don’t want to light your talent with a flo-fixture, but when you are lighting a green set the color temperature of the instrument is not as important as lighting your background evenly.

 3 - Light it Separately. Another important, but often overlooked, essential is lighting your subject independently of your set. This is important for two “key” reasons: shadows and reflections. Part of keeping your wall evenly lit is keeping your subject’s shadow from falling across it. To do this you need to position the talent at a distance of at least a few feet from the screen, and light him separately using three point lighting.

 4 - Lock it Down. Unless you are planning to use motion tracking markers and a complicated visual effects compositing software to motion track your subject into a virtual 3D environment so that the movement of the camera perfectly matches that of the digital background shot, even the smallest camera motion is unacceptable. This is a case where it’s got to be all or nothing. Either commit to motion tracking, or set your camera in cement, and take your hand off when you roll to prevent any vibration. The trick is to shoot your subject in a way that blends well with the background. In most green screen scenes the digital backdrops will be a still, a stable computer generated (CG) image or animation, or a clip of video shot from a locked down camera. If the camera moves or shakes, even a little, your subject will appear to bounce around in front of your keyed background scene, and that’s a chromakey faux pas to avoid at all costs.

 5 - Frame and Focus. For the same reason, do not zoom in or out as you record. This will make your subject appear to shrink or grow in size in relation to the background. Use the zoom to frame the subject where you need her to be, and once you have set up the shot, step away from the controls. Focus is another important consideration. The only way to know that you have the sharpest focus possible is to zoom all the way in to your lens’ maximum telephoto setting when you adjust your focus. I typically recommend setting your focus on the sparkle, or specular highlight, in the eyes, but in a chromakey setup, I make an exception. The best keys come from clean, sharp edges, so it is important that the edges of your talent be in crisp focus when you shoot on green.

 6 - Pull the Key. If you have successfully shot your subject in front of a smooth, bright, evenly-lit wall, lit your subject independent of the background to avoid shadows and spill, and shot with a crisp, clear focus, then pulling the key to reveal your digital backdrops or virtual backgrounds should be a relatively easy process.