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What Is a Green Screen?
A green screen is a backdrop against which footage is shot to later be superimposed over a separately filmed background, in a visual effects process called “chroma keying.” It is also known as chroma key compositing, color keying, or simply keying.
Green screen technology is a central part of this chroma key effect, allowing filmmakers to isolate the actor from their bright green background and then introduce a separate background in post-production—an essential piece of Hollywood movie magic.
Why Is a Green Screen Green?
A green screen works by providing a green backdrop that can ultimately be removed in post-production by telling the computer to remove anything that is the color green in the shot. While the special effect can technically be done with any color, green is usually the safest bet because it has little overlap with human skin tones. If, however, there’s a particular prop you’re using that needs to be green, a green screen won’t work, because the computer will remove the green prop from the shot. In cases where green won’t work, filmmakers will use blue screens.
How to Use a Green Screen
Ready to put your green screen to work? Here’s how to use a green screen:
- Set up your screen. Hang your green screen on a frame so that it will fill the entire background of your shot. Make sure it’s as flat as possible, with no wrinkles or tears. If the green screen is collapsible or has been folded up in storage, use an iron to smooth out the creases and wrinkles. If you don’t want to deal with wrinkles, look into purchasing or renting a wrinkle-resistant matte screen.
- Get the right lighting. Many beginners think that if there’s enough light on their subject, then the green screen will be fine—but actually, you need to have separate lighting set up to light your green screen, too. Otherwise, it will look patchy on camera and be more difficult to key out in post-production. The best lighting setup will be diffused, hitting your screen from above to avoid making any directional shadows. You’ll most likely need at least two 1000-watt bulbs, diffused through a softbox or even a DIY tool like white bedsheets, to get the best effect.
- Set up your subject. Whether you’re filming an actor or an object, you want to have as much space between the subject and the green screen as possible. If the subject is right in front of a green screen, it’s more likely that a greenish hue will reflect onto them, and the harder it will be to match the lighting of your subject with the lighting of the background you’ll be adding in in post.
- Film. Once the green screen set is ready, now comes the easiest part—filming the scene. Make sure to capture several takes, otherwise, you may not end up with the raw footage you need in post.
- Edit together the rough cut. Before you even think about adding in your non-green-screen background, focus on editing all of the footage into a rough cut, or, ideally, your final cut. You don’t want to spend tons of time keying out green screen footage only to realize you won’t be using that shot in the final.
- Edit out the green screen. Now that your footage is ready, it’s time to edit out the green screen using your editing software. There are many types of editing software you can use, and each one will follow a slightly different set of instructions. In general, there will be a button you press that will automatically remove anything green in the shot, and then a series of sliders you can use to further adjust and customize the keying out.
- Paste in your new background. After the green screen is edited out, all that remains is to add in your desired background image or background video on a new layer beneath the subject. Pay attention to the details, tweaking the lighting, sharpness, hue, and other levels to make the image as realistic as possible.
5 Tips for Using a Green Screen
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Here are a few filmmaking tips to help you make the most of the green screen effect:
- Pay close attention to your lighting levels. Lighting can make or break a green screen scene. To be convincing, your lighting must be consistent in both the foreground and background footage. If the sun is bright in the background footage and muted in the foreground footage, it will ruin the illusion as the background and foreground will not be cohesive.
- Don’t ignore the shadows. Lighting must also be even and soft: if the foreground footage is filmed in uneven light with shadows, different shades of green will appear on the green screen and will thus prove tough to isolate in post-production. The green background needs to appear as a single color and a solid color for the keying process to work, so the subject must be at least a few feet in front of the screen when filming, and should be lit by three-point lighting.
- Depth of field is essential. Camera distance is very important: the depth of field in both foreground and background footage should match, otherwise the composite image will look strange to the human eye and will immediately distract viewers.
- Use a tripod and keep it stable. To make your green screen work, your camera must remain stable and at the same focus. A vibrating or shaky video camera in the foreground will contrast the stability in the background. (Similarly, changing focus or zooming in will also contrast the background stability.)
- Leave green props and clothing at home. Clothing can ruin a good green screen scene—always ensure actors’ costumes do not have the same color as the screen behind them, or that part of their costume will also be replaced in chroma keying.
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