If you've watched the weather person on the news standing in front of an animated map or forecast for the week, you've seen green screen, or "chroma key" compositing in action. Originally something that could only be done by skilled visual effects artists working on larger budget film and television, anyone can now incorporate this technique using their mobile device and free apps like iMovie.
The process involves capturing film or video of a subject performing in front of a flat green background. That footage can then be manipulated in post-production to remove the green background and replace it with other footage.
The key to understanding chroma key or green screen compositing is understanding a few basic principles that drive the process.
The first is color. Every color image and video can be broken down into combinations of three colors (red, green, and blue) at different intensities. When we're working with digital video, the computer assigns numerical values to define how much red, green, and blue are in each pixel of the video (you might have seen a color identified by it's RGB values--that's those numerical values). When we have a large area of the image that is the same color (or a range of very similar colors), it is relatively easy to tell the computer to ignore those pixels and make those pixels transparent. The color you want to remove is known as the key color. Then you can replace them with anything else you want...still photos, other footage, animations, virtual backgrounds... the real or imaginary sky is truly the limit!
Do you have to use green for green screen? Technically, yes. You could also use blue, and many productions do, but that is technically called blue screen. (We're guessing you could have figured that one out.)
Blue and green are usually the colors used for this process because human skin tones fall under shades of red, and typically the subject of the footage you're shooting is a human being. And green is more common than blue because the very intense green used in backgrounds rarely shows up in clothes or other elements you want to keep. (Although sometimes it does, intentionally or not, with funny and creative results.)
Chroma keying--whether it's green screen compositing or blue screen or any other color—is easy to do when you follow some basic best practices.
You need to make sure you set up your backdrop and your camera to get a clean image that makes it easy for your editing program to differentiate between the foreground and background elements.
When you're setting up your green screen for production:
- A painted wall, paper backdrop, or a pop up screen that offers a flat surface are ideal choices. A curtain or sheet can work but you need to be very careful of folds and wrinkles that can create shadows that make getting a clean key in post difficult.
- You need to make sure that both your backdrop and your subject are evenly lit to get the best results. It’s especially important to avoid shadows on the backdrop as they can make it difficult for your video editing program to differentiate between the backdrop and the foreground. You want the background to be one consistent shade of color (the key color): left to right, top to bottom. (If you want to do a quick check, snap a quick photo and check RGB values of various areas of your background in a photo editing program. They should match as closely as possible.)
- Make sure to separate your subject from the backdrop as much as you are able to. This can be difficult at times, but it is really valuable to give as much space between your subject and the background as possible. The bright lights evenly lighting the green backdrop can cause the green or blue to reflect, or “spill,” onto your subject creating a highlight that color on them that can make getting a clean key difficult.
- Shoot at the highest frame size (HD 1080p or UHD 2160p for example) and bit rate that your camera is capable of so that you have as much information to work with as possible.
- In addition to needing a lot of light on your subjects to help separate them from the environment this will also allow you to shoot at a low ISO and avoid video noise, which can cause problems when trying to get a clean key.
- You need to shoot at a fast shutter speed to avoid motion blur in the footage you shoot. Motion blur can cause issues for getting a consistently clean key, especially if you have a lot of movement.
- Using a higher aperture (when your camera's iris is more closed and lets in less light) is also going to be important. Not only does your subject need to be in focus, so does your backdrop. Everything needs to be in focus to get the best results. If you want to have a shallow depth of field, you’ll need to fake that in post production with blur effects. A shallow depth of field when shooting on a green screen can lead to difficulty getting a clean key—especially around hair.
- Finally, make sure your talent isn’t wearing anything close to the same color as your backdrop. Even if the color is a different shade, it can make it difficult for your video editing program to differentiate between foreground and background. Avoiding similar colors will make the process of getting your key much easier and faster.
To learn more in-depth information about techniques for green screen compositing and how it works dive into this green screen guide and this set of tips from Premium Beat. To learn more about MNN and the Media Education Bootcamps, Professional Courses and Workshops we provide, visit us at: www.mnn.org/learn