Advanced Video Production Techniques : Inserting Virtual Backgrounds Using Chroma Key

If you've ever wondered how your local weatherman manages to stand in front of huge swirling weather maps, the answer is using a technology known as chroma key (also called green screen or blue screen). The weatherman isn't actually standing in front of those pictures. He's standing in front of a blank wall painted a very bright, unnatural green. Then, using what until recently was incredibly expensive technology, the green background is removed from the video image and replaced with the graphics that you see on television. When the weatherman is looking off to the side, he's actually looking at a small television monitor to figure out where to point.

Nowadays, chroma key is built into many video-editing platforms. Some require an additional plugin, but others include it as part of the basic functionality. To use this feature, however, you have to film yourself (or whoever your subject is) in front of a green (or bright blue) wall. The trick is to make sure the wall color is very uniform and is lit in such a way that there are no shadows on the wall. You can buy custom paint that the professionals use to paint their chroma key walls, or if you're budget constrained, you can buy a roll of bright green butcher paper at your local art supply store.

You need a large area to film against, because you have to stand far enough away from the green screen so that you don't cast any shadows on it. Lighting for a green screen shoot is an art form in itself. This is a good example of where calling in a professional to help you out is a great idea. After you've got a lighting setup established, you can reuse it for future shoots.

After you've shot your video against the green screen, the process for substituting the background depends on your video-editing software. Figure 1 illustrates the chroma key effect from Vegas. After you've specified what color to use as the key for the chroma key effect, that color is removed from the frame and another image or video is substituted where the key was.


Figure 1: Vegas chroma key effect

Tip One good reason to use chroma key is that the backgrounds are generally static, and as we'll find out later, static backgrounds encode best. Conversely, don't use backgrounds with motion in them if they can be avoided.

Most video-editing programs deal with video in terms of tracks, so when the chroma key effect is used, the video track beneath the main track is revealed. This is how the weatherman appears to be standing in front of the weather maps. In actuality, the weather maps are just showing through where the original chroma key color was.