Are you Lighting Right for Green Screen?

The Coolest Post-Production Tool

Among the powerful tools in the digital post-production toolbox, chromakey has to rank among the coolest. With a few clicks of your mouse, you can drop your foreground subject into any background you like. However, chromakeying can be quite frustrating if you're not a pro. Though it can be easier to handle in editing, it's an absolute bear in the studio, because the colored background requires that the process must be uniformly lit.

Very, very uniformly lit. And while your software might be magic, the key to a good key is good source footage. So let's review the basics of lighting for chromakey; but before we start deploying spots, floods and soft lights, we need to examine the chromakey process to see how it works and where the problems lie.

Chromakey Compositing
"Chroma" means color in video jargon, and chromakeying works by shooting a subject against a background of a very simple and uniform color. In post, the editing software replaces that color (and only that color) with a different shot. The most common example is the map (background) that fills the screen behind the meteorologist (foreground) on the local news. That person is actually standing in front of a solid green or (less frequently) blue screen, which is replaced in the control room with a background image of the jet stream, a map of the region, or some other graphic representation of the day's weather.

Sophisticated compositing software can compensate for slightly uneven lighting and even distinct shadows (on the Weather Channel, the meteorologists can actually touch the screen). Chances are, however, that your software isn't that smart, so uneven lighting affects the quality of the background image. In theory, you create a perfectly uniform background color by following just three simple rules:

• Light the background perfectly evenly.
• Light the subject separately from the background that you'll key out.
• Keep the subject and background as far apart as possible.

Those rules are simple enough in theory, but in practice they can be frustratingly hard to follow. To understand why, we need to look at the geometry of the background screen, the lens focal length, and distance between the camera, the subject and the screen.

Learn more at "Light Source: Lighting for Compositing"

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