Using Keying to Isolate Foreground Elements

There are two different methods used for keying: chroma keying and luma keying. Chroma keying is a method of keying on a particular hue of color. Although any color can be keyed on, the colors most frequently used for chroma keying are blue and green. Specific hues of blue and green with particular levels of saturation have been developed that provide the best results, and different companies have created commercially available paints, fabrics, and papers that use these colors.

The color you use—blue or green—depends largely on the color of your foreground subject. If you’re trying to create a key around a blue car, you probably want to use green as your background. Another advantage of using green, when possible, is that video formats generally preserve more information in the green component of the signal, resulting in slightly better keys.

Luma keying is based on a particular range of luma. Black is usually used, but you can also key on white. While keying out a white or black background may be more convenient in certain circumstances, it may be harder to correctly isolate your foreground subject because of shadows and highlights, which may have black or white values close to the luma range you’re keying out.

Regardless of the keying method you use, it’s important to start out with clips that key well. The decisions you make before and during your shoot affect how well your footage keys. Make sure that you:

Choosing an Appropriate Video Format

Ideal video clips for keying can be captured from footage in uncompressed or minimally compressed video formats, such as Betacam SP or Digital Betacam footage digitized with an uncompressed video capture interface, or DVCPRO 50 footage captured digitally with no additional compression added. Compression discards color information from a clip and can add artifacts around high-contrast edges in the picture (such as the edges surrounding the image to be keyed). If you use compressed video to create keying effects, you’ll frequently lose details around the edges of the keyed image, including hair, translucent cloth, reflections, and smoke.

If you must apply compression during capture, you can still pull good keys from clips with as much as a 2:1 compression ratio, but ideal source footage should be uncompressed. DV footage, which is compressed with a 5:1 ratio as it’s recorded, is less than ideal. This is because of compression artifacts that, while invisible during ordinary playback, become apparent around the edges of your foreground subject when you start to key. However, this doesn’t mean that you can’t key with DV footage.

With a high-quality DV camera and good lighting, it’s possible to pull a reasonable key using DV clips, but you cannot expect the kind of subtleties around the edges of a keyed subject that you can get with uncompressed or minimally compressed footage. For example, while you may be able to preserve smoke, reflections, or wisps of hair when keying uncompressed footage, with equivalent DV footage this probably won’t be possible. On the other hand, if your foreground subject has slicked-back hair and a crisp suit, and if there are no translucent areas to worry about, you may be able to pull a perfectly acceptable key.

Using Proper Lighting

The lighting you use when shooting blue- or green-screen footage plays a crucial role in determining whether or not you’ll be able to key out the background easily.

  • Blue or green background: Should be evenly lit, with no exceptionally bright areas (hot spots) or shadows. The material you use for the background screen should be smooth, with no bumps or wrinkles.
  • Video signal: Should have a minimum of film or video grain, since the “noisiness” that grain introduces can make it more difficult to pull a good key. Video can get grainy in low-light situations, so the lighting on your background screen should be bright enough that you don’t have to turn up the gain of your video camera.
  • Lighting of foreground subject: Should have close to a 1:1 ratio to the lighting of the background screen. This avoids overexposing or underexposing the background screen when the foreground subject is correctly lit.

    Once your background blue or green screen is properly lit, you should concentrate on lighting the foreground subject to match the scene into which you’re going to composite it. It’s especially important to make sure that the contrast between the shadows and highlights of your subject’s lighting is correct. While you can use the Final Cut Pro color correction filters to easily adjust the color and overall brightness of your subject, contrast is not so easy to change. This is not to say that you need to light your foreground subject flatly. Just make sure that the direction, quality, and contrast of the lighting you use works for the scene your subject will inhabit.

  • Distance between foreground subject and background screen: It’s a good idea to have some distance between the foreground subject and the background screen, to reduce the amount of colored light bouncing off the background blue or green screen and “spilling” on the foreground subject. In general, position your subject 5 to 10 feet away from the background screen.

Using Video Scopes to Help Correctly Light Your Background Screen

Because the Waveform Monitor shows you all variations in brightness in the video frame from the left to the right of the screen, you can use it to reveal hot spots in your background screen that may be difficult to see with the naked eye. If you connect your portable computer to a DV video camera with FireWire while you’re on the set, you can see the brightness level across the entire blue- or green-screen background on the Waveform Monitor’s graph. (You can also use an analog-to-DV converter to connect to the analog output of a camera with a different format.) Hot spots appear as spikes on the Waveform Monitor, and shadows appear as downward-pointing spikes, while shadowed areas appear as dips.

Figure. Viewer window showing hot spots and shadows on a background screen and the Waveform Monitor showing bright areas as tiny spikes and shadows as small downward-pointing spikes.

With this information, you can adjust your lighting until it appears even in the graph in the Waveform Monitor, ensuring the best possible background for keying. Once your lighting is adjusted, you can perform an additional test by capturing a clip and then using the Chroma Keyer filter to see how well it keys.