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.