The term chroma key is often used loosely to mean anything that refers to pulling a color difference matte from film or video footage. Even several manufacturers of matte composit ing hardware and software still refer to the process as chroma keying. Although chroma keyers have evolved to do basic keying quite well, the process refers to switching off certain color pixels in an image or footage. Thus, when selecting a green color from the background in an image layer, the keyer looks for all instances of that green color and switches them off (or makes them transparent), revealing the image or footage below. This often causes arti facts and bleed around the edges of the subject you're trying to key out. Most keyers attempt to minimize this effect by expanding the color range slightly and choking the subject to remove green edges. This approach doesn't work well to retain shadows, reflections, or transparency, nor does it handle spill suppression well.
A good hardwa re keyer or compositor has the technology to handle spill suppression,
transparency, and shadows, including motion blur in the foreground footage. Most soft ware keyers that a re included with non-linear editing (NLE) applications such as Final Cut Pro have minimal control over these issues and often rely on a good third-party plug-in to do the compositing.
For comparison, I've illustrated in Figure 2.6 the difference between the results
from a basic chroma keyer and a color-difference matte compositor. The first two images are the original green screen footage and then the chroma-keyed image against the back ground plate. The third and fourth images represent the matte generated with the Ultimatte AdvantEdge plug-in for Photoshop and the composited result