Chapter 6. Color Keying
Juxtaposing a person with an environment that is boundless, collating him with a countless
number of people passing by close to him and far away, relating a person to the whole world,
that is the meaning of cinema.
—Andrei Tarkovsky (Russian film director)
Color keying is uses a big lie—pretending a solid color is negative space—to allow you to place human char-
acters in virtually any location imaginable. Someday perhaps, when we have cameras that can perform full
3D scene reconstruction—not just what the lens lays down in a two-dimensional image, but a scan of the full
spatial and temporal information of the scene—we’ll laugh about the old days when we used to send actors out
on to a mostly empty stage of saturated blue or green.
Not only does this artificial environment create an often challenging situation for the compositor, who must
replace every trace of blue or green with subtle transparency, but in some cases it completely psyches out
actors. (It’s been compared to “acting in a void” or “acting in the desert” and was rumored to be a major reason
Liam Neeson announced his intention to retire following Star Wars, Episode One, a statement of frustration
he later recanted.)
Meanwhile, as they have for over half a century, blue screens (and their digital-era cousins, green screens)
remain the state of the art for situations in which you want to shoot figures—usually actors—in one setting
and composite them into another.
Novices often wonder if a background must be blue or green to be keyed. The answer is no. It just
happens that blue and green are primary colors not dominant in human skin tones and can be differ-
entiated clearly from most foreground colors, which contain a mixture of all three color channels.
The process goes by many names: color keying, blue screening, green screening, pulling a matte, color differ-
encing, and even chroma keying, a term that really belongs to television (think of a weather forecaster blowing
storm clouds across the eastern seaboard—that is, Bill Murray in Groundhog Day).
This chapter covers not only color keying of blue- and green-screen footage but all cases in which pixel val-
ues (hue, saturation, and/or brightness) stand in for transparency, allowing compositors to effectively separate
foreground from background based on color data.