Chapter 6. Color Keying
Color keying is the big lie that somehow manages to kinda, sorta reflect the truth. Someday perhaps, when we have cameras that can perform full 3D scene reconstructionnot just what the lens lays down in a two-dimensional image, but a scan of the full spatial and temporal information of the scenewe'll laugh about the old days when we used to send actors out on to a mostly empty stage of saturated blue or green, in some cases completely psyching out their performance (it's been compared to "acting in a void" or "acting in the desert"), so that later, the color could stand in for transparency. 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 figuresusually actorsin one setting and composite them into another.
The process goes by many names: color keying, blue screening, green screening, pulling a matte, color differencing, and even chroma keying, a term more likely to conjure the image of a weather forecaster on your local news, waving her hands to make the clouds clear out over Columbusor just Bill Murray in Groundhog Day. This chapter will discuss not only color keying with blue- and green-screen footage but also all cases in which pixel values (color, luminance, saturation, and so on) stand in for transparency, allowing compositors to effectively separate foreground from background based on channel levels. You will learn how to create several types of keys using techniques that create luminance information that is then applied to the alpha channel of a layer (or layers). The black areas will be transparent, the white areas opaque, and the gray areas gradations of semi-opacity.
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