Later, in postproduction, specialeffects
technicians substitute a different scene in place of the screen. So, a scene that looks like
it happened in midair or in France or on the moon actually happened in front of a huge blue
(or green) tarp. Separate footage is added in place of the tarp, and voil? . That??™s movie magic.
You can achieve the same kind of results yourself, whether you use a real blue screen or
jump in with no preplanning at all.
CHAPTER 15: Create Special Effects 307
Make a Weather Map
Let??™s start with the kind of shot you plan ahead for, which makes it a tad easier to get good results.
First, think about the evening news. You??™ve seen the weather forecaster stand in front of
a map of the United States and point to cold fronts a million times before, but have you ever
thought about how that bit of TV magic happens? Obviously, the forecaster isn??™t standing in front
of a real map. Instead, it??™s a blue or a green screen??”a solid-colored rectangle. An engineer uses
a technique called Chroma Key to replace the rectangle with video from another source. What
happens is this: the engineer selects the color of the blue screen and identifies it as a ???key.??? The
second video source then overwrites anything in the scene that has the key-valued color.
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