Pre-recorded footage was used to showcase the dramatic opening ceremony fireworks show.
From CNN –
Fireworks that burst into the shape of 29 gigantic footprints were shown trudging above the Beijing skyline to the National Stadium near the start of the ceremony.
Though the footprint-shaped fireworks were real, some of the footage shown to television viewers around the world and on giant screens inside the “Bird’s Nest” stadium featured a computer-generated three-dimensional image.
“It was confirmed that previously recorded footage was provided to the broadcasters for convenience and theatrical effects — as in many other big events,” Beijing organizing committee spokesman Wang Wei said. “On the day of the ceremony there were actual footprints of fireworks from the south to the north of the city.
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http://edition.cnn.com/2008/TECH/08/12/fake.fireworks.ap/index.html?imw=Y&iref=mpstoryemail