As the ceremony got under way with a dramatic, drummed countdown, viewers watching at home and on giant screens inside the Bird's Nest National Stadium watched as a series of giant footprints outlined in fireworks processed gloriously above the city from Tiananmen Square.

What they did not realise was that what they were watching was in fact computer graphics, meticulously created over a period of months and inserted into the coverage electronically at exactly the right moment.

The fireworks were there for real, outside the stadium. But those responsible for filming the extravaganza decided in advance it would be impossible to capture all 29 footprints from the air.

As a result, only the last, visible from the camera stands inside the Bird's Nest was captured on film.