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.

The trick was revealed in a local Chinese newspaper, the Beijing Times, at the weekend.

Gao Xiaolong, head of the visual effects team for the ceremony, said it had taken almost a year to create the 55-second sequence. Meticulous efforts were made to ensure the sequence was as unnoticeable as possible: they sought advice from the Beijing meteorological office as to how to recreate the hazy effects of Beijing's smog at night, and inserted a slight camera shake effect to simulate the idea that it was filmed from a helicopter.

"Seeing how it worked out, it was still a bit too bright compared to the actual fireworks," he said. "But most of the audience thought it was filmed live - so that was mission accomplished."

He said the main problem with trying to shoot the real thing was the difficulty of placing the television helicopter at the right angle to see all 28 footsteps in a row.

One advisor to the Beijing Olympic Committee (BOCOG) defended the decision to use make-believe to impress the viewer. "It would have been prohibitive to have tried to film it live," he said. "We could not put the helicopter pilot at risk by making him try to follow the firework route."

A spokeswoman for BOCOG said the final decision had been made by Beijing Olympic Broadcasting, the joint venture between the International Olympic Committee and local organisers that is responsible for providing the main "feeds" of all Olympic events to viewers around the world.

"As far as we are concerned, we let off the fireworks - that's what's important to us," she said.

Mr Gao said he was worried that technologically literate viewers who spotted the join might be critical, but comments online suggested more admiration of the result.

Although the event as a whole received rapturous reviews abroad, that has not been entirely the case at home. Some internet comments were hostile, saying that while it looked stunning the contents were vacuous.

Others focused on the sheer numbers of people involved - more than 16,000 performers, mostly from People's Liberation Army song and dance troops.

"That certainly showed China's unique character," said one comment. "Namely, that we have 1.3 billion people."

Chinese officials have admitted deceiving the public over another highlight of the Olympic opening ceremony: the picture-perfect schoolgirl who sang as the Chinese flag entered the stadium was performing to another girl's voice.

The girl in the red dress with the pigtails, called Lin Miaoke, 9, and from a Beijing primary school, has become a national sensation since Friday night, giving interviews to all the most popular newspapers.

But the show's musical designer felt forced to set the record straight. He gave an interview to Beijing radio saying the real singer was a seven-year-old girl who had won a gruelling competition to perform the anthem, a patriotic song called "Hymn to the Motherland".

At the last moment a member of the Chinese politburo who was watching a rehearsal pronounced that the winner, a girl called Yang Peiyi, might have a perfect voice but was unsuited to the lead role because of her buck teeth.

So, on the night, while a pre-recording of Yang Peiyi singing was played, Lin Miaoke, who has already featured in television advertisements, was seen but not heard.

"This was a last-minute question, a choice we had to make," the ceremony's musical designer, Chen Qigang, said. "Our rehearsals had already been vetted several times - they were all very strict. When we had the dress rehearsals, there were spectators from various divisions, including above all a member of the politburo who gave us his verdict: we had to make the swap."

Mr Chen's interview gave an extraordinary insight into the control exercised over the ceremony by the Games' political overseers, all to ensure the country was seen at its best.

Officials have already admitted that the pictures of giant firework footprints which marched across Beijing towards the stadium on Friday night were prerecorded, digitally enhanced and inserted into footage beamed across the world.

Mr Chen said the initial hopefuls to sing the anthem had been reduced to ten, and one, a ten-year-old, had originally been chosen for the quality of her voice. But she, too, had fallen by the wayside because she was not "cute" enough.

"We used her to sing in all the rehearsals," Mr Chen said. "But in the end the director thought her image was not the most appropriate, because she was a little too old. Regrettably, we had to let her go."

At that point Yang Peiyi stepped up to the plate.

"The main consideration was the national interest," he said. "The child on the screen should be flawless in image, in her internal feelings, and in her expression. In the matter of her voice, Yang Peiyi was flawless, in the unanimous opinion of all the members of the team."

That was until attention turned to Yang Peiyi's teeth. Nevertheless, Mr Chen thought the end result a perfect compromise.

"We have a responsibility to face the audience of the whole country, and to be open with this explanation," he said. "We should all understand it like this: it is a question of the national interest. It is a question of the image of our national music, our national culture.

"Especially at the entrance of our national flag, this is an extremely important, an extremely serious matter.

"So we made the choice. I think it is fair to both Lin Miaoke and Yang Peiyi - after all, we have a perfect voice, a perfect image and a perfect show, in our team's view, all together."

One question remains: why was Lin Miaoke allowed to give interviews in which she lapped up the praise for her singing. Mr Chen said she might not have known that the words she was singing could not be heard. She had, in fact, only known she was going to perform at all 15 minutes beforehand.

Yang Peiyi is said to have reacted well to the disappointment. "I am proud to have been chosen to sing at all," she is reported to have said.

Why couldn't they fix her teeth with CGI?

Turns out that Bill Gates was not the only famous Microsoft identity to be spotted at the Olympic opening ceremony.

Numerous bloggers also observed the cameo played by the so-called Blue Screen of Death (BSoD) - the Windows error screen that flashes up on your computer, signifying that it's time to reboot.

The screen was projected onto the roof of the National Stadium during the grand finale to the four-hour spectacular as veteran Chinese gymnast Li Ning, suspended 70 metres above the stadium floor, "jogged" towards the Olympic torch.

The BSoD message begins with the following sentence: "A problem has been detected and Windows has been shut down to prevent damage to your computer."

Usually, the only way to remove the BSoD is to restart your computer pressing the Ctrl+Alt+Delete buttons at the same time.

Images of the malfunction have popped up on Chinese forum sites showing the clearly visible error message projection.

The ceremony, which featured 14,000 performers depicting 5000 years of Chinese history, also involved the use of advanced technology to control the sound, lights and projectors.

At various times during the event, images of whales, clouds, and the faces of young children were superimposed on the roof the stadium - also known as the Bird's Nest.

And as Li Ning completed a lap of the stadium roof's rim, images of the torch relay were flashed up.

The spectacular opening ceremony also included 35,000 fireworks launched from 1800 sites around the city, to form patterns in the air.

Yesterday it was revealed that organizers digitally inserted computer-enhanced images of some of the firework displays broadcast during the opening ceremony.

The BSoD is a bit of an in-joke among geeks and is often used to poke fun at Microsoft's bug-prone Windows operating system.

Gates, who stepped down from his last official full-time position at Microsoft at the end of June, remains as the software company's part-time, non-executive chairman.

He has been spotted at several Olympic events, accompanied by his wife and is here is a private capacity. Microsoft is designated as an official supplier to the 2008 Olympic Games.

It's Windows XP operating system was chosen to run on all PCs used by the organizers a well as being the operating system installed on PCs supplied by Lenovo Group, the computer maker than is one of the major sponsors of the 2008 Olympics.

Microsoft's newer operating system, Vista, was not chosen.

Lenovo chairman, Yang Yuanqing, was quoted as saying that because of the complexity of the IT functions at the Games, it was decided to not use the the more recent operating system. "If it's not stable, it could have some problems," he said.



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