Wow, how did they do that?

For a film with not a single original idea, Roland Emmerich's mega-buck Stargate astounds with its special effects. They have used the lot to tell this tale of a threat to Earth by Ra, a time vampire played by Jaye Davidson, of The Crying Game fam e, whoplans to return as the leader of a lost race of ancient Egyptians. The Stargate of the title is a space gateway in the shape of a round door that instantly transports you millions of light years. To thwart the enemy's return, the US military ropes in DrDaniel Jackson, an Egyptologist played by James Spader, to activate the amazing artefact, pass through it into hyper-space, and confront the villain.

This plot provides a vehicle for the finest computer-generated effects to grace a cinema screen. This is not the only application of technology: the director was able to try out different camera angles before shooting a single frame thanks to computers that generated 3D storyboards setting out the film's sequences electronically. And to help earn back the studio's $50m (£32.8m) investment, a Stargate CD-Rom game will be released by Acclaim.

Only a few effects were achieved by physical means - notably a slobbering desert yak, consisting of a Clydesdale horse dressed in an electronically controlled contraption to increase its size and drooling ability. The most spectacular tricks were computer generated. The same morphing technology that turns a snail into a frog on a TV advert is used to reveal Ra's face beneath the forbidding headdress he uses to dominate his slaves in the desert city of Nagada. And the activation of the Stargate gave Mr Emmerich a chance to create a kind of self-illuminated spinning tunnel made of water that transforms itself into a rippling round doorway of vertical water.

To satisfy the expected public demand to know how such effects were achieved, the producers assigned Compton NewMedia to release a CD-rom called Secrets of Stargate, distributed in the UK via Gem for release within the next two months at £34.95. It claims to take you "behind the scenes" via test shots of effects and interviews with cast and production team. But it is more of an expensive promo for the movie.

So for some real how-did-they-do-that? secrets, I talked to Jeff Kleiser, co-founder of Kleiser-Walczak Construction Company, which, for a fee of $3m, was responsible for all the computer effects. "No one really knew what any of the effects were supposedto look like, so we got a chance to come in right at the beginning; we helped to design everything."

Mr Kleiser put a team of 15 on the set, with another 25 back at his company's offices. They wrote dedicated image-creation and compositing software, as well as using commercial digital packages such as Domino and Matador to create the Stargate itself. The same software was behind the morphing helmets worn by Ra and the Horus guards, the entire cityscape of Nagada, and the "clean-up" shots needed, at $50,000 a time, to erase footprints in the desert sands.

Mr Kleiser swears by a digital box-of-tricks called Wavefront. "You can hook on to it and write your own effects programs. To create the illusion of vertical water [on the Stargate] we had to make accurate reflections of the people as they approached, sowe set up a camera that looked up at the Stargate at a 45-degree angle. Then we shot with a second camera on the other side to get both points of view, which we then turned into the reflection paths."

Because the Stargate is a water plane, it had to ripple whenever anyone touched it. This presented Mr Kleiser with one of his greatest challenges.

"It all had to be digitised so the ripples would be accurate. Scanning lasers were lined up at the surface of the gate so when James Spader's face poked through, the image was captured. Then, using Matador [a digital matte program], we painted out the body and created mattes to obliterate the corresponding parts of the body that had gone through the gate. Then we repeated the process from the other side."

Matting is a method of cutting out images within any frame and replacing them with something else. These days it is done electronically, revolutionising the post-production process. When Mr Kleiser's computer-generated ripple mattes were digitally composited on to the live action of James Spader walking, he appeared to be passing through a gate of water.

The pastry-loving Mr Emmerich wanted Mr Kleiser to produce something spectacular when the gate is first activated but didn't know what. The closest he could come was that he wanted it to turn into a something with a swirling surface like a New York-stylestrudel. The result was the spinning water tunnel. To achieve it, Mr Kleiser set up an air cannon pointing down into a water tank and shot the burst of air that the cannon fired into the water. "Then we digitised the footage, turned it on its side and set it spinning. Roland was happy with his "strudel'!"

What a pity such dedication, enthusiasm and minute attention to detail was not invested in the script.

The writer is London editor of `The Film Journal', a US monthly.