Developers once had little time for such whizzery. A model and glossy brochures were enough. Tough times and a bad case of the British disease have changed all that, however.
"British tenants like to walk around finished buildings before making a decision," says Charles Stevens of property consultants Allsop & Co. Computers can now hurry things along.
Colin Hargreaves, of property consultants Healey & Baker, is manipulating a remote control. A gaggle of suits ride a jet high above the City of London. Suddenly they dive to weave through the corridors and malls of the Helicon, a futuristic glass City complex. This merging of video and virtual reality is designed to make viewers gasp.
Video clips, virtual graphics and statistics are manipulated from a remote control. The computer contains a large hard disk developed from the technology used to send video-on-demand services down phone lines.
"I can watch the audience and freeze the picture when someone asks a question - or appears to be going to sleep," he says.
Multiple "buttons" on the screen open specialised areas of information in pictures, video clips or virtual images. Another "button" moves into detailed technical analysis. The show, which can be tailored to a few minutes or more than an hour, was developed by the designers Sutton Young and cost close to pounds 100,000 - about a quarter of that going on the virtual fly-through.
But this outlay has to be set against the alternative of an audio-visual display (pounds 50,000), a full brochure (pounds 40,000) or a technical pack (pounds 10,000). Hargreaves believes he is saving money. And if viewers want to see the real thing, they can walk next door, where the Helicon is being built over a Marks & Spencer store.
Meanwhile, next to St Paul's Cathedral, Robert Game of the developer MEPC is setting up a show that draws even more heavily on computerised imagery to demonstrate the nearby Petershill office complex. The hardware is also slightly different, involving a pounds 12,000 video and photo-CD player with up to 600 static shots of the proposed development.
Again, the remote operator can stop and branch into areas of special interest. It can also be updated quickly as the building progresses, says BDG McColl, the firm that designed and produced the show. So can the video that visitors take away. Game would prefer to hand out CDs but says that CD-Rom players are rare. "This is not virtual reality," insists Phil Hutchinson of BDG McColl.
"It is computer modelling, running at 25 frames a second in high detail." This detail is crucial. Virtual reality would allow viewers to wander at will - but reduce what they saw to squares and triangles.
True virtuality will come as processing power grows. Hutchinson says they are using two linked Pentium PCs running AutoCAD to model from drawings and 3D Studio to render and light the sequences. "Only two years ago we needed to link 12 PCs to do this. In another two years, a 166mhz Pentium could be the basic standard machine."
But PC power may not be sufficient. "You also need theatricality," says Peter Russell of Rosswood Computer Services.
Rosswood is knee-deep in animators with showbiz roots. It is working on proposals for one of the country's largest shopping developments. This kind of approach, aimed at planners and public nervous about the visual impact of development, could be a big growth area for virtual techniques.Reuse content