If you're passing the Waterloo Imax cinema in London in the next few weeks, you'll see its giant rotunda has been draped in an equally outsized picture representing the seven ages of man.

The image is made up of thousands of other tiny pictures, all of them portraits of Canon Europe employees. It is being touted by the imaging company as the biggest photomosaic in the world.

The mammoth montage is one of Canon's biggest publicity posters yet and was commissioned to launch the company's new people-empowerment 'You can' campaign. It was created by Robert Silvers, artist and head of Runaway Technologies.

Silvers is the originator of the now famous, and heavily patented, Photomosaic technology. With it he has created images that have graced the covers of Time, Newsweek and Life and have been hung in art galleries around the world.

The software, which Silvers developed while a research student at the Massachusetts Institute of Technology, divides a picture into a fine grid, then trawls through a pool of thousands of other images to match them for colour, brightness and shape to each of the squares in the grid, rendering the picture in a mosaic.

The technique is raised above the merely mathematical if the pool of photographs used for the tiles is selected to add greater significance to the finished image. For instance, Silvers has produced a portrait of the late Princess Diana made out of images of (English) roses. Likewise, using Canon Europe employees to make up the seven ages of man 'emphasises the universality of opportunity' the company aims to push in its campaign.

But the process can also be used to produce fiendishly difficult jigsaw puzzles.

When Silvers devised the software in 1995, using it was rather processor-intensive and images could take hours to produce. Now, however, the average PC can churn one out in less than an hour.

Unfortunately, Silvers has not released his coveted software on to the market, nor does he plan to. "That's not the direction I want to take the company," he says. "I am an artist first and foremost." But it's hard to keep a good thing quiet in today's world, and plenty of copycat programs have popped up and are available as shareware on the internet.

Many of them are not nearly as impressive, however, as they use a different technique.

Their effectiveness depends less on choosing the best image for each tile, and more on inserting a faint overlay image of the original picture over the mosaic, to tinge each tile with the right tone. Where Silvers prides himself on making the best selection possible, often discarding the first and second choices his software offers him, other programs often rely on a Photoshop-style cheat.

There's an irony here, since if a competitor package comes too close to the quality of Silvers' program, it is likely to infringe on one of his large collection of patents. As he says, "The better the imitation [software] is, the more I'd regard it as illegal."

This obviously constitutes a sizeable grey area, as most of the shareware available includes both image-trawling algorithms of the photomosaic type and the easy cheat overlay system. Buyers must use their own judgement of what is legal and what is effective.

If you're interested in finding out more about photomosaics, check out Runaway Technologies' site or search for shareware at www.imagespro.com.