Super screen

Super-bright, large-scale, and very high-resolution digital projectors are becoming indispensable tools of modern communication. They are also very expensive. Prices for high-end projectors run into tens or even hundreds of thousands of pounds, which keeps these devices from being widely used.

But HP Labs' Nelson Chang and Niranjan Damera-Venkata had a hunch they could make a much cheaper projection system by combining the outputs of several smaller projectors to create a single, high-quality image.

As the researchers saw it, only one thing stood in their way: What they were proposing was widely thought to be impossible.

Earlier work seemed to show that you could combine two images to create one higher-definition image only when the images were offset a very small and precise distance from each other. But projectors simply can't reliably throw images onto a screen that precisely - so no one had done it.

What if, the researchers wondered, you could find a way around this and combine two, or indeed 10, very imperfectly aligned projections to create a single, super-bright and super-resolution image?

'People were saying that mathematically you couldn't do it,' recalled Damera-Venkata, but we found some loopholes no one had seen.'

The result: a flexible projector technology called Pluribus that allows HP's customers to project crystal-clear, wall-sized images for one-tenth the price of current projection systems.

How did they do it? First, said researcher Chang, the system uses a camera to capture the image that each single projector throws onto a screen.

'With the camera in the loop,' Chang said, 'we can use a very precise and automated calibration process that figures out exactly how the projectors are misaligned and accounts for differences among them.'

Even identical projectors will project images that vary in size, colour, luminance and contrast, noted Chang. But send them through the Pluribus system, and the outputs from up to 12 projectors can be calibrated - with the best portions of each recombined to create a single picture of startling brightness and resolution.

Although Pluribus looks great, its true appeal lies in the cost savings it offers anyone in the business of projecting large images. For example, Pluribus can combine ten off-the-shelf projectors costing $1,000 each to project an image as bright and sharp as that created by a single high-grade projector costing $100,000.

At the same time, Pluribus offers advantages over other multi-projector systems in image quality and labour savings. In conventional systems, it is not uncommon for two trained technicians to spend about six hours calibrating just two projectors, said Chang. In contrast, Pluribus' automated calibration process makes it possible for a single engineer to complete the task for all projectors in the system in about 10 minutes.

But Pluribus has other attractive features, noted Damera-Venkata.

'Say you are a big movie theatre and you have a $100,000 projector,' he suggested, 'what happens if the projector fails? You are left without a show. Here you have redundancy, so that when one light goes off you still have nine projectors running. It's going to be little dimmer, but the show goes on.'

Pluribus is also device neutral - it works with any number of projectors of any kind, which makes it easier for businesses to update and upgrade. In addition, it works on anything from a desktop to a stadium scale.

The system allows multiple projected images to be superimposed upon each other to create a single image of unique brightness and resolution, for example. But it is also designed to work in a "tile" pattern, which is how multiple projectors have typically have been used together.

Users don't have to choose between the tile or overlay patterns, but can adopt whatever display configuration best fits particular need. This allows a display designer to trade off screen ratio with brightness, resolution, redundancy, and cost, for example.

From the system's perspective, said Chang, 'we don't really care about the exact projector configuration; Pluribus can optimise the final displayed image regardless. Basically what you have instead is a flexible canvas onto which you can just render things. And it all works in real time, so what you render can be anything from a single image to multiple videos at once.'

He and Damera-Venkata are now working with others at HP to explore business opportunities for the system.