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Warping images using your PC graphics card

By projecting images onto contoured surfaces you get a virtual experience that puts you at the center of the action. Now, a quick and easy calibration technique could help the concept find a wider application beyond the planetarium.

It's almost as if you were weightless. You can reach out and touch the Big Dipper and all around you are multitudes of twinkling stars. The glowing band of the Milky Way is clear in the sky, leaving you with the impression of actually being enveloped within it. In short, the perfect illusion – numerous projectors broadcasting a recreated version of the actual night sky onto the huge dome of the planetarium. The biggest of these domed projection screens measure over 20 meters in diameter. Now, scientists from the Fraunhofer Institute for Open Communication Systems FOKUS in Berlin have joined forces with graphics card manufacturer NVIDIA to integrate the technology required to project images of this type using a standard PC graphics card. "Desktop warping" allows any Windows desktop display to be projected onto curved screens – just like in a planetarium.

But how exactly do you go about "warping" screens? Manuel Schiewe, a FOKUS researcher who has dedicated a lot of time to the topic, is on hand to provide some answers. To get an even picture on a curved screen, each of the projectors must be precisely calibrated with one another. Each of them projects a part of the image, parts which together form the whole. "It's a bit like a puzzle. Every piece has to slot in exactly with the others to get the right picture," explains Schiewe. Projectors must be set up so that the images they project are of an equal brightness and suitable for a curved surface. This requires Schiewe to recalibrate the images, which were originally intended for a flat screen.

Initially, the only way this could be done was to calibrate manually. "As soon as the position of the projectors moved even slightly, we had to step in and make manual adjustments," recalls Schiewe. The solution came from Fraunhofer FOKUS's laboratories in Berlin, where researchers developed a piece of software that automatically calibrates images to match the projection screen. To do this, cameras capture test images generated by the projectors. The software then uses image processing algorithms to calculate the current calibration of the images on the projection screen. It then has all the information it needs to make auto- matic corrections to the calibration and brightness of the images. "Today, planetariums, theme parks and simulators use the `Projector auto-alignment' tool to guarantee an even picture on large contoured surfaces," says Schiewe.