One of the toughest computing tasks in animation--for games, movies, or even medical imaging--is also one of the most important for realism: accurately rendering reflections and shadows. But today's best mass-market software only approxi­mates the play of light. Now researchers at companies like Adobe and Intel are developing software that can, almost in real time, change the appearance of moving objects as they pass through shadow or reflect new aspects of their surroundings. In effect, the software determines what path the light in the scene would have taken to reach each pixel. If it identifies a reflective surface or an obstruction, it changes the pixel's color value accordingly. The technology can handle multiple shadows and reflections--even reflections of reflections. Earlier versions, which took hours to execute these steps, were used mainly by architects and Hollywood animation studios. ­Daniel Pohl, an Intel researcher, says the tech­nology needs fine-­tuning, but he thinks that future personal computers with multiple processing cores could use it for everything from traversing virtual worlds like Second Life to viewing 3-D medical images. The software could reach consumers within five years.