Virtual ghosts have been unleashed in the machine by a team of computer scientists at Nottingham University, writes Steve Farrar.

The scientists have developed a way to mix past and present action within virtual environments, adding a new layer of content to entertainment, education, simulation and training systems.

The system's flexibility will offer many new options to virtual environment designers, allowing users to watch past events.

It also offers an opportunity to link other media such as film and television to the virtual environment.

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The system of temporal links has been created by Chris Greenhalgh, reader in computer science, Steve Benford, professor of collaborative computing, and PhD students Martin Flintham and Jim Purbrick.

Their latest results have been presented to the IEEE virtual reality conference in the US.

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Relatively little attention has previously been given to exploring the use of temporal links within virtual environments. Most focus purely on live action.

However, the Nottingham team believes its approach will produce a richer experience.

The system is very flexible, allowing the temporal links to run backwards and forwards while squeezing the past action into whatever spaces are desired by the designers, from being directly superimposed over the live world to being confined within a globe.

It works by recording a full snapshot of the world and every subsequent change. This time-stamped data is then made available for future use.

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"This opens up a lot of possibilities, some practical, such as analysis and debriefing in a training simulator, and others more esoteric like flashbacks that increase narrative possibilities," Professor Benford said.

The team created a test called Avatar Farm in the multi-user collaborative virtual environment known as Massive-3.

Four players explored this world and were able to learn more about their environment through a series of intermittent flashbacks of pre-recorded scenes that filled in the history and tensions between their various characters.

One experiment involved using a large 360-degree surround cinema in Germany to project each player's view of their shared virtual environment while infrared cameras tracked their movements within the dome.

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