Graphics and multimedia:Data compression

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Data compression

In order to reduce the amount of space occupied by video images, data compression techniques are employed. Intel's DVI chip set, for example, can achieve compression ratios of around 160:1. The speed of compression (to store the image) and decompression (to reproduce the image) are very fast, and we are not far off the day when full-motion high-resolution video can be reproduced by a PC from compressed files.

Data compression uses the fact that a great deal of information in an image or sequence of images is redundant. For example, expanses of sky do not change from pixel to pixel within an individual frame, nor from frame to frame within a sequence of frames. This redundant data can therefore be thrown out, all that is needed is the data for a single pixel together with the area of the frame, and the sequence of frames, to which it applies.

A variety of mathematical techniques are used to increase the amount of compression. For example, the Philips CD-I system employs a compression technique called Discrete Cosine Transform (DCT), which breaks down each frame into blocks, and compares how these change from frame to frame. If there is little change from frame to frame the blocks can be bigger, and less code is therefore needed to produce a succession of frames.

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Virtual reality systems

You have probably seen examples of interactive video material, such as the BBC's Domesday Project, perhaps on TV. If so, you will know that many videodiscs include numerous shots of buildings, streets, or towns taken from many angles, and that the computer-driven interactive part of the system allows you to 'travel' around the building or town. Unlike an ordinary film, with this system you can choose which way to go (by perhaps pointing and clicking with the mouse), and you can pause in any location and look around before proceeding.

It's a bit like real life, except that it only impacts your visual senses - you can't, for example, touch any of the objects you are looking at, and you are always aware that you are not really 'there' but are in fact sat in front of a monitor. But what if you could 'touch' the objects, and what if you could see them in three dimensions on a wrap-around screen. And what if you could move around by some more natural means than pushing and clicking a mouse?

Well, systems are aroid that can do this, and they transport the user into a 'virtual reality' - something that closely simulates real life.

Virtual reality systems were first developed by NASA in the mid 1980s, to solve the problem of repairing space stations. Ideally, it wanted to use robots to work in the hostile environment of space, but there are repair situations where the human skills are essential. Its solution was to feed signals from the video camera in the robot's 'head' to the human astronaut on the ground, and relay back to the robot the movements of the astronaut's hands. So a headset was developed containing two tiny TV sets, one for each eye, to give stereo vision. It also contained tracking circuitry to transmit movements of the astronaut's head to the robot, so that the camera automatically pointed wherever the astro­naut wished to look. The astronaut also wore a data glove which could transmit the movements of his hands and fingers to the robot.

Today, half a decade later, virtual reality systems have progressed considerably:

• The headset now contains stereo earphones for sound.

• The user may sit in a console of some kind which simulates the movement of aircraft or other trans­ portation.

• In many virtual reality applications, the whole system is hooked up to a computer, which generates the simulated environment. In these systems the data glove will be programmed to interpret certain movements as com­mands. For example, if you wish to move through the simulated environment, you simply point your finger in the desired direction.

• Instead of a data glove it is now possible to don an entire 'datasuit' so that movements of the entire body can be electronically sensed and transmitted.

Virtual reality applications include:

• Flight test simulators.

• Control of robots and other devices in environments that are impossible for humans. Space station mainten­ance has already been mentioned, but future possibilities seem limitless, from microsurgery (by controlling a tiny surgical device inserted in the patient's body) to testing the design of buildings by walking through computer simulations of them.

• Entertainment and leisure applications, such as com­ puter 'arcade' games.

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