ABSTRACT
The cost of data storage may have tumbled to a fraction of what it was even five years ago, and the Internet’s bandwidth is constantly increasing, but the requirement for storing and transmitting data in ever smaller packages remains. We now have effective audio data reduction technologies, but before that came data reduction and compression of images. JPEG compression for still images is the basis of MPEG, the moving picture equivalent. But before we try to understand image compression, it is probably a good idea to take a look at what an uncompressed image consists of. One way to do this is to think in terms of computer displayed images as it provides a simple starting point, although this is relevant to all digitized image systems. If you look back to an earlier age of computing, particularly to the old compact Macintosh range or Atari ST, you may remember images in which each pixel (picture element) was described by a single bit of data. That 1 bit could be either 1 or 0 and the pixel therefore could only be either fully black or fully white. These screens did not show any levels of grey, except by mixing areas of black and white pixels. When you upgraded your 1 bit monochrome computer to a more modern colour-capable model your dealer would have informed you that the monitor was capable of showing 16 colours, 256 colours, ‘thousands’ of colours or ‘millions’ of colours. Colour monitors are in fact capable of any of these; it is the amount of RAM that is dedicated to video inside the computer that is the limiting factor. If you want each pixel to be capable of 16 colours then fairly obviously 4 bits of RAM per pixel are required, multiplied by the number of pixels in the screen. 256 colours require 8 bits, 32 768 colours (thousands) require 15 and 16 777 216 (millions) require 24. ‘Millions of colours’ is becoming the standard these days and it is easier to think of it as 8 bits for each of the primary colours. This explains why 15 bits are used for 32 768 colours rather than 16 for 65 536; 5 bits each are used for the red, blue and green primaries. You may be asking at this stage whether it is really necessary to have millions of colours. Can the eye really distinguish this many? For most intents and purposes thousands of colours on a computer screen are indistinguishable from millions, but the extra colours are used as ‘professional headroom’ just as we like to have
extra bits – when we can get them – above and beyond the 16 bit domestic CD standard. Even at the thousands of colours standard, a 640 480 pixel image demands more than half a megabyte of data. A moving image which consumed data storage at this rate would require a data rate of well over 100 megabits per second. This is impractical for any consumer medium.
