From the preceding discussion it should be apparent that real world images take up a lot of memory in the computer so something should be said about moving this data around. For example, a photographic quality image of the kind obtainable from a 35mm colour print has a resolution of 3000x2000 (or 6Megapixels) and if stored with 24bits per pixel would need 18Megabytes! To give this figure some kind of perspective, the maximum data transfer rate of the ISA bus found in ageing PCs is about 2.4Megabytes/second while even that of the PCI bus is only 33Megabytes/second. If you are considering showing photographic quality movies from disc on even a high-end PC don't be surprised if it doesn't work. Much work has been put into developing data compression techniques that allow the display of at least TV quality images in real time which means a resolution of about 320x200 at a frame-rate of 50Hz, with the sort of data rates that can be sustained by a CD-ROM drive and a PC and you will often see the acronym MPEG (Motion Picture Experts Group) in this context but so far these techniques are of little benefit to vision researchers. Of more interest are the data compression algorithms designed for still images such as JPEG or PCX which can be used to compress images for storage on disc. They are often used as a common file interchange format between graphic and image processing programs but the computation required in compression and decompression renders them unsuitable for real-time use.

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