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Archiving and Storage

Images should be archived at the highest reasonable resolution possible after taking into consideration the amount of storage space that can be afforded (see section on PhotoCD also). Several types of media for archiving exist; DAT (up to 8 GB compressed data), recordable CD (660 MB only but the data is incorruptible), re-writable optical discs (up to 1.2 GB).

For archiving purposes, colour images should be stored at 24 bit colour or in the case of grey-scale images at 256 levels of grey. Although the eye cannot recognise distinct colours or shades of grey beyond these numbers the computer will need these numbers from which to select the best possible colours when performing colour reduction. Some image formats, however, do not accommodate 24 bit colour and therefore the format chosen for archiving may also determine the colour depth at which the image can be stored.

A back-up strategy will also be required for the images once they have been converted to the desired format, resolution and number of colours.


To summarise the basic steps once the image has been captured and archived/stored:
  • Re-size the image to the required screen resolution for delivery
  • Convert to the number of colours or shades of grey as required
  • Save the file in the appropriate format

Make sure any manipulation is carried out before the steps above are followed. For further guidelines on the practical aspects of image capture see Section 3, Conclusions.

The choices made in the above three steps will be determined by the lowest technical specification of machine available to your audience or end users.

Storage Capacity and Compression

In an analogue system, storage capacity is much less restrictive than digital. A videodisc has a capacity of 54,000 still images per side. Digital imaging, however, makes significant demands on RAM and disc storage space. Typically, a 640 by 480 image with 256 colours will take up roughly 250KB (1/4 of a MByte) of disc space. Although disc storage is becoming cheaper and capacity of the "standard" hard disc is increasing, in practice space is usually limited. To counter this, data compression techniques are used to reduce the volume of data in order to minimise the demands made on storage and processing power and improve time taken to load or save files. Compression is achieved using algorithms (mathematical formulae), which identify the information that needs to be recorded and store it. It is then 'reconstructed' during decompression.

There are two types of compression:

  • lossless, in which all the data is preserved and typically will compress images 2:1
  • lossy, in which the data is degraded, the more so the greater the compression ratio used.
Lossless techniques are mainly used for text-based data where compression rates are quite high due to common letter groupings, etc. For images, techniques such as run length encoding are employed within some image formats such as PCX and BMP to reduce file size. Run-length encoding takes stretches of pixels sharing the same colour and stores the information for these pixels in just two bytes; one for the colour and the other for the number of adjacent pixels. Ratios of typically 2 or 3:1 can be achieved with this technique.

Many of the lossy compression techniques seek a compromise between quality and quantity and rely on human ability to compensate for losses, exploiting the way we perceive. However, there are some subject areas where the use of lossy techniques demands serious attention and research, particularly in the medical field. Many of these techniques are designed to compress moving video as well as still images. Such techniques include JPEG/MPEG, Fractal compression (still and video), Video for windows and Apple Quicktime. It is not within the scope of this document to provide any great detail of video compression as this area demands its own publication. However, all these terms will be explained in the following paragraphs.

JPEG: As with image formats, the widespread need for compression methods has resulted in the emergence of a plethora of techniques. Consequently the International Standards Organisation (ISO) set up two groups, the Joint Photographic Expert Group and the Motion Picture Expert Group (MPEG) to establish international standards for the compression/decompression of still and moving video and associated audio.

A number of capture cards will save the image as a JPEG format although this can be performed in software alone; many of the image processing programs offer this facility and JPEG is now commonly recognised as another file format. JPEG also comes in two flavours, JPEG for still images and motion JPEG. Motion JPEG is a modified version of standard JPEG that calculates the differences between frames instead of storing every frame. A whole key frame is stored every 8th frame. JPEG can offer still image compression ratios of 25:1. MJPEG capture boards are available but beware; there are various non-compatible versions of MJPEG around.

MPEG: Already we have two standards for MPEG - these are MPEG I (a sub-set of which has been defined for VideoCD/White Book CD and CD-I) and MPEG II. MPEG II is designed to offer higher quality at a bandwidth of 1.2 Mbit/second at 704 x 480 pixels and 30 frames per second (fps) and is used for images of high definition TV size. MPEG I has been developed to fit into a bandwidth of 1.5 Mbit/second to allow data retrieval from single speed CD-ROMs at 320 x 240 pixels at 30 fps. Compression ratios from 30:1 to 200:1 are obtainable.

MPEG is even more advanced that Motion JPEG and uses a process called predictive calculation. Information in the current frame is used to predict the information in following frames. A further standard, MPEG IV (combining MPEG III ) is under development. Algorithms for playing MPEG I movies in software alone are available for use under Video for Windows and Apple's Quicktime but will suffer the same scalar problems (see Video for Windows section). Boards for providing hardware-assisted playback are also available, allowing full-screen, full-motion video. However, MPEG is still not mainstream technology and several computer magazines are reporting on incompatibility problems between computer, CD-ROM drive and MPEG playback cards.

One of the drawbacks of MPEG is that a great deal of processing power needs to be applied to perform the compression in the first place. Traditionally this has meant expensive, dedicated MPEG editing systems, or paying a lot of money to a bureau to do it for you, although now relatively low-cost expansion cards are beginning to emerge which claim to do the job for you.

Video for Windows: Developed by Microsoft, this sets a standard for incorporating digital video under Windows. For many, Video for Windows is confusing; however, Video for Windows is more of a container for new technologies. Video for Windows created a new file standard called AVI (Audio Video Interleaved). The AVI format merely defines how the video and audio will be stored on your hard disc, that is, interleaved with the audio for frame 1 followed by the video for frame 1, the same for frame 2 and so on. This may appear simple but is important, as without interleaving, programs would have to jump from place to place on your hard disc to find the next bit in the sequence. This slows things down and so anything that reduces the demands made on the hard disc by video is important. What AVI doesn't do, however, is define how the video will be captured, compressed or played back. This means that as new technology for video is introduced, it can be incorporated into Video for Windows. AVI files are played using the Media Player supplied with Windows 3.1.

Video CODECs are software compression/decompression algorithms that define how the video is captured, compressed and played back. A number of these exist for Video for Windows and include Indeo 3 (Intel), Cinepak (licensed by Microsoft from SuperMac), Microsoft Video 1, offering compression ratios of up to 50 to 60:1.

  • Indeo: One of the most important advantages of Indeo is that it can be used for real-time compression but needs dedicated hardware such at IntelĂs Smart Video Recorder (specifically IntelĂs I-750 chip) to perform the compression, although decompression is in software only. That is, the video is compressed as the data is captured. However, if the video requires extensive editing, real-time compression should not be used. Indeo is a 24 bit CODEC and dithers well to 256 colours when displayed on an 8 bit display; there is no need to reduce the number of colours as the CODEC copes by itself. Needless to say, to reap the benefits, Indeo video should be played on 16 bit or higher displays. Compression ratios of 10:1 are obtainable. The latest version of the CODEC is 3.2.
  • Cinepak: As with Indeo, Cinepak also offers very good image quality and is capable of higher compression ratios than Indeo. It is best used for high action sequences. Decompression of Cinepak, however, is not as good, taking 4 to 10 times slower than Indeo when using software playback alone, that is without a hardware board to assist playback. Both compression and decompression are performed in software. Compression ratios of 10-20:1 are obtainable. Cinepak was originally developed by SuperMac for integration into AppleĂs Quicktime (see below) but has been licensed to Microsoft for Video for Windows.
  • Microsoft Video 1: Is not in the same league as Indeo and Cinepak but can be used as a straight 8 bit CODEC; the advantage is in reducing file size. Only to be used when hardware is limited.

Quicktime Apple have provided their own video-plus-audio file format equivalent to Video for Windows, called Quicktime. Quicktime provides a basic set of software schemes that meet a range of compression needs for still images, animation and video. Quicktime provides support for both Cinepak and Indeo video. A CODEC for playing Quicktime movies is available for Video for Windows (although Quicktime movies play back slower on PCs than AVI files) as is a converter for Quicktime to the AVI and MPEG formats. Quicktime 2.0 will capture movies at 30fps and quarter screen resolution (320 x 240). If 15 fps is acceptable you can show movies at full screen but performance will depend on the power of your MAC.

AVI and Quicktime files are played back in software alone and because of this the speed of playback and the size of the video window will depend on the power of the processor and graphic capability of the machine and the video will be scaled accordingly. A clip that looks perfectly acceptable on a Pentium 90 system may be barely recognisable as a piece of video on a 20mhz 386. On a Pentium Cinepak and Indeo can achieve 30 fps at 320 x 240 pixels and can be enlarged to 640 x 480 using graphics acceleration.

Fractal Compression Developed by Iterated Systems, it offers greater compression using algorithms based on fractal transforms. Still images can be compressed by up to 100:1. For video Iterated Systems have developed ˘Softvideo,÷ providing full screen colour video at 30 frames per second on a PC using software alone. However it takes 15 hours to compress one minute but decompression is fast. The movies are also scaleable, as with still images, the videoĂs resolution being independent of the size of the window. Fractal compression is a proprietary format and standard image processing software will not read this format.


It is not within the scope of this document to discuss copyright in any great depth. However, with the great advances that have been made with technology it is extremely easy to breach copyright. Here are a few helpful tips to avoid this:
  • Ideally, only capture material of which you are the copyright holder, or for which the copyright holder is known such that permission can easily be sought.
  • The 1988 Copyright Act states that where a copyright work has been produced by someone Šin the course of his employment, his employer is the first owner of any copyright in the work subject to any agreement to the contrary.. Your images may therefore be the copyright of your institution. There are some instances where the institution has given back these rights to the individual employees. Check your own situation.
  • Do not scan images from books and other published work. Even if it is for in-house educational use only, you are still breaking the law. If you are the author you may still need to check with the publishers whether you hold copyright.
  • In this country all work is automatically copyrighted even if it does not bear the copyright symbol.
  • If you do wish to use one or two images from a book containing thousands, it is worth contacting the publisher. Some publishers will give reproduction rights for one or two images for non-profit educational projects for an agreed use. Check with publishers first.
  • Always gain copyright/reproduction rights before you embark on image capture. This could avoid needless expenditure of time and money at a later date.

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