Archiving and StorageImages 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.
Summary:To summarise the basic steps once the image has been captured and archived/stored:
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.
There are two types of compression:
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.
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.