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Editing and displaying images:

Analogue model

Once recorded, analogue images cannot be edited; if the wrong image has been recorded the only alternative is to re-record. With tape, certain frames can be erased but with recordable videodisc the technology employed is "Write Once Read Many" (WORM) or "Analogue Write Once" (AWO) technology and in this case the images cannot be deleted. The standard videodiscs are read only. In some cases the cataloguing software can be programmed not to access particular images. The capacity of tape and videodisc is large and the loss of a few frames is not disastrous. However, re-writable videodisc technology is now available; although the original recorded image cannot be edited, it can be erased and re-recorded.

Interactive video may be a one-screen or two-screen system. In the case of the latter a second display device is required to display the video image. Some videodisc players possess a character-generating set, allowing arrows, for example, to be drawn on the videodisc image. For more powerful interaction a one-screen system is needed but this necessitates an extra piece of hardware: an overlay card. The overlay card or board combines the incoming analogue signal and digital information from the computer and displays the information on one screen. There are a few overlay cards available on the market, the price ranging from £200 to approximately £1,000 depending on additional functions required. The majority of overlay cards work by digitising the incoming analogue signal, allowing manipulation of the video image. Commands and routines, or in some cases interfaces, are supplied with these boards, enabling capture and storage of the digitised video signal, the boards doubling up as a frame grabber/digitising board.

To include video images in computer-based learning material, drivers will be needed to control the videodisc player from the computer. In some instances the drivers will be supplied with the authoring software. If not, the videodisc player manufacturers should be able to supply the appropriate driver. For Windows development, the drivers can also be obtained from the relevant electronic bulletin boards.


                                   Analogue                            Digital

Advantages                 Very high quality, unlimited        Can be edited and manipulated
                           palette                               at any time 

                           Large storage capacity              Multiple copying without loss of 
                                                               quality
  
                          Full-screen, full motion, high-      Workstations much cheaper and
                          quality video                         common 

                          Highly developed technology


Disadvantages             Difficult to edit or manipulate      Quality loss on initial
                         (although sequence in which images    digitisation in some instances
                          appear can be)

                         Copying results in loss of            large files - reduced storage 
                         quality                               capacity 
 
                         Workstations can be expensive         Full-screen, full-motion video,
                         and therefore delivery machines       currently available only with 
                         rarer                                  hardware assisted playback

                                                               Standards for moving video 
                                                               remain in conflict
Table 2.1. outlining the advantages and disadvantages of analogue and digital images

Digital model:

When the images have been captured, using the appropriate hardware, they are not necessarily ready for importing directly into computer-based learning authoring software. The employment of image processing software may be needed for further image manipulation.

With digital images three issues need to be addressed:

  • Image formats
  • Resolution
  • Number of colours
Image processing software provides the means to change the format, resolution and number of colours of the captured image, as well as a variety of other complex features. Packages vary in cost depending on the complexity of the package. Adobe Photoshop is now regarded as being the industry standard but remains one of the most complex and expensive packages around. A cheaper package may provide all the functions and more besides that you require.

Image File Formats:

What image file formats does my authoring software recognise?

Computer graphics (including images) come in two different forms: vector based (also called object-oriented) graphics and bit-mapped (also called raster based) graphics. Vector based images are described by formulae. They look smooth on a display and because of the way the information about the image or graphic is stored they are rendered evenly at any size or orientation. The mathematical formula that describes the file contains specifications about both the dimension and direction that is associated with them. Thus the images can be scaled or resized without distorting the object. Software systems that allow the creation of vector graphics are called draw programs. Typical examples of vector based images are technical illustrations, floor plans, maps, diagrams and charts.

For photorealistic images we are really concerned with bit-mapped images. Bit-mapped images are made up of a number of pixels to form a mosaic. When enlarged, the individual pixels themselves are enlarged and hence they reveal stepped edges often referred to as "jaggies". Each pixel or picture element has a value, made up of 1's and 0's, that is stored. For this reason, bit-mapped images can become very large in terms of file size (see also section on compression). The advantage of bit-mapped images is that the individual picture elements can be manipulated and controlled. However, this requires considerable processing power. Software programs that manipulate bit maps are known as paint programs. Applications include imaging, photo retouching and other art based techniques.

Simple paint programs are ineffective for manipulating digital images because they do not contain the tools necessary to manipulate images globally or to work with a region of the image. Colour image processing software has a wide variety of tools for extensive image editing. This must be taken into consideration before purchasing software and your choice will depend on the functions you require. As previously mentioned, quite sophisticated image processing software may be provided or come bundled with capture boards and scanners.

Image (graphic) file formats abound, each one having evolved for a reason. Fortunately there are many common file formats and the majority of software (image processing and authoring software) recognise the common formats. In some instances the software that drives and enables image capture from your scanner or video board may allow you to save the image as one of the more common file formats directly.

Other hardware may capture an image as a proprietary format. In these cases software should be supplied as part of the package that converts the proprietary format to one of the more common formats.

There is a third class of digital files that accommodates both vector and bitmapped information. These are called metafiles, although both types of information are rarely written into a metafile.

The most common formats include:

  • Windows BMP: Windows native bitmap format. This format came into use with Windows 3.0. It is an uncompressed format so file sizes can be quite large and therefore seldom the choice for large high-resolution images. However, it has widespread support in the Windows world.
  • PCX: One of the oldest and most common raster formats for PCs (PC Paintbrush format) using RLE (run-length encoding) for compression. It is starting to be replaced by other, newer formats.
  • TIF(PC)/TIFF(MAC): Tagged Image File Format, a general purpose multi-platform raster format developed by Aldus and Microsoft designed specifically to provide a basis for importing scanned images into DTP packages. Comes in various versions and may be compressed using LZW compression. Both PC and MAC
  • TGA: TARGA file format. The first popular format for high-resolution images. No compression methods or tags are employed and hence TGA files are large. A PC format.
  • GIF: Graphics Interchange format, owned and used by Compuserve. A sophisticated format, providing amongst other capabilities LZW compression, allowing small files to be created that can easily be transferred over networks. Software packages' support for GIF is increasing and several shareware viewers/converters are available. One of the standards for the World Wide Web network information system.
  • PICT: Macintosh format. All Macintosh programs save and read PICT files.
  • EPS: Encapsulated post script developed by Adobe Systems Inc. Metafile, supporting both vector and raster information. Multi-platform. EPS files can be easily converted to other formats but the reverse is not so easy. EPS was developed to enable images to be sent to a printer supporting Postscript.
  • CGM: Computer Graphics Metafile, one of the most widely supported metafile formats created by the ANSI in order to provide a common ground for graphical information interchange. Supports both vector and bitmapped information.
  • WMF: Windows Metafile. A vector format allowing images to be resized proportionately on screen. Not commonly used and support for the format outside Windows is limited.
  • JPEG: Joint Photographic Expert Group. Designed for maximal image compression. JPEG uses a new kind of compression called "lossy" compression - information is lost on decompression but in such a way that the human eye won't miss it. Lossy is not to be confused with lousy! JPEG images may also degrade further if manipulated further and saved. Make sure that compressing the image as a JPEG image happens last after all manipulation or keep the original as a lossless format.
  • RIFF: Resource Interchange File Format. A family of file structures rather than a single file format supporting multimedia. Developed by Microsoft and IBM. Includes PAL (Palette file format), WAV (Waveform Audio format), MID (MIDI format) and DIB files (see next format).
  • DIB: Device Independent Bitmap. Popularised by Windows. Files saved in this format can be displayed on a variety of devices. Similar to BMP images and can be opened in some packages, although not offering DIB as a choice, by setting the file type to BMP. Used mostly by programmers who need to worry about displaying images on a variety of devices.
  • PhotoCD: A proprietary format developed by Kodak. See subsection on PhotoCD.

Note: The GIF format incorporates LZW compression technology developed by the Unisys Corporation. In early 1993 Compuserve were notified by Unisys of patent rights granted to LZW. Compuserve have negotiated licenses for themselves and the software developers who work with them. However, this situation won't immediately affect users in higher education. Unisys have recently issued a statement saying that they don't require licensing, or fees to be paid, for non-commercial, non-profit GIF-based applications, including those for use on on-line services. Concerning developers of software for the Internet, the same principle applies. Unisys will not be pursuing previous inadvertent infringement by developers producing versions of software products prior to 1995.

Compuserve are now co-ordinating the development of GIF24 - a 24-bit lossless compression format with an open specification available without cost.

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