Editing and displaying images:
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.
Table 2.1. outlining the advantages and disadvantages of analogue and digital images
With digital images three issues need to be addressed:
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:
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.