HUMAN-COMPUTER INTERACTION
SECOND EDITION
HUMAN-COMPUTER INTERACTION
SECOND EDITION
A typical screen resolution is about 50 dpi compared with a laser printer at 300 dpi. Some packages can show magnified versions of the document in order to help in this. On the other hand, even monochrome screens can show several levels of greyness or brightness, and many screens are in colour. Good colour printers are rare and at best printers show greyness by half-tones: black dots intermingled with white ones. In addition, the sizes and aspect ratios are very different. An A4 page is about 11 inches tall by 8 wide (297 ¥ 210 mm), whereas a screen is often of similar dimensions, but wider than it is tall.
These differences cause problems when designing software. Should you try to make the screen image as close to the paper as possible, or should you try to make the best of each? One approach to this would be only to print what could be displayed, but that would waste the extra resolution of the printer. On the other hand, one can try to make the screen as much like paper as possible, which is the intention behind the paper-white A4 (or double A4) display. This is a laudable aim, but cannot get rid of all the problems.
A particular problem is with fonts. Imagine we have a line of 'm's, each having a width of 0.15 inch (4 mm). If we print them on a 50 dpi screen, then we can make the screen character seven or eight dots wide, in which case the screen version will be narrower or wider than the printed version. Alternatively, we can print the screen version as near as possible to where the printed characters would lie, in which case the 'm's on the screen would have different spaces between them: 'mm mm mm mm m'. The latter looks horrible on the screen, so most software chooses the former approach. This means that text which aligns on screen may not do so on printing. Some systems use a uniform representation for screen and printer, using the same font descriptions and even, in the case of the Next machine, PostScript for screen display as well as printer output. However, this simply exports the problem from the application program to the operating system.
The differences between screen and printer mean that different forms of graphic design are needed for each. For example, headings and changes in emphasis are made using font style and size on paper, but using colour, brightness and line boxes on screen. This is not usually a problem for the display of the user's own documents as the aim is to give the user as good an impression of the printed page as possible, given the limitations. However, if one is designing parallel paper and screen forms, then one has to trade off consistency between the two representations with clarity in each. An overall similar layout, but with different forms of presentation for details, may be appropriate.
Another important area is electronic publishing for multimedia and the World Wide Web. Whereas in desktop publishing the scanned image usually ends up (after editing) back on paper, in electronic publishing the scanned image is destined to be viewed on screen. These images may be used simply as digital photographs or may be made active whereby clicking on some portion of the image causes pertinent information to be displayed (see Chapter 3 for more on the point and click style of interaction). One big problem when using electronic images is the plethora of formats for storing graphics (see Section 2.6.5). Another problem is the fact that different computers can display different numbers of colours and that the appearance of the same image on different monitors can be very different.
One application of this technology is mail order catalogues. The order form is printed with a glyph. When completed, forms can simply be collected into bundles and scanned in batches, generating orders automatically. If the customer faxes an order the fax-receiving software recognizes the glyph and the order is processed without ever being handled at the company end. Such a paper user interface may involve no screens or keyboards whatsoever. It is paradoxical that Xerox PARC, where much of the driving work behind the WIMP interface began, have also been the developers of this totally non-screen and non-mouse paradigm. However, the common principle behind each is the novel and appropriate use of different media for graceful interaction.
What input and output devices would you use for the following systems? For each, compare and contrast alternatives, and if appropriate indicate why the conventional keyboard, mouse and CRT screen may be less suitable.
(i) LCD screen -- low-power requirement.
(ii) Small dedicated LED display (LCDs often can't be read in sunlight and large screens are fragile).
This calls for very high-precision input and output facilities. It is similar to CAD in terms of the screen facilities and printing, but in addition will require specialized data capture.
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