A Soft-edged Character Set and its Derivation

Characters are normally displayed on raster display devices in the form of dot arrays. Humble VDUs often use 6 x 10 arrays spaced on an 8 x 11 grid of positions. Each character is then defined by 60 bits and the entire character set is stored in a read only memory of, say, 96 (characters) x 6 (bits wide) x 10 (bits high). This is 5760 bits all together and accounts for only a tiny proportion of the component cost of the unit electronics. In some of the 'better' terminals more characters may be provided. Some character sets may be held in random access memory and thus be loaded dynamically from a user's definitions, but it remains the case that the character generator logic is relatively small compared with everything else. Indeed, this is increasingly true with more and more upmarket displays—most of the increased function being devoted to picture storage and manipulation. A binary dot array is not in fact the best way to render a letterform on a CRT display. The dottiness produces a pretty crude and unsatisfactory image which isagly and often hard to read. Raster displays do, it is true, tend always to be seen showing dotty and ugly pictures but diagrams are not intended for 'reading' in quite the same manner as running text. Fortunately, this dottiness is not intrinsic and solutions are known for arbitrary images, including text' Quite stunning improvements can be achieved by simple methods which can be cheaply implemented for character displays. This paper discusses the techniques and presents a complete alphabet definition as an example of the method in use.